- What is a CTF?
- Common Categories
- Useful Tools
- Benefits of Playing
As I’m helping to organize the BSides San Francisco CTF this weekend, I thought I’d share a little primer for CTFs for those who have not gotten into them before.
What is a CTF?
I suspect that most people in the information security (“cybersecurity”) space have already heard of Capture the Flag (or CTF) competitions, but in case you haven’t, I wanted to provide a short overview.
Capture the Flag competitions are a timed and scored set of security-related challenges. They may take many forms and durations, but there are some pretty common styles, and the majority of CTFs are 24-72 hours long. Some are associated with conferences or other events, while others are run entirely online.
The most common style of CTF is called “Jeopardy Style”, named after the TV show. In these CTFs, players complete challenges from an assortment of categories to earn points. Most often, completing each challenge awards the “flag” that is entered into the scoreboard to receive points. These may be run for individual players or teams of players.
Another style is the “Attack-Defense CTF”, in which teams of players have a network to defend while also being able to attack the networks of other players. They may involve stealing flags off the opponent’s network or planting ones own flags to earn points.
Most players’ first CTF experience will be with a Jeopardy-style CTF competition, such as the one we’re running this weekend, so I’ll focus on that style for the remainder of this post.
While CTFs may present challenges that are widely varied, there are some categories that are fairly common across the board. Beyond those described below, you’ll see varied topics like Mobile, Cloud, or even “Miscellaneous” where you’ll be doing some potentially obscure task.
As a general rule, these challenges usually have an expectation of gaining some kind of privileged access, usually in the form of code execution or a shell. Many of them involve some form of memory corruption (buffer overflow, use-after-free, double free, etc.), and some CTF participants would say that pwn should even be limited to memory corruption. Usually, it will be a networked binary/service written in a relatively low-level language. In any case, these would be the common “exploitation of a priviliged service”.
Solving these may also involve some reverse engineering to understand the binary, but the focus here is normally on the exploitation of the bug(s) more than the reverse engineering. Occasionally, a challenge will provide source code or other resources.
Web challenges are incredibly common and only continuing to grow in popularity with challenge authors over time. Probably has something to do with nearly everything being a web app these days. These challenges involve compromising a web application, either by server-side vulnerabilities (SQL injection, request splitting, auth bypass, SSRF) or through client-side vulnerabilities (XSS, CSRF), or some combination.
For the client-side exploits, most challenges involve some kind of automated
browser visiting the relevant application to be exploited. (Having to exploit a
browser bug is less common, but also possible.) For the BSidesSF CTF, we call
webbot”, and use a headless Chrome driven by the puppeteer library.
Web challenges can have a special complexity for challenge authors: shared state, such as databases, make it easier for players’ attempts to interfere with each other. (Personally, I try to avoid such state if I can.)
Forensics challenges have a wide variety of challenges to recover some kind of data from an underlying media. This might be packet captures, disk images, memory dumps, steganography or even log analysis. It’s encouraging to see more responder/blue team oriented content appearing in CTFs, even if I’m personally terrible at them.
These challenges are great for those in (or looking to get into) SOC, digital forensics, or other related fields. Also great for those transitioning from something like network administration, as some of the topics should be quite familiar.
Crypto means cryptography! These challenges typically involve some kind of custom cryptosystem, though I’ve also seen bad key generation or incorrect application of well-known cryptographic primitives. If you think you’re good at math, spotting patterns, or figuring out weird formats, this might be a category you can use to test yourself.
Crypto challenges can range from basic Caesar ciphers to mis-applications of cryptosystems like AES or RSA. They’re also a great opportunity to practice scripting, as you’ll often need to apply your approach to a large volume of data.
Reverse Engineering challenges mostly involve a program that needs to be reverse-engineered to figure out the hidden flag. Often, they’ll ask for an input and use that input to produce or decrypt the flag and then display them. These usually tend to be native code (C/C++ or even something like Rust or Go), but you might also encounter managed code (such as .NET, Java, or Python bytecode). Disassemblers and decompilers will be your friend here.
These are great for those who want to understand malware, or want to extend their reversing skills for better exploitation or other security practices. I’ve personally learned so much about the internals of operating systems and application security from reversing challenges.
For a variety of reasons, Linux CTF challenges are far more common than Windows challenges. Consequently, you’ll probably want some flavor of Linux VM. It doesn’t have to be something security-specific like Parrot or Kali Linux, but something you can test things on and run things on. Don’t forget to snapshot this VM, as it’ll give you a clean start each time, as well as reduce the risk of something going wrong with a challenge attempt completely screwing things up.
For web challenges, having a web proxy that lets your replay and modify HTTP requests is incredibly useful. BurpSuite is a bit of a gold standard, but OWASP Zap and mitmproxy are other options. Having a VPS or using a “Request Bin” to receive requests online can also be useful.
For reversing, pwnable, and other challenges, you’ll want a disassembler like Ghidra,
radare2/rizin, BinaryNinja or IDA. You’ll also probably want a debugger – I
gdb with the
gef script on Linux, and
windbg on Windows.
For forensics challenges, there’ll be a variety of tools that depend on the circumstance. Commonly, though, you’ll see challenges involve packet captures (PCAP), for which Wireshark is just about the only answer.
You might need to be able to host files or receive reverse shells. In such a case, having a system with a public IP can be incredibly useful. I tend to use a VPS for this, as I’m often at a conference or on another network doing Network Address Translation, which makes receiving incoming connections more difficult. I mostly use DigitalOcean because they’re relatively low cost and easy to spin up in a variety of regions, but you can get some really cheap VPSs if you look on a site like LowEndBox. For something like playing in a CTF, the lower reliability of a cheaper option is not a significant concern.
GCHQ’s CyberChef can also be a great tool during CTFs, along with familiarity with some sort of scripting language. If you’re using Python, I can highly recommend pwntools when doing reversing or exploitation challenges.
Benefits of Playing
Probably the most prevalent benefit of playing in CTF competitions is the fun and enjoyment brought about by solving challenges. Just like any kind of puzzle, there is a sense of accomplishment on solving a challenge. (Especially something new or difficult to you.)
There is definitely an educational benefit to participating in a CTF as well. They provide a great opportunity to reinforce an existing skill or try out something new during a CTF – the stakes are low, and if well-designed, it should be possible to solve the puzzle.
Even if one doesn’t learn new technical skills, puzzle-like games can stretch the mind and help improve the ability to think “outside the box.”
I’ve also met some great people through playing and building CTF challenges. This can be a good networking opportunity, as most of them will be in the information security space or related areas.
I recommend just giving it a try. Look at a challenge that looks fun, check it out, and give it a try. You can get far just by Googling a few things in a lot of cases, and you might just learn something!