The Curl_input_negotiate function in http_negotiate.c in libcurl 7.10.6 through 7.21.6, as used in curl and other products, always performs credential delegation during GSSAPI authentication, which allows remote servers to impersonate clients via GSSAPI requests.
https://bugzilla.redhat.com/show_bug.cgi?id=711454
http://www.ubuntu.com/usn/USN-1158-1
http://www.securitytracker.com/id?1025713
http://www.redhat.com/support/errata/RHSA-2011-0918.html
http://www.mandriva.com/security/advisories?name=MDVSA-2011:116
http://www.debian.org/security/2011/dsa-2271
http://support.apple.com/kb/HT5130
http://security.gentoo.org/glsa/glsa-201203-02.xml
http://secunia.com/advisories/48256
http://secunia.com/advisories/45181
http://secunia.com/advisories/45144
http://secunia.com/advisories/45088
http://secunia.com/advisories/45067
http://secunia.com/advisories/45047
http://lists.fedoraproject.org/pipermail/package-announce/2011-June/061992.html
http://lists.fedoraproject.org/pipermail/package-announce/2011-July/062287.html
http://lists.apple.com/archives/security-announce/2012/Feb/msg00000.html