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CAPTCHA Fixation Attack<br />

Client 1 GET /captcha.php + SESSIONID Server 2<br />

HTTP/1.1 302 Moved Temporarily<br />

Location: /get_captcha.php?id=captchaID<br />

3<br />

Pick a random<br />

CAPTCHA ID<br />

from finite set<br />

of CAPTCHA<br />

values<br />

GET /get_captcha.php?id=evil_ID+ SESSIONID 5<br />

4<br />

CAPTCHA 6<br />

< CAPTCHA Verification ><br />

Set CAPTCHA<br />

ID and/or<br />

solution in<br />

HTTP Session<br />

www.foundstone.com<br />

© 2010, McAfee, Inc.

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