Breaking ledgerctf's AES white-box challenge

Introduction

About a month ago, my mate b0n0n was working on the ledgerctf puzzles and challenged me to have a look at the ctf2 binary. I eventually did and this blogpost discusses the protection scheme and how I broke it. Before diving in though, here is a bit of background.

ledger is a french security company founded in 2014 that is specialized in cryptography, cryptocurrencies, and hardware. They recently put up online three different puzzles to celebrate the official launch of their bug bounty program. The second challenge called ctf2 is the one we will be discussing today. ctf2 is an ELF64 binary that is available here for download (if you want to follow at home). The binary is about 11MB, written in C++ and even has symbols; great.

Let's do it!

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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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First dip into the kernel pool : MS10-058

Introduction

I am currently playing with pool-based memory corruption vulnerabilities. That’s why I wanted to program a PoC exploit for the vulnerability presented by Tarjei Mandt during his first talk “Kernel Pool Exploitation on Windows 7” [3]. I think it's a good exercise to start learning about pool overflows.

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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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Pinpointing heap-related issues: OllyDbg2 off-by-one story

Introduction

Yesterday afternoon, I was peacefully coding some stuff you know but I couldn't make my code working. As usual, in those type of situations you fire up your debugger in order to understand what is going on under the hood. That was a bit weird, to give you a bit of context I was doing some inline x86 assembly, and I've put on purpose an int3 just before the piece of assembly code I thought was buggy. Once my file loaded in OllyDbg2, I hit F9 in order to reach quickly the int3 I've slipped into the inline assembly code. A bit of single-stepping, and BOOM I got a nasty crash. It happens sometimes, and that's uncool. Then, I relaunch my binary and try to reproduce the bug: same actions and BOOM again. OK, this time it's cool, I got a reproducible crash in OllyDbg2.

I like when things like that happens to me (remember the crashes I've found in OllyDbg/IDA here: PDB Ain't PDD), it's always a nice exercise for me where I've to:

  • pinpoint the bug in the application: usually not trivial when it's a real/big application
  • reverse-engineer the codes involved in the bug in order to figure out why it's happening (sometimes I got the sources, sometimes I don't like this time)

In this post, I will show you how I've manage to pinpoint where the bug was, using GFlags, PageHeap and WinDbg. Then, we will reverse-engineer the buggy code in order to understand why the bug is happening, and how we can code a clean trigger.

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Some thoughts about code-coverage measurement with Pin

Introduction

Sometimes, when you are reverse-engineering binaries you need somehow to measure, or just to have an idea about how much "that" execution is covering the code of your target. It can be for fuzzing purpose, maybe you have a huge set of inputs (it can be files, network traffic, anything) and you want to have the same coverage with only a subset of them. Or maybe, you are not really interested in the measure, but only with the coverage differences between two executions of your target: to locate where your program is handling a specific feature for example.

But it's not a trivial problem, usually you don't have the source-code of the target, and you want it to be quick. The other thing, is that you don't have an input that covers the whole code base, you don't even know if it's possible ; so you can't compare your analysis to that "ideal one". Long story short, you can't say to the user "OK, this input covers 10% of your binary". But you can clearly register what your program is doing with input A, what it is doing with input B and then analyzing the differences. With that way you can have a (more precise?) idea about which input seems to have better coverage than another.

Note also, this is a perfect occasion to play with Pin :-)).

In this post, I will explain briefly how you can build that kind of tool using Pin, and how it can be used for reverse-engineer purposes.

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Regular expressions obfuscation under the microscope

Introduction

Some months ago I came across a strange couple of functions that was kind of playing with a finite-state automaton to validate an input. At first glance, I didn't really notice it was in fact a regex being processed, that's exactly why I spent quite some time to understand those routines. You are right to ask yourself: "Hmm but the regex string representation should be in the binary shouldn't it?", the thing is it wasn't. The purpose of this post is to focus on those kind of "compiled" regex, like when the author transform somehow the regex in a FSM directly usable in its program (for the sake of efficiency I guess). And to extract that handy string representation, you have to study the automaton.

In this short post, we are going to see how a regular expression looks like in assembly/C, and how you can hide/obfuscate it. I hope you will enjoy the read, and you will both be able to recognize a regular expression compiled in your future reverse-engineering tasks and to obfuscate heavily your regex!

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