The HashContext class in hphp/runtime/ext/ext_hash.cpp in Facebook HipHop Virtual Machine (HHVM) before 3.3.0 incorrectly expects that a certain key string uses '\0' for termination, which allows remote attackers to obtain sensitive information by leveraging read access beyond the end of the string, and makes it easier for remote attackers to defeat cryptographic protection mechanisms by leveraging truncation of a string containing an internal '\0' character.