frp has an authentication bypass in HTTP vhost routing when routeByHTTPUser is used for access control

Summary

frp contains an authentication bypass in the HTTP vhost routing path when routeByHTTPUser is used as part of access control. In proxy-style requests, the routing logic uses the username from Proxy-Authorization to select the routeByHTTPUser backend, while the access control check uses credentials from the regular Authorization header. As a result, an attacker who can reach the HTTP vhost entrypoint and knows or can guess the protected routeByHTTPUser value may access a backend protected by httpUser / httpPassword even with an incorrect Proxy-Authorization password.

This issue affects deployments that explicitly use routeByHTTPUser. It does not affect ordinary HTTP proxies that do not use this feature.

Details

The issue is in pkg/util/vhost/http.go.

In proxy-style requests using an absolute URI, the routing path extracts the username from Proxy-Authorization and stores it as the request HTTPUser, which is then used for routeByHTTPUser route selection.

More specifically, injectRequestInfoToCtx() derives the routing user from Proxy-Authorization, while the original ServeHTTP() implementation used req.BasicAuth() for the authentication check.

Because routing and authentication use different credential sources, a request can be routed to a protected backend based on the Proxy-Authorization username while the authentication check is not performed against the same credentials. This creates an authentication bypass when routeByHTTPUser, httpUser, and httpPassword are used together.

This is not a universal anonymous bypass for all frp HTTP proxies; it is specific to deployments that use routeByHTTPUser and where the target user value is known or can be inferred.

A minimal fix is to make the authentication check in proxy mode use the same credential source as route selection, i.e. to derive proxy-mode credentials from Proxy-Authorization consistently.

From local Git history analysis, this logic appears to have been introduced by commit 4af85da0c2c6eb981142a8fdb44f885d26cb9d08, with the earliest containing release tag appearing to be v0.43.0.

PoC

I reproduced the issue with the official frp_0.68.0_linux_amd64.tar.gz release binaries both locally and on an internet-reachable test server under my control.

Minimal setup:

• frps exposes an HTTP vhost entrypoint.
• One HTTP proxy is configured with:
  • customDomains = ["example.test"]
  • routeByHTTPUser = "alice"
  • httpUser = "alice"
  • httpPassword = "secret"
• The protected backend returns a constant marker string: PRIVATE.

Minimal request flow:

1. Direct unauthenticated request:

   • curl -i --proxy '' -H 'Host: example.test' http://<FRPS_HOST>:<VHOST_HTTP_PORT>/
   • Result: 404 Not Found

2. Direct request with correct backend credentials:

   • curl -i --proxy '' -u alice:secret -H 'Host: example.test' http://<FRPS_HOST>:<VHOST_HTTP_PORT>/
   • Result: 200 OK, body contains PRIVATE

3. Proxy-style request with incorrect Proxy-Authorization:

   • curl -i --noproxy '' -x http://<FRPS_HOST>:<VHOST_HTTP_PORT> --proxy-user alice:wrong http://example.test/
   • Result: 200 OK, body contains PRIVATE

Observed minimal result summary:

• DIRECT_NOAUTH -> 404
• DIRECT_BASICAUTH_GOOD -> 200 PRIVATE
• PROXY_PROXYAUTH_WRONGPASS -> 200 PRIVATE

This was reproduced against the official binary, not only against a local source build.

Impact

This is an authentication bypass leading to unauthorized access to a protected backend.

The practical impact depends on what service is behind the protected route. Examples include private application endpoints, internal administration panels, loopback-only local services, or development and operations interfaces.

Important boundary: if the protected backend is an frpc admin API that is separately protected by its own webServer.user / webServer.password, this issue only bypasses the outer vhost restriction and does not automatically bypass the inner admin authentication. In that case, the request may still reach the backend but correctly receive 401 Unauthorized from the inner layer.

There is also a deployment-specific downstream impact path. If the bypassed backend is an frpc admin API without separate inner authentication, and if that frpc instance permits store-based proxy management, an attacker may be able to create additional plugin-based proxies through the admin API. In deployments where a unix_domain_socket proxy can be used to expose Docker's Unix socket, this may further expose the Docker API and potentially enable host-level command execution through Docker. This follow-on consequence depends on multiple additional deployment conditions and should be treated as a conditional downstream impact rather than the core vulnerability itself.

Because exploitation requires a deployment to explicitly use routeByHTTPUser, and because the attacker must know or be able to guess the target routeByHTTPUser value, the issue is better classified as a configuration-dependent authentication bypass rather than a default-configuration issue.