GitHub Security Advisory: GHSA-g3hg-j4jv-cwfr
Package: github.com/traefik/traefik/v3 (GO)
Severity: MODERATE (CVSS 0.0)
CWE: CWE-208: Observable Timing Discrepancy
Vulnerable versions: >= 3.7.0-ea.1, < 3.7.0-ea.2
Patched in: 3.7.0-ea.2

Summary: Traefik Affected by BasicAuth Middleware Timing Attack Allows Username Enumeration

Details:
## 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](https://github.com/traefik/traefik/issues).

<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:
```yaml
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:
```python
#!/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, [CREDENTIAL_REDACTED]"):
    times = []
    for _ in range(SAMPLES):
        start =