The SplObjectStorage unserialize implementation in ext/spl/spl_observer.c in PHP before 7.0.12 does not verify that a key is an object, which allows remote attackers to execute arbitrary code or cause a denial of service (uninitialized memory access) via crafted serialized data.
http://php.net/ChangeLog-7.php
https://bugs.php.net/bug.php?id=73257
https://github.com/php/php-src/commit/61cdd1255d5b9c8453be71aacbbf682796ac77d4