The mcrypt_create_iv function in ext/mcrypt/mcrypt.c in PHP before 4.4.7, 5.2.1, and possibly 5.0.x and other PHP 5 versions, calls php_rand_r with an uninitialized seed variable and therefore always generates the same initialization vector (IV), which might allow context-dependent attackers to decrypt certain data more easily because of the guessable encryption keys.