GryphonCTF_2016: lolc0ded

Category: Pwn Points: 35 Description:

$ curl http://play.spgame.site:13337/README.lol
HAI 1.337
    CAN HAS STDIO?
    I HAS A file ITZ I IZ STDIO'Z OPEN YR "README.lol" AN YR "r" MKAY
    VISIBLE I IZ STDIO'Z LUK YR file AN YR 13337 MKAY

    OBTW 
       Flag is at /home/lolc0ded/flag.lol.
    TLDR
KTHXBYE

The service is running at http://play.spgame.site:13337/. Feel free to check out http://play.spgame.site:13337/index.lol to learn more about lolc0ded.

Write-up

Possibly one of the most nugget brained challenge for this year, a HTTP server that runs on garbled crap. In a nutshell, we need to do directory tranversals. Since we have a very verbose 404 page, we just keep trying http://play.spgame.site:13337/home/../home/lolc0ded/flag.lol. Apparently, this filters out to become http://play.spgame.site:13337/home/lolc0ded/flag.lol. No matter how many ../ you put, it just seems to redirect back.

So let's try //. With http://play.spgame.site:13337/home//../home/lolc0ded/flag.lol, you get redirected to, http://play.spgame.site:13337/home/home/lolc0ded/flag.lol. Interesting pattern now. Is doubling the symbol working? Let's try // and .... now! With http://play.spgame.site:13337/home//....//home/lolc0ded/flag.lol You get back the original file at http://play.spgame.site:13337/home/lolc0ded/flag.lol except this time around, the url didn't redirect!

Now let's try iterating it upwards. With http://play.spgame.site:13337/home//....//home/lolc0ded/flag.lol we have the fake flag file. With http://play.spgame.site:13337/home//....//....//home/lolc0ded/flag.lol we have a brutal 404 page with

Not Found The requested URL /home//../../home/lolc0ded/flag.lol was not found on this server.

Perfect. At no. 4 we get the flag.

Therefore, the flag is GCTF{d0nT_c0d3_L0nG_COd3_uS1Ng_L0LcOd3}.

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