From: Dave Ferguson (gmdavefgmail.com)
Date: Fri Apr 07 2006 - 09:50:47 CDT

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I implemented a Captcha challenge for a web app and can recommend it
wholeheartedly. I used the Java-based JCaptcha implementation
(http://sourceforge.net/projects/jcaptcha) and was quite happy with
it. The captcha image was displayed after a certain number of failed
login attempts and the image changed after each attempt. It's
certainly a hurdle for brute force attacks but without the problems
associated with account lockouts. BTW, audio/sound captchas are also
available for those concerned about ADA issues.

-Dave Ferguson

On 4/6/06, Hemil <hemilnet-square.com> wrote:
> I think implementing CAPTCHA can be very handy in stopping all of these
> bots and automated tools to do BF. No matter whatever error message web
> application gives, whatever response code it returns, CAPTCHA will stop
> automated scripts and tools.
>
> ---Hemil
> [Net-square]
> Rogan Dawes wrote:
> > Ryan Barnett wrote:
> >> Correct. The returned HTTP status codes is but one of many methods of
> >> enumerating valid account credentials. The most common mistake is
> >> differences in the error message details provided to the user upon
> >> successful/failed login attempts. Web apps should not inform the user
> >> whether or not the problem was with the username or password, but
> >> rather that they failed to authenticate. The 2nd most obvious sign is
> >> passing parameters in URL or cookie variables (such as
> >> STATUS=Authenticated).
> >>
> >> This being said, there are still problems with using 302 redirects and
> >> that it is still possible to enumerate successful/unsuccessful
> >> authentication attempts based on the Location header data returned
> >> with the 302 status code. If the authentication fails, it will send a
> >> 302 and the location most likely will be back to the login page. A
> >> successful attempt, however will send a 302 but the new Location will
> >> be something other than the login page. This is enough data for a
> >> scanner/script to automate and trigger on.
> >>
> >
> > You mean, other than the fact that there is no longer a login form on
> > the resulting page?
> >
> > Mmmm.
> >
> > Rogan
> >
> >
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