CallBackery versions before 0.62.0 for Perl allow session cookie forgery via a signing secret generated with rand(). The secret that signs session cookies is built from two calls to the built-in rand(), which is seeded with 32 bits. The secret therefore has at most 2**32 possible values, and one signed cookie is enough to test each candidate offline. An attacker who obtains a single session cookie can recover the secret and sign cookies for any user, including an administrator.