ext/session/session.c in PHP before 5.6.25 and 7.x before 7.0.10 skips invalid session names in a way that triggers incorrect parsing, which allows remote attackers to inject arbitrary-type session data by leveraging control of a session name, as demonstrated by object injection.
http://openwall.com/lists/oss-security/2016/09/02/9
http://rhn.redhat.com/errata/RHSA-2016-2750.html
https://bugs.php.net/bug.php?id=72681
https://github.com/php/php-src/commit/8763c6090d627d8bb0ee1d030c30e58f406be9ce?w=1
https://security.gentoo.org/glsa/201611-22
https://www.tenable.com/security/tns-2016-19
http://www.php.net/ChangeLog-5.php