sql/password.c in Oracle MySQL 5.1.x before 5.1.63, 5.5.x before 5.5.24, and 5.6.x before 5.6.6, and MariaDB 5.1.x before 5.1.62, 5.2.x before 5.2.12, 5.3.x before 5.3.6, and 5.5.x before 5.5.23, when running in certain environments with certain implementations of the memcmp function, allows remote attackers to bypass authentication by repeatedly authenticating with the same incorrect password, which eventually causes a token comparison to succeed due to an improperly-checked return value.