In BSidesSF CTF, calc.exe exploits you! (Author writeup of launchcode)

Hey everybody,

In addition to genius, whose writeup I already posted, my other favourite challenge I wrote for BSidesSF CTF was called launchcode. This will be my third and final writeup for BSidesSF CTF for 2019, but you can see all the challenges and solutions on our Github releases page.

This post will be more about how I developed this, since the solution is fairly straight forward once you know how it's implemented.
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BSidesSF CTF author writeup: genius

Hey all,

This is going to be an author's writeup of the BSidesSF 2019 CTF challenge: genius!

genius is probably my favourite challenge from the year, and I'm thrilled that it was solved by 6 teams! It was inspired by a few other challenges I wrote in the past, including Nibbler. You can grab the sourcecode, solution, and everything needed to run it yourself on our Github release!

It is actually implemented as a pair of programs: loader and genius. I only provide the binaries to the players, so it's up to the player to reverse engineer them. Fortunately, for this writeup, we'll have source to reference as needed!
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Technical Rundown of WebExec

This is a technical rundown of a vulnerability that we've dubbed "WebExec". The summary is: a flaw in WebEx's WebexUpdateService allows anyone with a login to the Windows system where WebEx is installed to run SYSTEM-level code remotely. That's right: this client-side application that doesn't listen on any ports is actually vulnerable to remote code execution! A local or domain account will work, making this a powerful way to pivot through networks until it's patched.

High level details and FAQ at https://webexec.org! Below is a technical writeup of how we found the bug and how it works.

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Solving b-64-b-tuff: writing base64 and alphanumeric shellcode

Hey everybody,

A couple months ago, we ran BSides San Francisco CTF. It was fun, and I posted blogs about it at the time, but I wanted to do a late writeup for the level b-64-b-tuff.

The challenge was to write base64-compatible shellcode. There's an easy solution - using an alphanumeric encoder - but what's the fun in that? (also, I didn't think of it :) ). I'm going to cover base64, but these exact same principles apply to alphanumeric - there's absolutely on reason you couldn't change the SET variable in my examples and generate alphanumeric shellcode.

In this post, we're going to write a base64 decoder stub by hand, which encodes some super simple shellcode. I'll also post a link to a tool I wrote to automate this.

I can't promise that this is the best, or the easiest, or even a sane way to do this. I came up with this process all by myself, but I have to imagine that the generally available encoders do basically the same thing. :)
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BSidesSF CTF wrap-up

Welcome!

While this is technically a CTF writeup, like I frequently do, this one is going to be a bit backwards: this is for a CTF I ran, instead of one I played! I've gotta say, it's been a little while since I played in a CTF, but I had a really good time running the BSidesSF CTF! I just wanted to thank the other organizers - in alphabetical order - @bmenrigh, @cornflakesavage, @itsc0rg1, and @matir. I couldn't have done it without you folks!

BSidesSF CTF was a capture-the-flag challenge that ran in parallel with BSides San Francisco. It was designed to be easy/intermediate level, but we definitely had a few hair-pulling challenges.

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Going the other way with padding oracles: Encrypting arbitrary data!

A long time ago, I wrote a couple blogs that went into a lot of detail on how to use padding oracle vulnerabilities to decrypt an encrypted string of data. It's pretty important to understand to use a padding oracle vulnerability for decryption before reading this, so I'd suggest going there for a refresher.

When I wrote that blog and the Poracle tool originally, I didn't actually know how to encrypt arbitrary data using a padding oracle. I was vaguely aware that it was possible, but I hadn't really thought about it. But recently, I decided to figure out how it works. I thought and thought, and finally came up with this technique that seems to work. I also implemented it in Poracle in commit a5cfad76ad.
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dnscat2 0.05: with tunnels!

Greetings, and I hope you're all having a great holiday!

My Christmas present to you, the community, is dnscat2 version 0.05!

Some of you will remember that I recently gave a talk at the SANS Hackfest Summit. At the talk, I mentioned some ideas for future plans. That's when Ed jumped on the stage and took a survey: which feature did the audience want most?

The winner? Tunneling TCP via a dnscat. So now you have it! Tunneling: Phase 1. :)

Info and downloads.
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SANS Hackfest writeup: Hackers of Gravity

Last weekA few weeks ago, SANS hosted a private event at the Smithsonian's Air and Space Museum as part of SANS Hackfest. An evening in the Air and Space Museum just for us! And to sweeten the deal, they set up a scavenger hunt called "Hackers of Gravity" to work on while we were there!

We worked in small teams (I teamed up with Eric, who's also writing this blog with me). All they told us in advance was to bring a phone, so every part of this was solved with our phones and Google.

Each level began with an image, typically with a cipher embedded in it. After decoding the cipher, the solution and the image itself were used together to track down a related artifact.

This is a writeup of that scavenger hunt. :)
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dnscat2: now with crypto!

Hey everybody,

Live from the SANS Pentest Summit, I'm excited to announce the latest beta release of dnscat2: 0.04! Besides some minor cleanups and UI improvements, there is one serious improvement: all dnscat2 sessions are now encrypted by default!

Read on for some user information, then some implementation details for those who are interested! For all the REALLY gory information, check out the protocol doc!
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Why DNS is awesome and why you should love it

It's no secret that I love DNS. It's an awesome protocol. It's easy to understand and easy to implement. It's also easy to get dangerously wrong, but that's a story for last weeka few weeks ago. :)

I want to talk about interesting implication of DNS's design decisions that benefit us, as penetration testers. It's difficult to describe these decisions as good or bad, it's just what we have to work with.

What I DON'T want to talk about today is DNS poisoning or spoofing, or similar vulnerabilities. While cool, it generally requires the attacker to take advantage of poorly configured or vulnerable DNS servers.

Technically, I'm also releasing a tool I wrote a couple weeks ago: dnslogger.rb that replaces an old tool I wrote a million years ago.
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How I nearly almost saved the Internet, starring afl-fuzz and dnsmasq

If you know me, you know that I love DNS. I'm not exactly sure how that happened, but I suspect that Ed Skoudis is at least partly to blame.

Anyway, a project came up to evaluate dnsmasq, and being a DNS server - and a key piece of Internet infrastructure - I thought it would be fun! And it was! By fuzzing in a somewhat creative way, I found a really cool vulnerability that's almost certainly exploitable (though I haven't proven that for reasons that'll become apparent later).

Although I started writing an exploit, I didn't finish it. I think it's almost certainly exploitable, so if you have some free time and you want to learn about exploit development, it's worthwhile having a look! Here's a link to the actual distribution of a vulnerable version, and I'll discuss the work I've done so far at the end of this post.

You can also download my branch, which is similar to the vulnerable version (branched from it), the only difference is that it contains a bunch of fuzzing instrumentation and debug output around parsing names.
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Defcon quals: wwtw (a series of vulns)

Hey folks,

This is going to be my final (and somewhat late) writeup for the Defcon Qualification CTF. The level was called "wibbly-wobbly-timey-wimey", or "wwtw", and was a combination of a few things (at least the way I solved it): programming, reverse engineering, logic bugs, format-string vulnerabilities, some return-oriented programming (for my solution), and Dr. Who references!

I'm not going to spend much time on the theory of format-string vulnerabilities or return-oriented programming because I just covered them in babyecho and r0pbaby.

And by the way, I'll be building the solution in Python as we go, because the first part was solved by one of my teammates, and he's a Python guy. As much as I hated working with Python (which has become my life lately), I didn't want to re-write the first part and it was too complex to do on the shell, so I sucked it up and used his code.

You can download the binary here, and you can get the exploit and other files involved on my github page.
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Defcon Quals: babyecho (format string vulns in gory detail)

Welcome to the third (and penultimate) blog post about the 2015 Defcon Qualification CTF! This is going to be a writeup of the "babyecho" level, as well as a thorough overview of format-string vulnerabilities! I really like format string vulnerabilities - they're essentially a "read or write anywhere" primitive - so I'm excited to finally write about them!

You can grab the binary here, and you can get my exploit and some other files on this Github repo.
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Defcon Quals: Access Control (simple reverse engineer)

Hello all,

Today's post will be another write-up from the Defcon CTF Qualifiers. This one will be the level called "Access Client", or simply "client", which was a one-point reverse engineering level. This post is going to be mostly about the process I use for reverse engineering crypto-style code - it's a much different process than reversing higher level stuff, because each instruction matters and it's often extremely hard to follow.

Having just finished another level (r0pbaby, I think), and having about an hour left in the competition, I wanted something I could finish quickly. There were two one-point reverse engineering challenges open that we hadn't solved: one was 64-bit and written in C++, whereas this one was 32-bit and C and only had a few short functions. The choice was easy. :)

I downloaded the binary and had a look at its strings. Lots of text-based stuff, such as "list users", "print key", and "connection id:", which I saw as a good sign!
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Defcon Quals: r0pbaby (simple 64-bit ROP)

This past weekend I competed in the Defcon CTF Qualifiers from the Legit Business Syndicate. In the past it's been one of my favourite competitions, and this year was no exception!

Unfortunately, I got stuck for quite a long time on a 2-point problem ("wwtw") and spent most of my weekend on it. But I did do a few others - r0pbaby included - and am excited to write about them, as well!

r0pbaby is neat, because it's an absolute bare-bones ROP (return-oriented programming) level. Quite honestly, when it makes sense, I actually prefer using a ROP chain to using shellcode. Much of the time, it's actually easier! You can see the binary, my solution, and other stuff I used on this github repo.

It might make sense to read a post I made in 2013 about a level in PlaidCTF called ropasaurusrex. But it's not really necessary - I'm going to explain the same stuff again with two years more experience!
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dnscat2 beta release!

As I promised during my 2014 Derbycon talk (amongst other places), this is an initial release of my complete re-write/re-design of the dnscat service / protocol. It's now a standalone tool instead of being bundled with nbtool, among other changes. :)

I'd love to have people testing it, and getting feedback is super important to me! Even if you don't try this version, hearing that you're excited for a full release would be awesome. The more people excited for this, the more I'm encouraged to work on it! In case you don't know it, my email address is listed below in a couple places.
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GitS 2015: Huffy (huffman-encoded shellcode)

Welcome to my fourth and final writeup from Ghost in the Shellcode 2015! This one is about the one and only reversing level, called "huffy", that was released right near the end.

Unfortunately, while I thought I was solving it a half hour before the game ended, I had messed up some timezones and was finishing it a half hour after the game ended. So I didn't do the final exploitation step.

At any rate, I solved the hard part, so I'll go over the solution!
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GitS 2015: Giggles (off-by-one virtual machine)

Welcome to part 3 of my Ghost in the Shellcode writeup! Sorry for the delay, I actually just moved to Seattle. On a sidenote, if there are any Seattle hackers out there reading this, hit me up and let's get a drink!

Now, down to business: this writeup is about one of the Pwnage 300 levels; specifically, Giggles, which implements a very simple and very vulnerable virtual machine. You can download the binary here, the source code here (with my comments - I put XXX near most of the vulnerabilities and bad practices I noticed), and my exploit here.

One really cool aspect of this level was that they gave source code, a binary with symbols, and even a client (that's the last time I'll mention their client, since I dislike Python :) )! That means we could focus on exploitation and not reversing!
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