CVE-2026-40575
CRITICAL9.1EPSS 0.09%OAuth2 Proxy has an Authentication Bypass via X-Forwarded-Uri Header Spoofing
Description
### 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
Affected packages (2)
- Bitnami/oauth2-proxy>= 7.5.0, < 7.15.2
- Go/github.com/oauth2-proxy/oauth2-proxy/v7>= 7.5.0, < 7.15.2
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | CRITICAL9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |