Authelia is an open-source authentication and authorization server providing two-factor authentication and single sign-on (SSO) for applications via a web portal. In versions 4.36.0 through 4.39.19, due to lack of canonicalization of domains in very specific edge cases, an access control rule may be skipped when it should match a request. The specific conditions that could lead to a security issue for vulnerability are: 1. The specific target resource of the attack must be using the forwarded authorization integration; 2. The requested domain must have two additional segments compared to a session domain i.e. "a.b.example.com" is requested, but the session domain is "example.com"; 3. There access control rules must specify two separate rules which both contain inexact domain matches such as "*.b.example.com" and "*.example.com" i.e. wildcards, username matches, group matches; 4. The rules must be in order of most specific domain to least specific domain; 5. The second rule must be more permissive than the first rule; 6. The attacker must specifically request a URL for the more specific domain, with the second part containing one or more capitalized letters i.e. "https://a.B.example.com" and no other segment with capitalized letters; 7. The integration used must not be the Envoy ExtAuthz integration; and 8. The proxy must not canonicalize the requested host name in the relevant header before sending it to the relevant authorization endpoint. The kind of configuration used to produce this issue and result in a "bypass" rule being matched has long been highly discouraged. Essentially hosts which should be bypassed entirely should not be secured by having the proxy check them with the authorization handlers. Upgrade to 4.39.20 to receive a patch.