MantisBT before 1.2.18 uses the public_key parameter value as the key to the CAPTCHA answer, which allows remote attackers to bypass the CAPTCHA protection mechanism by leveraging knowledge of a CAPTCHA answer for a public_key parameter value, as demonstrated by E4652 for the public_key value 0.
https://www.mantisbt.org/bugs/view.php?id=17811
https://github.com/mantisbt/mantisbt/commit/7bb78e45
https://exchange.xforce.ibmcloud.com/vulnerabilities/99004
http://www.securityfocus.com/bid/71321
http://www.openwall.com/lists/oss-security/2014/11/27/8
http://www.openwall.com/lists/oss-security/2014/11/26/19