X-MAS CTF 2018 - Krampus' Code Block

misc
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This challenge took us a lot more time than it should have. We’re only given a netcat connection, where we can “talk” to Krampus.

Solution

Writing help we discover that Krampus is simply a Python 2.7 interpreter, so we need to find a bug in some function in order to exploit it. Playing a bit with him we can see that we can use chr(), input(), print(), eval() and some useless functions, but none of these with strings, dots, commas, underscores, etc..

After a lot of time we figure out that the right strategy is to use eval() encoding the string as a concatenation of chr() results.

Again, after some trial and error we found that
eval(chr(95)+chr(95)+chr(105)+chr(109)+chr(112)+chr(111)+chr(114)+chr(116)+chr(95)+chr(95)+chr(40)+chr(39)+chr(111)+chr(115)+chr(39)+chr(41)+chr(46)+chr(108)+chr(105)+chr(115)+chr(116)+chr(100)+chr(105)+chr(114)+chr(40)+chr(39)+chr(46)+chr(39)+chr(41)) works and lists the files in Krampus’ directory (it is eval("__import__('os').listdir('.')")). Then the struggle begins: we have some py files with the source, an instruction.txt file that says that we need to connect to server.jar (that is another file) to play with Krampus, and some setting files for the server. Using eval("open('server.properties').read()") (obviously encoded as before) we found this

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#Minecraft server properties
#Sat Dec 22 14:06:58 UTC 2018
player-idle-timeout=0
motd=A Minecraft Server
force-gamemode=false
server-ip=
pvp=true
spawn-npcs=true
spawn-animals=true
generate-structures=true
gamemode=0
server-port=25565
difficulty=1
prevent-proxy-connections=false
use-native-transport=true
resource-pack=
resource-pack-sha1=
online-mode=true
allow-flight=false

So it is a Minecraft server! From now, we spent about an hour to try to encode in base64 the server and download it, but it was too big and he stopped answering after a few seconds. Then we tried to run the server on Krampus’ as a jar file, but nothing happened. Then we had the idea that it was a real Minecraft server, so we donwloaded the client and… It worked!

From now, it was only 90 painful minutes of playing minecraft to find the flag in the map.