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Showing posts with the label Assembly

Guy's 30 Reverse Engineering Tips & Tricks

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Good morning lovely people!

During April I challenged myself to tweet 1 reverse engineering tip every day. For your viewing pleasure, here I aggregated all 30 tips.

Be sure to follow me @va_start for my latest tweets and more reverse engineering extravaganza.
Leave a comment on this post or tag me on Twitter - I reply pretty quickly :)

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Tips & Tricks Tip 1 *Reverse Engineering Tip 1/30*
long branch-less functions w/many xors & rols are usually hash functions. IDA view of MD5 func:#BinReversingTipspic.twitter.com/cLSGfxNupK — Guy๐Ÿ‚ (@va_start) April 1, 2020 Tip 2 -Reversing Tip 2/30-
Building on the last tip, after finding a hash function, google its constant to identify the exact hash algorithm. #BinReversingTipspic.twitter.com/MJJIBY9pde — Guy๐Ÿ‚ (@va_start) April 2, 2020 Tip 3 -Reversing Tip 3/30-

Calling Arbitrary Functions In EXEs: Performing Calls to EXE Functions Like DLL Exports

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Motivation When reversing or fuzzing an executable, being able to run an arbitrary function with controlled data is extremely helpful. Through iteratively playing with the function's parameters and examining the output, we can better understand the function's logic.
Background A dll (Dynamic Linked Library) with our target function would allow us to conveniently review and test the function as we wish. The only problem is that usually the function we want to examine resides in an exe, not a dll. Converting¹ an exe to a dll is a solvable challenge. After all, both an exe and a dll share the same PE (Portable Executable) file format.  So let's explore, how can we convert¹ an exe to a dll?
Spoiler: there are a few more steps than just changing the extension ๐Ÿ˜‰
¹ "convert to DLL" = fundamentally behave like a DLL.

I'll use this exe created from the following code and target the decode_string function for demonstration purposes throughout this post.

Challenges Th…

Stack Overflow CVE-2019-17424 Vulnerability Write-Up and RCE Exploit Walk Through

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Stack Overflow CVE-2019-17424 Vulnerability Write-Up and RCE Exploit Walk Through This is Part 2 in a 4 part series about my process hunting for vulnerabilities in a network auditing tool (used to protect networks by detecting and fixing security holes), and fully exploiting one of the vulnerabilities I found. I recommend reading the series in ascending numeric order. Link to part 1 here. Links to parts 3, and 4 at the end of this post.

This post describes how I found CVE-2019-17424 and successfully exploited the vulnerability in the precompiled, packaged product.
Vulnerability ⚡ Reader’s Exercise ๐Ÿ”Ž I found CVE-2019-17424 by manually reviewing the source code of nipper-ng. Provided below is an excerpt from the source code containing only the vulnerable function. You are welcome to take it as an exercise to find the vulnerability in the code below:

Notice: The vulnerability in the code above is identified in the paragraph below. If you want to try to find the vulnerability yourself, o…

Simplest Fibonacci Assembly Code

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Tl;Dr There is an insanely cool, simple and elegant way to calculate Fibonacci numbers in assembly using only 2 opcodes!

Full disclosure: this post is inspired by chapter two of the book "xchg rax, rax".
Fibonacci Numbers Just a simple review: Fibonacci Numbers are calculated with the formula below.


So for example to get the 3'rd Fibonacci number, we need to sum the 2nd and 1st Fibonacci numbers.
The Code ๐Ÿ‘จ‍๐Ÿ’ป๐Ÿ‘ฉ‍๐Ÿ’ป Behold! Below is the most elegant code you will ever see for in assembly


Source available here.
Explanation๐Ÿง  The magic happens in the XADD opcode which is an "xchg (exchange)" and "add"  operation in one opcode. It works exactly as you would expect: first exchange the two operands, and then add them saving the result to the first operand. Official Intel documentation here.

Next, the "loop" opcode changes the code flow to jump back and re-execute the xadd opcode multiple times.
Understanding ✔ To completely understand why and ho…