DerbyConCTF Flag: MayUrG0atsBeFr33

October 9, 2014 — 2 Comments

I knew the approach to this CTF would be different from those in the past based on the instructions.

In years past, the directions instructed you on which devices you were NOT to touch and which devices were the targets.  This year, participants were simply directed which devices were off-limits.  These off-limit devices were identified either by subnet or IP Address.

The first thing I did was copy the contents of these addresses/subnets into a file I would use as my “exclusion” file during any network or vulnerability scans I would run.  This was the contents of my “exclude.txt” file:

From here, I kicked off a series of nmap scans.  I started a rather large one using the command:

nmap -sn --excludefile /root/Desktop/CTF/exclude.txt -oG /root/Desktop/CTF/nmap_10.10.0.0.txt

But between the network latency/downtime, it became pretty apparent that it would take forever, if at all, for this scan to finish — so I broke it up into smaller chunks.

I decided to target the subnet first for basic services that I was confident would be running and attempt to get at the “easier” stuff first:

nmap -n -p21 -T5 --excludefile /root/Desktop/CTF/exclude.txt -oN /root/Desktop/CTF/nmap_10.10.146.0.ftp --open
nmap -n -p80 -T5 --excludefile /root/Desktop/CTF/exclude.txt -oN /root/Desktop/CTF/nmap_10.10.146.0.http --open
nmap -n -p443 -T5 --excludefile /root/Desktop/CTF/exclude.txt -oN /root/Desktop/CTF/nmap_10.10.146.0.https --open
nmap -n -p22 -T5 --excludefile /root/Desktop/CTF/exclude.txt -oN /root/Desktop/CTF/nmap_10.10.146.0.ssh --open
nmap -n -p23 -T5 --excludefile /root/Desktop/CTF/exclude.txt -oN /root/Desktop/CTF/nmap_10.10.146.0.telnet --open

Later I scanned the subnet, but only came up with one system.

nmap -Pn -n -sV -T5 --excludefile /root/Desktop/CTF/exclude.txt -oG /root/Desktop/CTF/ --open

Any subsequent scans of the entire subnets where there were off-limit devices (, or piratically everything between didn’t turn up any live hosts.

I won’t dissect those findings in this blog post, but if you’re interested, you can get a copy of the output from these scans if you roll back to a previous blog post.

After I had identified the devices running http/https, I kicked off a ‘wget’ to mirror the contents of those systems on my local computer:

wget -m

When that was completed, I used ‘grep’ to search through all the files for an instance of ‘flag’ (not case-sensitive):

grep --color -r -i "flag" .

I believe those commands are correct, working off my poor memory here. Given what I said yesterday about the flags not being obvious, you can see the flaw in my methodology here.  What if ‘flag’ is mangled with additional characters?  Well, I would return to these files later to step through them manually when I was bored, but this search turned up a flag in the html code which was good for 20 points:
flag MayUrG0atsBeFr33

Also, based on the contents of the BBoard.html file on this server, I suspect there was more going on here.  The comment <script>alert('xss');</script> lead me to believe there might be some cross-site scripting vulnerability going on here, but XSS is out of my skilset, so I was unable to confirm.  There was also the curious string ZmxhZzogMVNoZWVwODI2SHVnZ2VyMjMKVGhlcmUgc3RpbGwgbG9naW4gaXNz\ndWVzLCBzZWU6Cq6kqa/y6P2cuq2tgL2vr626/v38u/3/6KCprOi8p+i/ramj\nrabou6CprKe/6LituqW7\n that I was unable to identify.  I’m still not certain what that might have been, but the frequent ‘\n’ in the text left me wondering if those were newlines or not.

Anyone care to share any hints or otherwise help me out with this one?

Aaron Melton


2 responses to DerbyConCTF Flag: MayUrG0atsBeFr33

  1. The string is Base64 encoded.

    • But of course it is. How embarassing. :/

      I was too fixated on the \n’s in the string and completely overlooked the possibility of it being base64 encoded because the string didn’t end in the typical “=” sign. Thanks for the tip:

      aaron@computer ~ $ echo “ZmxhZzogMVNoZWVwODI2SHVnZ2VyMjMKVGhlcmUgc3RpbGwgbG9naW4gaXNz\ndWVzLCBzZWU6Cq6kqa/y6P2cuq2tgL2vr626/v38u/3/6KCprOi8p+i/ramj\nrabou6CprKe/6LituqW7\n” | base64 -d
      flag: 1Sheep826Hugger23
      There still login issbase64: invalid input

      Dare I ask how many points I lost out on that one?

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