Linux Reverse Engineering CTFs for Beginners

After a while, I decided a write a short blog post about Linux binary reversing CTFs in general. How to approach a binary and solving for beginners. I personally am not a fan of Linux reverse engineering challenges in general, since I focus more time on Windows reversing. I like windows reverse engineering challenges more. A reason me liking Windows is as a pentester daily I encounter Windows machines and it’s so rare I come across an entire network running Linux. Even when it comes to exploit development it’s pretty rare you will manually develop an exploit for a Linux software while pentesting. But this knowledge is really useful when it comes to IoT, since almost many devices are based on Linux embedded. If you want to begin reverse engineering and exploit development starting from Linux would be a good idea. I too started from Linux many years ago. Saying that since some people when they see a reverse engineering challenge they try to run away. So if you are a newbie I hope this content might be useful for you to begin with.

The ELF Format

Let’s first have a look at the ELF headers. The best way to learn more about this in detail is to check the man pages for ELF.

Here’s in more detail. The “e_shoff” member holds the offset to the section header table. The “sh_offset” member holds the address to the section’s first byte.
(more…)

Advertisements

Haxing Minesweeper

Recently I tweeted a screenshot where I won the Minesweeper game by looking at the mine field from the memory. I posted this for no reason, just for fun since I was happy that I finally won this game. I used to play this game back in 2002 in Windows XP and I never won this game, I never even understood how this game works until today when I read how it really works 😀

In few minutes my notifications were flooded, I didn’t expect to get this much of likes. Some people asked me a tutorial on this. I thought of writing a very quick blog post on this. Pardon me if I missed anything.
(more…)

A Basic RSA Encrypter

This is a small post about implementing a basic RSA encrypter to encrypt sections in an exe. We can use this to exchange exes with people. We will encrypt the section using the public key and the user has to use his private key to decrypt the exe. This can be applied in evading anti-viruses too.

I will use multiplication instead of an exponent. Since it would be easy to implement in few lines in assembly. However, this will allow breaking the private key easily hence the complete scheme is broken.

Enc = (m*e) \text{ mod } N

Dec = (c*d) \text{ mod } N

The correctness of this scheme depends on the fact that

Dec(Enc(m)) = (m*e*d) \text{ mod } N = m \text{ mod } N

(more…)

Windows Kernel Exploitation: Stack Overflow

Introduction

This post is on exploiting a stack based buffer overflow in the HackSysExtremeVulnerableDriver.
There’s lot of background theory required to understand types of Windows drivers, developing drivers, debugging drivers, etc. I will only focus on developing the exploit while explaining some internal structures briefly. I would assume you have experience with assembly, C, debugging in the userland.
This driver is a kernel driver. A driver is typically used to get our code into the kernel. An unhandled exception will cause the famous BSOD. I will be using Windows 7 32-bit for this since it doesn’t support SMEP (Supervisor Mode Execution Prevention) or SMAP (Supervisor Mode Access Prevention). In simple words, I would say that when SMEP is enabled the CPU will generate a fault whenever the ring0 tries to execute code from a page marked with the user bit. Basically, due to this being not enabled, we can map our shellcode to steal the ‘System’ token. Check the Shellcode Analysis part for the analysis. Exploiting this vulnerability on a 64-bit system is different.
You can use the OSR Driver Loader to load the driver into the system.
If you want to debug the machine itself using windbg you can use VirtualKD or LiveKD

You can add a new serial connection using VirtualBox or VMware, so you can debug the guest system via windbg. I will be using a serial connection from VMware.
For kernel data structures refer to this. I have used it mostly to refer the structures.
After you have registered the driver you should see this in ‘msinfo32’.

If you check the loaded modules in the ‘System’ process you should see our kernel driver ‘HEVD.sys’.


(more…)

Lab01-02 Analysis

This program is packed using UPX and can be easily unpacked.

At the start we see a call to ‘StartServiceCtrlDispatcher’ which is used to implement a service and the service control manager will call the service entry point provided. In here I have labeled the service entry point as ‘ServiceMain’. The name of the service created would be ‘Malservice’.

(more…)

Patching Windows Media Player

I’m writing this post on the request of @rudr4_sarkar. This is a very simple patch in which you can open multiple instances of wmplayer. It basically uses the ‘CreateMutexW’ API to create a mutex object with the string “Local\Microsoft_WMP_70_CheckForOtherInstanceMutex”.

screenshot_2

The pseudo code would be something like this

HANDLE hMutex = CreateMutex(NULL, FALSE, L"Local\Microsoft_WMP_70_CheckForOtherInstanceMutex");
if (GetLastError() == ERROR_ALREADY_EXISTS) {

}

You just need to patch the ‘JNZ’ to a ‘JMP’ instruction, that will always jump to the good boy 🙂

disass (more…)

Random CrackMe

This is an interesting crackme I found randomly. You can download it from here: http://www.mediafire.com/file/5r3a3uqsg1pbp4v/CrackMe1.zip

The algorithm uses the PID of the application for the serial key calculation. It also uses the ‘GetComputerName’ and ‘GetUserName’ win32 APIs in generating the serial key. The length of both results are used for the loops and each ascii value is added. These results are used in the final calculation in the serial key.
This graph is from the OllyGraph plugin.


(more…)