CVE-2026-81509: Unvalidated r parameter allows post-authentication open redirect

nforwardauth is a forward auth service that allows the use of a single authentication middleware for multiple sites. Versions prior to 1.5.1 accept an unvalidated r query parameter on the /login endpoint and use them as the post-authentication redirect destination. An unauthenticated attacker can create a legitimate nforwardauth login URL that redirects a victim to an attacker-controlled website after the victim successfully authenticates. This issue may facilitate phishing and other social-engineering attacks by abusing the trust associated with the legitimate authentication domain. This vulnerability does not provide an unauthenticated ForwardAuth bypass. Version 1.5.1 contains a patch.