OAuth2 Proxy has an Authentication Bypass via X-Forwarded-Uri Header Spoofing

Impact

A configuration-dependent authentication bypass exists in OAuth2 Proxy.

Deployments are affected when all of the following are true:

• OAuth2 Proxy is configured with --reverse-proxy
• and at least one rule is defined with --skip_auth_routes or the legacy --skip-auth-regex

OAuth2 Proxy may trust a client-supplied X-Forwarded-Uri header when --reverse-proxy is enabled and --skip-auth-route or --skip-auth-regex is configured. An attacker can spoof this header so OAuth2 Proxy evaluates authentication and skip-auth rules against a different path than the one actually sent to the upstream application.

This can result in an unauthenticated remote attacker bypassing authentication and accessing protected routes without a valid session.

Patches

This issue is addressed as part of the newly introduced --trusted-proxy-ip flag in v7.15.2. If you leave it unset, OAuth2 Proxy will continue to trust ALL source IPs (0.0.0.0/0) for backwards compatibility, which means a client may still be able to spoof forwarded headers. Therefore after upgrading we urge you to use the new --trusted-proxy-ip flag to set the IPs or CIDR ranges of the reverse proxies that are allowed to send X-Forwarded-* headers and furthermore implement the mitigation steps outlined below to properly configure your load balancer infrastructure.

Mitigation

• Strip any client-provided X-Forwarded-Uri header at the reverse proxy or load balancer level

• Explicitly overwrite X-Forwarded-Uri with the actual request URI before forwarding requests to OAuth2 Proxy

  Example nginx mitigation for the auth subrequest:

    location /internal-auth/ {
      internal; # Ensure external users can't access this path
  
      # Make sure the OAuth2 Proxy knows where the original request came from.
      proxy_set_header Host       $host;
      proxy_set_header X-Real-IP  $remote_addr;
      # set the value to the actual $request_uri and therefore strip any user provided X-Forwarded-Uri
      proxy_set_header X-Forwarded-Uri $request_uri;
  
      proxy_pass http://oauth2-proxy:4180/;
    }
  

• Restrict direct client access to OAuth2 Proxy so it can only be reached through a trusted reverse proxy

• Remove or narrow --skip-auth-route / --skip-auth-regex rules where possible