Anubis: Policy bypass via client controlled X-Original-URI headerGHSA-6wcg-mqvh-fcvg

Any HTTP client can bypass Anubis bot protection on the default configuration by adding a single request header. No challenge needs to be solved. Affected versions: v1.22.0 through v1.25.0 (introduced in commit d1d631a, PR #1015)

The root cause is in lib/policy/checker.go, PathChecker.Check():

func (pc *PathChecker) Check(r *http.Request) (bool, error) {
    originalUrl := r.Header.Get("X-Original-URI")
    if originalUrl != "" {
        if pc.regexp.MatchString(originalUrl) {
            return true, nil
        }
    }
    if pc.regexp.MatchString(r.URL.Path) {
        return true, nil
    }
    return false, nil
}

The header value comes directly from the client request. The middleware chain never strips it. In reverse proxy mode an attacker fully controls it. The default policy imports data/common/keep-internet-working.yaml, which contains path-only ALLOW rules with no other conditions:

- name: well-known
  path_regex: ^/\.well-known/.*$
  action: ALLOW

When X-Original-URI matches one of those regexes, the rule fires as ALLOW and the request is forwarded upstream without any challenge or JWT check.

Proof of concept

Normal request, gets challenged:

curl -s https://anubis.techaro.lol/ | grep -o "<title>.*</title>"

Bypass, gets upstream content:

curl -s -H "X-Original-URI: /.well-known/x" https://anubis.techaro.lol/ | grep -o "<title>.*</title>"

The first command returns the Anubis challenge page title. The second returns the real site title directly, with no cookie set and no challenge issued. Verified live against anubis.techaro.lol.

Suggested fix

The fix should probably be to strip the header from incoming client requests in the middleware chain before policy evaluation. In auth_request mode the header is set by the proxy after Anubis processes the middleware, so stripping it at ingress does not break that deployment mode.