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Traefik: A timing side-channel vulnerability allows for valid username enumeration via BasicAuth middleware

GHSA-6x2q-h3cr-8j2h · CVE-2026-41263

Published · Modified

Description

Summary

There is a timing side-channel vulnerability in Traefik's BasicAuth middleware that allows an attacker to enumerate valid usernames through response-time differences.

The variable intended to hold a constant-time fallback secret always resolves to an empty string, causing the constant-time comparison to short-circuit in microseconds rather than performing a full bcrypt evaluation. This restores the original timing oracle and makes it possible to distinguish existing users from non-existing ones by measuring authentication response times.

Patches

For more information

If there are any questions or comments about this advisory, please open an issue.

Original Description

BasicAuth Timing Regression: CVE-2026-32595 Fix Is a No-Op Due to Map Key/Value Confusion

TL;DR

The patch for CVE-2026-32595 is a no-op. Line 49 of basic_auth.go has a
map key/value confusion that makes notFoundSecret always "". The
"constant time" fallback calls goauth.CheckSecret(password, ""), which
fast-fails in ~1us instead of running bcrypt (~60ms).

Evidence (HEAD 786f7192e, 2026-04-09)

Black-box PoC against live traefik binary on port 28080:

bucket n median min
existing user (wrong pw) 240 62.85 ms 57.54 ms
nonexistent user (wrong pw) 400 0.48 ms 0.35 ms

Median ratio: 130.4x. Classification: 8/8 correct.

Go in-tree test: goauth.CheckSecret direct ratio 12,746x.

Root cause (4-step trace)

  1. basic_auth.go:49: users[slices.Collect(maps.Values(users))[0]] -- looks
    up a hash as a username key, returns "".
  2. basic_auth.go:119-120: calls goauth.CheckSecret(password, "").
  3. go-http-auth/basic.go:87: empty string matches no prefix, falls to default
    compareMD5HashAndPassword.
  4. basic.go:107-109: bytes.SplitN("", "$", 4) returns length 1, function
    returns instantly.

Files

  • poc/exploit.py -- black-box Python timing oracle
  • poc/basic_auth_timing_regression_test.go -- Go in-tree test
  • poc/traefik.yml + poc/dynamic.yml -- traefik config
  • poc/live_http_poc_output_head.txt -- verbatim PoC output on HEAD

Koda Reef

References

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