Anubis Web AI Firewall Policy Bypass Vulnerability via X-Original-URI Header

A policy bypass vulnerability has been identified in Anubis, a Web AI Firewall Utility, in versions 1.22.0 prior to 1.26.0-pre1. The vulnerability arises because the PathChecker.Check() function in lib/policy/checker.go improperly trusts the client-controlled X-Original-URI header when matching request paths. This flaw allows HTTP clients to bypass Anubis challenges by exploiting default ALLOW rules that match certain paths, such as those under the well-known directory. As a result, requests can be forwarded to upstream resources without the usual bot protection measures.

Impact

Exploitation of this vulnerability allows any HTTP client to bypass Anubis bot protection by manipulating the X-Original-URI header, effectively disabling the firewall's challenge response for the duration of the request.

Reproduction

To reproduce this vulnerability, send a request to an Anubis-protected endpoint that includes the X-Original-URI header with a value that matches an allowed path regex, such as the well-known directory. The response will bypass the Anubis challenge and return the upstream content directly.

Remediation

Users can update to Anubis version 1.26.0-pre1, which addresses this vulnerability by modifying the PathChecker to properly handle the X-Original-URI and X-Forwarded-Uri headers, ensuring that path-based rules are correctly applied even when using Traefik as a reverse proxy.