1. Overview
Is an elf32 static and stripped binary, but the good news is that it was compiled with gcc and it will not have shitty runtimes and libs to fingerprint, just the libc ... and libprhrhead
This binary is writed by Ricardo J Rodrigez
When it's executed, it seems that is computing the flag:
But this process never ends .... let's see what strace say:
There is a thread deadlock, maybe the start point can be looking in IDA the xrefs of 0x403a85
Maybe we can think about an encrypted flag that is not decrypting because of the lock.
This can be solved in two ways:
- static: understanding the cryptosystem and programming our own decryptor
- dynamic: fixing the the binary and running it (hard: antidebug, futex, rands ...)
At first sight I thought that dynamic approach were quicker, but it turned more complex than the static approach.
2. Static approach
Crawling the xrefs to the futex, it is possible to locate the main:
With libc/libpthread function fingerprinting or a bit of manual work, we have the symbols, here is the main, where 255 threads are created and joined, when the threads end, the xor key is calculated and it calls the print_flag:
The code of the thread is passed to the libc_pthread_create, IDA recognize this area as data but can be selected as code and function.
This is the thread code decompiled, where we can observe two infinite loops for ptrace detection and preload (although is static) this antidebug/antihook are easy to detect at this point.
we have to observe the important thing, is the key random?? well, with the same seed the random sequence will be the same, then the key is "hidden" in the predictability of the random.
If the threads are not executed on the creation order, the key will be wrong because is xored with the th_id which is the identify of current thread.
The print_key function, do the xor between the key and the flag_cyphertext byte by byte.
And here we have the seed and the first bytes of the cypher-text:
With radare we can convert this to a c variable quickly:
And here is the flag cyphertext:
And with some radare magics, we have the c initialized array:
radare, is full featured :)
With a bit of rand() calibration here is the solution ...
The code:
https://github.com/NocONName/CTF_NcN2k15/blob/master/theAnswer/solution.c
3. The Dynamic Approach
First we have to patch the anti-debugs, on beginning of the thread there is two evident anti-debugs (well anti preload hook and anti ptrace debugging) the infinite loop also makes the anti-debug more evident:
There are also a third anti-debug, a bit more silent, if detects a debugger trough the first available descriptor, and here comes the fucking part, don't crash the execution, the execution continues but the seed is modified a bit, then the decryption key will not be ok.
Ok, the seed is incremented by one, this could be a normal program feature, but this is only triggered if the fileno(open("/","r")) > 3 this is a well known anti-debug, that also can be seen from a traced execution.
Ok, just one byte patch, seed+=1 to seed+=0, (add eax, 1 to add eax, 0)
before:
after:
To patch the two infinite loops, just nop the two bytes of each jmp $-0
Ok, but repairing this binary is harder than building a decryptor, we need to fix more things:
- The sleep(randInt(1,3)) of the beginning of the thread to execute the threads in the correct order
- Modify the pthread_cond_wait to avoid the futex()
- We also need to calibrate de rand() to get the key (just patch the sleep and add other rand() before the pthread_create loop
Adding the extra rand() can be done with a patch because from gdb is not possible to make a call rand() in this binary.
With this modifications, the binary will print the key by itself.
Related word
- Pentest Tools For Mac
- How To Install Pentest Tools In Ubuntu
- How To Hack
- Hacker Tools Hardware
- Hack Tools For Ubuntu
- Hacking Tools 2020
- Tools 4 Hack
- Hacking Tools Windows 10
- Hacker Tools Hardware
- Pentest Tools Open Source
- Hackrf Tools
- Hack Tool Apk
- Pentest Tools Android
- Hacking Tools For Windows 7
- Easy Hack Tools
- Ethical Hacker Tools
- Hacker Security Tools
- Hack And Tools
- Hacker Techniques Tools And Incident Handling
- Pentest Tools Port Scanner
- Nsa Hacker Tools
- Android Hack Tools Github
- Tools For Hacker
- Hacker Tools Github
- Hackers Toolbox
- Pentest Tools Linux
- Hacking Tools Name
- Kik Hack Tools
- Hacker Tools List
- Nsa Hack Tools
- Hacker Tools 2020
- Beginner Hacker Tools
- Game Hacking
- How To Hack
- Github Hacking Tools
- Hacker Tools For Windows
- Wifi Hacker Tools For Windows
- Hacker Tools Software
- Hacking Tools Download
- Pentest Tools Github
- Hack Tool Apk
- Tools For Hacker
- Hacker Tools Hardware
- Hack Apps
- Hack App
- Hack Tools Github
- Hack Website Online Tool
- World No 1 Hacker Software
- Pentest Box Tools Download
- Pentest Automation Tools
- Beginner Hacker Tools
- Pentest Tools Online
- Hacking Tools Pc
- Hacking Apps
- Pentest Tools Bluekeep
- Hacking Tools Download
- Pentest Tools Windows
- How To Make Hacking Tools
- Hacking Tools For Windows 7
- Pentest Tools Tcp Port Scanner
- Pentest Tools Windows
- Hackers Toolbox
- Hacking Tools Mac
- Hacking Tools Pc
- Tools 4 Hack
- What Is Hacking Tools
- Hacker Techniques Tools And Incident Handling
- Hacking Tools Name
- Hack Tool Apk No Root
- Hacker Tools Free Download
- Hack Tools
- Hacker Tools Free Download
- Github Hacking Tools
- Hacking Tools 2019
- Pentest Reporting Tools
- Hacking Tools For Games
- Hack Tools For Windows
- Underground Hacker Sites
- Android Hack Tools Github
- Hacker Hardware Tools
- Hacking Tools For Kali Linux
- Pentest Tools Find Subdomains
- Hacker Tools Hardware
- Hacking Tools Windows
- Hacking Tools Online
- Hacking Tools For Kali Linux
- Pentest Tools Android
- Hacking Tools Windows
- Hack Rom Tools
- Pentest Tools Port Scanner
- What Are Hacking Tools
- Pentest Tools Open Source
- World No 1 Hacker Software
- Hack Tools 2019
- Hackers Toolbox
- How To Hack
- Top Pentest Tools
- Hack Apps
- Github Hacking Tools
- Hacking Tools For Windows Free Download
- Hack Tools
- Hack Tools For Windows
- Pentest Tools Bluekeep
- Hacking App
- Hack Rom Tools
- Hacking Tools Pc
- Pentest Tools Android
- Pentest Tools Kali Linux
- Hacker Tools Linux
- Hacker Tools Online
- Hacker
- Hack Tools For Mac
- Pentest Tools Bluekeep
- Pentest Tools Website Vulnerability
- World No 1 Hacker Software
- Hacking Tools Kit
- Hack Tools
- Nsa Hack Tools Download
- Easy Hack Tools
- Hack Tools Online
- Hack Tools Github
- Hacking Tools Kit
- Physical Pentest Tools
- Hack Tools For Pc
- Hacking Tools For Kali Linux
- Hacker Tools For Ios
- Android Hack Tools Github
- Best Hacking Tools 2020
- Underground Hacker Sites
- World No 1 Hacker Software
- Hack Tools For Pc
- How To Install Pentest Tools In Ubuntu
- Usb Pentest Tools
- Hacking Tools
- Pentest Tools Github
- How To Make Hacking Tools
- Hacker Tools Free Download
- Best Hacking Tools 2019
- Hacking Tools Pc
- Install Pentest Tools Ubuntu
- Pentest Tools Open Source
- Nsa Hack Tools
- Hacker Tools Free
- Pentest Tools For Windows
- Hacker Tools Apk Download
- Underground Hacker Sites
- Hack Tools Github
- Hacking Tools Windows
- Hacking Tools Mac
- Hackers Toolbox
- Hacking Tools Hardware
- Hack Tools
- Hack Tool Apk
- Termux Hacking Tools 2019
- Pentest Tools List
- Hack Tools Pc
- Pentest Tools For Ubuntu
- Hack Tools 2019
- Hack Website Online Tool
- Easy Hack Tools
- Hack Tools For Mac
- Hacking Tools For Pc
- Pentest Tools Free
- Hacker Tools For Ios
- Hacker Tools Windows
No hay comentarios:
Publicar un comentario