Invoice Ninja through 5.13.26 contains an open redirect vulnerability in the client portal login that allows unauthenticated attackers to redirect authenticated victims to attacker-controlled external URLs by injecting a malicious value into the intended query parameter. Attackers can craft a client login link with an external URL in the intended parameter, which is stored in the session without host validation and emitted verbatim via a bare redirect in the ContactLoginController authenticated() handler after the victim completes a legitimate login, enabling phishing attacks.