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pam_captcha username harvest vulnerability
pam_captcha is visual text-based CAPTCHA challenge module for PAM that
uses figlet to generate the CAPTCHAs.
A site with a screen shot:
I found a security problem with the pam_captcha. If you enter a username
that is not a valid user followed by the correct CAPTCHA, you do not get
prompted for a password. You simply get prompted for another CAPTCHA.
However, if you enter a username that is a valid user followed by the
correct CAPTCHA, you will get prompted for a password. This means an
attacker, or a script/bot could easily harvest a list of valid usernames
simply by whether or not it prompts for a password after a valid captcha
entry. I have duplicated this behavior in FreeBSD 8.0 which uses BSD's
OpenPAM. From what I have seen this module is not compatible with Linux-PAM.
I don't know enough C Fu to propose a patch. Until it is patched the
solution is to disable pam_captcha in your pam config file. The creator
of this module seems to think that using this module isn't really even