OAuth2 Proxy has an Authentication Bypass via Fragment Confusion in skip_auth_routes and skip_auth_regex

Impact

A configuration-dependent authentication bypass exists in OAuth2 Proxy.

Deployments are affected when all of the following are true:

• Use of skip_auth_routes or the legacy skip_auth_regex * Use of patterns that can be widened by attacker-controlled suffixes, such as ^/foo/.*/bar$ causing potential exposure of /foo/secret * Protected upstream applications that interpret # as a fragment delimiter or otherwise route the request to the protected base path

In deployments that rely on these settings, an unauthenticated attacker can send a crafted request containing a number sign in the path, including the browser-safe encoded form %23, so that OAuth2 Proxy matches a public allowlist rule while the backend serves a protected resource.

Deployments that do not use these skip-auth options, or that only allow exact public paths with tightly scoped method and path rules, ARE NOT affected.

Patches

A fix has been implemented to normalize request paths more conservatively before skip-auth matching so fragment content does not influence allowlist decisions.

Released as part of v7.15.2

Workarounds

Users who cannot upgrade immediately can reduce exposure by tightening or removing skip_auth_routes and skip_auth_regex rules, especially patterns that use broad wildcards across path segments.

Recommended mitigations:

• Replace broad rules with exact, anchored public paths and explicit HTTP methods
• Reject requests whose path contains %23 or # at the ingress, load balancer, or WAF level
• Avoid placing sensitive application paths behind broad skip_auth_routes rules