The libxml_disable_entity_loader function in runtime/ext/ext_simplexml.cpp in HipHop Virtual Machine for PHP (HHVM) before 2.4.0 and 2.3.x before 2.3.3 does not properly disable a certain libxml handler, which allows remote attackers to conduct XML External Entity (XXE) attacks.
https://github.com/facebook/hhvm/commit/95f96e7287effe2fcdfb9a5338d1a7e4f55b083b