The unserialize implementation in ext/standard/var.c in PHP 7.x before 7.0.14 allows remote attackers to cause a denial of service (use-after-free) or possibly have unspecified other impact via crafted serialized data. NOTE: this vulnerability exists because of an incomplete fix for CVE-2015-6834.
http://lists.opensuse.org/opensuse-updates/2017-01/msg00034.html
https://access.redhat.com/errata/RHSA-2018:1296
https://bugs.php.net/bug.php?id=72978
https://github.com/php/php-src/commit/b2af4e8868726a040234de113436c6e4f6372d17