MoinMoin 1.7.1 allows remote attackers to bypass the textcha protection mechanism by modifying the textcha-question and textcha-answer fields to have empty values.
https://euvd.enisa.europa.eu/vulnerability/EUVD-2010-1268
http://www.vupen.com/english/advisories/2010/0831
http://www.ubuntu.com/usn/USN-925-1