Facebook HipHop Virtual Machine (HHVM) before 3.1.0 does not drop supplemental group memberships within hphp/util/capability.cpp and hphp/util/light-process.cpp, which allows remote attackers to bypass intended access restrictions by leveraging group permissions for a file or directory.
https://github.com/facebook/hhvm/commit/851fff90a9b7461df2393af32239ba217bc25946