beVX challenge on the operation table

Introduction

About two weeks ago, my friend mongo challenged me to solve a reverse-engineering puzzle put up by the SSD team for OffensiveCon2018 (which is a security conference that took place in Berlin in February). The challenge binary is available for download here and here is one of the original tweet advertising it.

With this challenge, you are tasked to reverse-engineer a binary providing some sort of encryption service, and there is supposedly a private key (aka the flag) to retrieve. A remote server with the challenge running is also available for you to carry out your attack. This looked pretty interesting as it was different than the usual keygen-me type of reverse-engineering challenge.

Unfortunately, I didn't get a chance to play with this while the remote server was up (the organizers took it down once they received the solutions of the three winners). However, cool thing is that you can easily manufacture your own server to play at home.. which is what I ended up doing.

As I thought the challenge was cute enough, and that I would also like to write on a more regular basis, so here is a small write-up describing how I abused the server to get the private key out. Hope you don't find it too boring :-).

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Keygenning with KLEE

Introduction

In the past weeks I enjoyed working on reversing a piece of software (don't ask me the name), to study how serial numbers are validated. The story the user has to follow is pretty common: download the trial, pay, get the serial number, use it in the annoying nag screen to get the fully functional version of the software.

Since my purpose is to not damage the company developing the software, I will not mention the name of the software, nor I will publish the final key generator in binary form, nor its source code. My goal is instead to study a real case of serial number validation, and to highlight its weaknesses.

In this post we are going to take a look at the steps I followed to reverse the serial validation process and to make a key generator using KLEE symbolic virtual machine. We are not going to follow all the details on the reversing part, since you cannot reproduce them on your own. We will concentrate our thoughts on the key-generator itself: that is the most interesting part.

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Taming a wild nanomite-protected MIPS binary with symbolic execution: No Such Crackme

As last year, the French conference No Such Con returns for its second edition in Paris from the 19th of November until the 21th of November. And again, the brilliant Eloi Vanderbeken & his mates at Synacktiv put together a series of three security challenges especially for this occasion. Apparently, the three tasks have already been solved by awesome @0xfab which won the competition, hats off :).

To be honest I couldn't resist to try at least the first step, as I know that Eloi always builds really twisted and nice binaries ; so I figured I should just give it a go!

But this time we are trying something different though: this post has been co-authored by both Emilien Girault (@emiliengirault) and I. As we have slightly different solutions, we figured it would be a good idea to write those up inside a single post. This article starts with an introduction to the challenge and will then fork, presenting my solution and his.

As the article is quite long, here is the complete table of contents:

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Dissection of Quarkslab's 2014 security challenge

Introduction

As the blog was a bit silent for quite some time, I figured it would be cool to put together a post ; so here it is folks, dig in!

The French company Quarkslab recently released a security challenge to win a free entrance to attend the upcoming HITBSecConf conference in Kuala Lumpur from the 13th of October until the 16th.

The challenge has been written by Serge Guelton, a R&D engineer specialized in compilers/parallel computations. At the time of writing, already eight different people manage to solve the challenge, and one of the ticket seems to have been won by hackedd, so congrats to him!

woot.png
According to the description of the challenge Python is heavily involved, which is a good thing for at least two reasons:

In this post I will describe how I tackled this problem, how I managed to solve it. And to make up for me being slow at solving it I tried to make it fairly detailed.

At first it was supposed to be quite short though, but well..I decided to analyze fully the challenge even if it wasn't needed to find the key unfortunately, so it is a bit longer than expected :-).

Anyway, sit down, make yourself at home and let me pour you a cup of tea before we begin :-).

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Having a look at the Windows' User/Kernel exceptions dispatcher

Introduction

The purpose of this little post is to create a piece of code able to monitor exceptions raised in a process (a bit like gynvael's ExcpHook but in userland), and to generate a report with information related to the exception. The other purpose is to have a look at the internals of course.

--Exception detected-- ExceptionRecord: 0x0028fa2c Context: 0x0028fa7c Image Path: D:\Codes\The Sentinel\tests\divzero.exe Command Line: ..\tests\divzero.exe divzero.exe PID: 0x00000aac Exception Code: 0xc0000094 (EXCEPTION_INT_DIVIDE_BY_ZERO) Exception Address: 0x00401359 EAX: 0x0000000a EDX: 0x00000000 ECX: 0x00000001 EBX: 0x7ffde000 ESI: 0x00000000 EDI: 0x00000000 ESP: 0x0028fee0 EBP: 0x0028ff18 EIP: 0x00401359 EFLAGS: 0x00010246 Stack: 0x767bc265 0x54f3620f 0xfffffffe 0x767a0f5a 0x767ffc59 0x004018b0 0x0028ff90 0x00000000 Disassembly: 00401359 (04) f77c241c IDIV DWORD [ESP+0x1c] 0040135d (04) 89442404 MOV [ESP+0x4], EAX 00401361 (07) c7042424304000 MOV DWORD [ESP], 0x403024 00401368 (05) e833080000 CALL 0x401ba0 0040136d (05) b800000000 MOV EAX, 0x0

That's why I divided this post in two big parts:

  • the first one will talk about Windows internals background required to understand how things work under the hood,
  • the last one will talk about Detours and how to hook ntdll!KiUserExceptionDispatcher toward our purpose. Basically, the library gives programmers a set of APIs to easily hook procedures. It also has a clean and readable documentation, so you should use it! It is usually used for that kind of things:
  • Hot-patching bugs (no need to reboot),
  • Tracing API calls (API Monitor like),
  • Monitoring (a bit like our example),
  • Pseudo-sandboxing (prevent API calls),
  • etc.
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Breaking Kryptonite's obfuscation: a static analysis approach relying on symbolic execution

Introduction

Kryptonite was a proof-of-concept I built to obfuscate codes at the LLVM intermediate representation level. The idea was to use semantic-preserving transformations in order to not break the original program. One of the main idea was for example to build a home-made 32 bits adder to replace the add LLVM instruction. Instead of having a single asm instruction generated at the end of the pipeline, you will end up with a ton of assembly codes doing only an addition. If you never read my article, and you are interested in it here it is: Obfuscation of steel: meet my Kryptonite.

home-made-adder.png

In this post I wanted to show you how we can manage to break that obfuscation with symbolic execution. We are going to write a really tiny symbolic execution engine with IDAPy, and we will use Z3Py to simplify all our equations. Note that a friend of mine @elvanderb used a similar approach (more generic though) to simplify some parts of the crackme ; but he didn't wanted to publish it, so here is my blog post about it!

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