DEF CON CTF 2018 Finals


Let’s talk about the DEF CON Capture-the-flag
Finals 2018. I never thought I would actually participate
in this prestigious event as the best teams in the world travel to Las Vegas for it. Even just qualifying for it means your team
belongs to the top in the world. But I wasn’t there for that reason, I was
only here because the german team Eat Sleep Pwn Repeat collaborated with other german
teams. We played a few CTFs together for practice
and then to qualify for the finals. We couldn’t even agree on a name so you
might have seen us as Germanys Next ROP Model, Krautstrike or at the finals as Sauercloud. As you know my channel has a lot of videos
about solving CTF challenges. And if you don’t really know what CTFs are,
I have linked two videos here. But I have only ever played jeapordy or wargame
style CTFs where you have single challenges to solve. And they have the problem that nowadays you
just need big teams of skilled people if you want to compete for top ranking. Because certain challenges can just take a
single person over 15 hours to solve. And in a CTF with a dozens of challenges you
need man-power. But the DEFCON Finals CTF is different. It’s an attack and defense CTF which I have
never played before. So how does that work? So attack and defense CTFs can vary how they
work. And this years CTF also had different rules
and ideas were tweaked to balance the game. It’s important that we look at the rules
and the setup for this CTF closely, so that you can understand the nuances in strategies. So don’t take this as a general description
how every A&D ctf works, but they do share some similarities, and we are able to make
some assumptions about that for preparations. So typically an attack and defense CTF works
like this. Every team gets a machine. This machine runs services, so it runs programs
that can be exploited. Exploiting means like with every CTF challenge,
your goal is it to steal the flag on said system. So that might be a flag.txt file or whatever
with a unique flag to proof you got access to the system. So your team has to analyse, reverse engineer
and eventually create the exploit to get the flag from a target system. That’s like every other jeapordy ctf game. But the twist is, that every team has such
a vulnerable machine running. So you have to use your exploit against all
the other machines. By doing so you are stealing the flag from
each team. And that gives points. Now there are a few additional things that
make the attack and defense very different from regular jeapordy. First. The defense part. Teams can defend themselves from exploits. How this is done can vary from CTF to CTF
but often you are required to patch the vulnerability or maybe you can even deploy firewall rules. This means while you are analysing and hunting
for the vulnerability to create your exploit, at the same time you want to think about,
and eventually implement a patch for your own machine. But of course you want to prevent so called
superman defenses. You could just kill your own service, or completely
block any input, or just remove entire functionalities of the program in order to prevent other teams
stealing your flag. So you can see that defense can be very creative
and quickly can become unfair. Thus often a so called SLA (Service Level
Agreement), is implemented. So that means a game server will constantly
execute testcases against your service to ensure that your service is still running
as intended. But even then a clever defender could try
to create a patch that allows the game server tests, but blocks all other players. You can imagine that defense ideas and strategies
can go wild and to keep the game fair, you try to prevent that. How exactly defcon CTF did that we will learn
shortly. Another important detail is the concept of
the game tick. A clock. So if you would just have to steal the flag
once from all teams, there would be a race to the first exploit and then, all teams get
exploited and that’s it? That doesn’t sound too exciting. So typically the game is ran in ticks or intervals. For example every 5 minutes the flags on all
team’s machines are exchanged. So you run your exploits every five minutes
against all teams and hope to still find a vulnerable service. Then you take the flags, submit them to the
scoreboard and it then awards you points for how many flags you got this round. This actually makes attack and defense exciting. So you are sitting there and you see that
a team has a working exploit against you, and they get points each round for it, so
now you are trying to fix the bug in order to stop them from gaining those points. And then crazy strategies might emerge. Could you exploit your own service and submit
the flag yourself for one additional point? Is your team the only one with a working patch,
do you want to share the patch with other teams so that the current top team can’t
gather even more points? It can go crazy. Then sometimes you have access to the whole
network traffic. Sometimes you can log traffic yourself or
the game itself logs it and shares the pcaps, but that means now traffic analysis becomes
super important. You could maybe steal exploits from other
teams by simply replaying the packets. You could try to extract flags that teams
have leaked through the network traffic and submit it yourself. And to protect your own exploits, you now
start to think about obscuring your exploits in the traffic. Maybe you know bugs in wireshark that you
can include in your traffic to make analysis for other teams harder. Okay, so now you have a good general idea. And based on this teams prepare for the defcon
finals. Basically every team has a software developed
to automatically execute exploits against each team every tick, you don’t want to
do that by hand, as well as taking then taking the flags that are returned and automatically
submit them to the scoreboard. You also don’t want to gather and submit
maybe dozens of flags every 5 minutes by hand. Here is for example the flag submitter service
from the team Mhackaroni, the italian team. And you can tell that some teams might totally
overengineer them with fancy stats and so forth. And maybe even develop a lot more tools for
example analyse pcaps, maybe you don’t want to use wireshark for various reasons. So as mentioned in the previous video, our
team got a suite in Cesars palace to play from because there is only space for 8 people
at the table in the CTF area. So in order to connect everybody, quite some
network engineering had to be done. And luckily our team had some awesome people
who took care of that. I don’t know exactly the details but it
all worked flawlessly, but I have heard that one person stayed up all night to get it working. Thanks so much. For internet we actually had planned to use
an LTE router, but the connection was just terrible. We had of course wifi and I think even ethernet
in the hotel room, but that network was also terribly slow. So I think in the end we abused the ethernet
connection of the smart TV in the room. That network was apparently separate and you
could get really good speeds. And I believe we were not the only team who
figured that out. I think here on this picture from the post-ctf
shellphish party in their team suite, you can also see some network cables hooked up
around the TV. Super funny. Before the CTF every team got 8 bags with
a badge for DEF CON and some other goodies. Let’s have quick look at that. The most important thing was the golden CTF
coin. It wa used to allow you into the CTF area
and it was just kinda special. It had the Defcon 26 CTF logo on the front
and the logo of the ORDER OF THE OVERFLOW on the back. That is the name of the team who organized
this years CTF. In previous years it was organized by the
Legit Business Syndicate and this year by the order of the overflow. So that was their first time and I think they
did an awesome job. These names may sound weird or mysterious
to you, I totally love it, but behind them are basically some university professors,
phd students and other academics. I think most of them are also part of shellphish? But yeah. Then we have this bag with stuff. So first we have the lanyward for the badge
here. This CD I believe contains the defcon soundtrack,
a pretty neat gimmick. Then we have a booklet with all the information
about defcon 26. The schedule, the talks, all the events around
it and generally contains everything you need to know. Here is the page about the CTF. Then we have some stickers from this awesome
art, and finally the dc 26 badge. The Dark Tangent presents Def Con 26. If we put in some batteries the front lights
up with what I believe was a game. There are some controls. But I havent looked into it at all. I later heard that you can solve small puzzles
and challenges to unlock a different color for the letters. Cool. So now it’s the morning of the CTF. We have setup everything in the suite, everybody
is ready, our services are deployed and running, now we just wait for the start. Then we finally get the rules. I won’t go over everything in detail, you
can read it yourself here, but just a few notes. First of all there are hours. So only during this time the network was open
for attack and defense. But that doesn’t mean you had nothing to
do at night. In fact you used that time to keep working
on exploits and patches for some challenges to have them ready in the morning. So each team got an ethernet cable they could
extend with some switches. Based on those isolated networks each team
could simply visit 10.100.0.2 and that service then knew which team it is. So no need for usernames and passwords. The game proceeds in ticks of a fixed time. At the beginning of a tick, new flags will
be distributed to all services. Successful exploitation and redemption of
this flag will increase your score and decrease the score of others. And this CTF also had another twist. To further your prosperity, the Order has
developed not one, but two types of services: Attack/Defense and King of the Hill. King of the Hill was pretty cool, they were
more like a programming challenges? you compete against other teams for the best solution,
which depends on the service in question. COuld shortest, fastest, or most complete
or whatever. Not all services were available from the start. They were slowly released one at a time and
also deprected after a while. That was indicated with colors green, yellow
and red. So if you have sunk a lot of time in a challenge
and it turns yellow you might want to switch to somethign else as the points you would
get in the remaining time is maybe too little. For each tick you were not exploited by a
team you would get defense points for it. And if you got and submitted a flag for another
team, you would get an attac points. And king of the hill also gave points the
longer you were at the top of that leaderboard. Later points were normalized and weighted
accordingly. Patching was also very itnersting. We didn’t have access to our services or
machine directly. The only way to interact with our own machine
was by uploading a patched binary version. And then the system would run certain functionality
tests to ensure your binary still is functioning properly. So you can just reject any input. And also often patches were limited to a certain
amount of bytes. So that required very careful planning on
what and how to patch it. And if you would deploy some kind of superman
defense you might get punished for it by revoking your ability to patch. Of course denial of service and so forth is
not allowed and if you vioalte the rules you might even get banned. I think during the CTF two teams were blocked
from the network for a period time as a punishment. So here is the internal interface for our
team. This was right when the CTF started. We could use this form to submit flags, but
of course we would actually use our automated service. I don’t know but I believe somebody from
us quickly looked at how the form is submitted and made the necessary changes for our flag
submitting service. The CTF actually started with a King of The
Hill Challenge. You get all the information where to reach
the service and you could download the client binary. Basically it was a multiple choice command-line
quiz where you had to select the matching disassembly or assembly. It started pretty simple because you had the
raw bytes of the opcosed and could use an disassembly library such as capstone. So this was mostly a programming challenge
because it’s kinda annoying to interact with this ANSI text interface, parsing the
questions and answers. But there were several ways to improve your
script. At first you might write a simple version
that calls the client binary and uses it to interact with the server, then in parallel
somebody might reverse engineer the client and server protocol and implements that directly
to get more control and speed over solving the quiz and then eventually you might also
find bugs that allow you to advance further and further in the game which would have been
impossible otherwise. So I actually quite liked that concept. Another example challenge I participted in
was the oooeditor. It was a very simple editor, kinda looked
like radare, and by simply playing around with it executing different commands in various
orders, I found what looked like a use after free issue, where it was possible to leak
pointers from the heap. And the mein exploitable bug was that your
file content was loaded onto the heap and you could easily read or write out of bounds
with negative offset numbers. But that bug was also so simple, that every
team had patched that very quickly and we couldn’t find another one. As far as I know each attack and defense challenge
had a fairly simple bug and a much harder bug. So even if a team patched something you had
a chance to use a much more obscure and more difficult to find bug for an exploit. Overall the amount of challenges that were
released, and the fact that pcaps of the network traffic only became available VERY late in
the lifetime of a service, the organizers managed to figure out rules that meant that
having a huge team wasn’t really necessary. I heard that last year analyzing pcas was
very powerful and that just required a lot of labour. So with these changes and only 1-3 challenges
up at the same time, it was easy to manage by a team of only 8 people. And I think that really worked out well. But that also meant that me, being not the
most experience, fastest and skilled person, I couldn’t really contribute much. Others were just rocking it like crazy. But I had still fun. But I was obviously not the only person that
had that issue, others just took breaks and checked out Def Con, participated in other
CTFs going on or spent time developing our own personal scoring system to quickly see
if we get attacked and who has currently a working exploit and so forth. The CTF platform exposed these details through
an API. In the end we ended up on the fourth place,
which is super impressive. But I also know how skilled the people in
our team are and they totally had the ability to win. But that’s competition, a lot of things
factor into who will win in the end. So I hope you liked that overview of the Defcon
CTF and that I was able to share with you how it felt like participating in it. I really hope I get another chance of doing
this as I really enjoyed playing with this team. And congrats to all teams who participated
and thanks to the order of the overflow for creating this excellent game.

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