OAuth2 Proxy Authentication Bypass Vulnerability (CVE-2026-41059)

OAuth2 Proxy, a reverse proxy providing authentication using OAuth2 providers, is susceptible to an authentication bypass vulnerability (CVE-2026-41059) affecting versions 7.5.0 through 7.15.1. This vulnerability arises when specific configurations are in place: the use of skip_auth_routes or the legacy skip_auth_regex with patterns vulnerable to attacker-controlled suffix widening (e.g., ^/foo/.*/bar$), and protected upstream applications that interpret # as a fragment delimiter. An attacker can exploit this by crafting requests containing a number sign (# or its encoded form %23) to match public allowlist rules, while the backend inadvertently serves protected resources. Organizations using OAuth2 Proxy within this vulnerable range should upgrade to version 7.15.2 or implement mitigations.

Attack Chain

1. An attacker identifies an OAuth2 Proxy instance running a vulnerable version (7.5.0 - 7.15.1) and configuration, specifically utilizing skip_auth_routes or skip_auth_regex with overly permissive patterns.
2. The attacker crafts a malicious HTTP request targeting a protected resource, embedding a # or %23 within the URI path. For example, a request to /foo/secret%23bar.
3. OAuth2 Proxy evaluates the request against the configured skip_auth_routes or skip_auth_regex rules. The crafted path bypasses the authentication check because the initial segment matches the allowed pattern (e.g., /foo/).
4. OAuth2 Proxy forwards the request to the upstream application without authentication.
5. The upstream application receives the request. Due to the presence of # or %23, the application might interpret the URL differently, possibly ignoring the fragment and routing the request to the protected resource (e.g., /foo/secret).
6. The upstream application, believing the request is legitimate, processes the request and potentially returns sensitive data.
7. The attacker receives the unauthorized response from the upstream application, successfully bypassing authentication.

Impact

Successful exploitation of CVE-2026-41059 allows unauthenticated attackers to access protected resources and sensitive data behind OAuth2 Proxy. The impact is highly dependent on the nature of the protected resources, potentially leading to data breaches, unauthorized access to administrative interfaces, and other security compromises. The number of affected organizations is unknown but depends on the prevalence of vulnerable OAuth2 Proxy configurations.

Recommendation

• Upgrade OAuth2 Proxy to version 7.15.2 or later to patch CVE-2026-41059.
• Tighten or remove skip_auth_routes and skip_auth_regex rules, especially patterns that use broad wildcards across path segments, as mentioned in the advisory for CVE-2026-41059.
• Implement ingress, load balancer, or WAF rules to reject requests whose path contains %23 or #, as recommended in the CVE-2026-41059 advisory.
• Deploy the Sigma rule "OAuth2 Proxy Authentication Bypass Attempt via URL Fragment" to detect exploitation attempts.