Punk::OAuth2 versions before 0.03 for Perl allow an attacker-chosen off-site redirect after login because sameoriginpath accepts a backslash or tab in the return parameter.

oauth2login reads the return parameter from the initiation request, runs sameoriginpath over it, and stores the survivor in the session flow record as the post-login redirect target. That check rejects a value that does not begin with a slash, one with a slash as its second byte, and one containing CR or LF. A backslash and a tab pass. The URL Standard treats a backslash as equivalent to a slash for special schemes, so /\evil.example parses with the authority evil.example. It also strips ASCII tab before parsing, so a tab between two leading slashes leaves //evil.example.

A crafted link to the application's own login route lands the victim on the attacker's site after a genuine authentication. The redirect carries no authorization code or access token.