The default session serializer in PHP 5.2 through 5.2.13 and 5.3 through 5.3.2 does not properly handle the PS_UNDEF_MARKER marker, which allows context-dependent attackers to modify arbitrary session variables via a crafted session variable name.
http://www.vupen.com/english/advisories/2010/3081
http://www.redhat.com/support/errata/RHSA-2010-0919.html
http://www.debian.org/security/2010/dsa-2089
http://secunia.com/advisories/42410
http://lists.opensuse.org/opensuse-security-announce/2010-10/msg00000.html
http://lists.opensuse.org/opensuse-security-announce/2010-09/msg00006.html