Firefox before 1.0 and Mozilla before 1.7.5, when configured to use a proxy, respond to 407 proxy auth requests from arbitrary servers, which allows remote attackers to steal NTLM or SPNEGO credentials.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A9578
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A100049
https://exchange.xforce.ibmcloud.com/vulnerabilities/19174
https://bugzilla.mozilla.org/show_bug.cgi?id=267263
http://www.securityfocus.com/bid/12407