PHP 4.4.4, 5.1.6, and other versions, when running on Apache, allows local users to modify behavior of other sites hosted on the same web server by modifying the mbstring.func_overload setting within .htaccess, which causes this setting to be applied to other virtual hosts on the same server.
https://www.redhat.com/archives/fedora-package-announce/2009-May/msg01465.html
https://www.redhat.com/archives/fedora-package-announce/2009-May/msg01451.html
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A11035
http://www.securitytracker.com/id?1021979
http://www.redhat.com/support/errata/RHSA-2009-0350.html
http://www.openwall.com/lists/oss-security/2009/02/25/3
http://www.openwall.com/lists/oss-security/2009/02/03/3
http://www.openwall.com/lists/oss-security/2009/01/30/1
http://www.debian.org/security/2009/dsa-1789
http://secunia.com/advisories/35306
http://secunia.com/advisories/35007
http://secunia.com/advisories/35003
http://secunia.com/advisories/34830
http://secunia.com/advisories/34642
http://lists.opensuse.org/opensuse-security-announce/2009-04/msg00003.html