Potential Captcha Validate Bypass in flask-session-captcha

Impact

flask-session-captcha is a package which allows users to extend Flask by adding an image based captcha stored in a server side session.

The captcha.validate() function would return None if passed no value (e.g. by submitting a request with an empty form).

If implementing users were checking the return value to be False, the captcha verification check could be bypassed.

Sample vulnerable code:

if captcha.validate() == False:
    ... # abort
else:
   ... # do stuff

Patches

A new version (1.2.1) is available that fixes the issue.

Workarounds

Users can workaround the issue by not explicitly checking that the value is False.

Checking the return value less explicitly should still work.

if not captcha.validate():
    ... # abort
else:
   ... # do stuff

if captcha.validate():
    ... # do stuff
else:
   ... # abort

References

https://github.com/Tethik/flask-session-captcha/pull/27

For more information

If you have any questions or comments about this advisory:

• Open an issue in the github repo