Punk versions before 0.18 for Perl allow session cookie forgery via an empty default HMAC key when a session is declared without a secret.
The session keyword freezes its options onto the application as given: it does not require a secret, warn, or refuse to start when one is absent. The cookie read and the write-back both default that key to the empty string, so a declaration with no secret option, or with an undefined or empty one, signs and verifies with a zero-length HMAC-SHA256 key.
An attacker who knows the cookie format can then mint one offline carrying any contents the session holds, such as a user identifier or a role. Nothing marks the misconfiguration at runtime: cookies are well formed and sessions round-trip as expected.
For deployments that cannot upgrade to 0.18, declare the session keyword with a non-empty secret. The secret keyword fails closed, so sourcing the key through it turns a missing configuration path or an unset environment variable into a startup error.
Upgrade to Punk 0.18 or later.