CVE-2026-20896 Gitea Auth Bypass X-WEBAUTH-USER Admin Account Creation

This rule detects unauthorized exploitation attempts against Gitea environments vulnerable to CVE-2026-20896. When Gitea is configured with REVERSE_PROXY_TRUSTED_PROXIES set to permit all sources, an attacker can bypass authentication by injecting the 'X-WEBAUTH-USER: admin' header in HTTP requests. This allows the attacker to gain administrative privileges and perform unauthorized actions, such as creating new admin accounts or accessing restricted API endpoints.