CVE-2026-41263

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.

For more information

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

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:

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

CVE-2026-41263 has a CVSS score of 3.7 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.7.0-rc.2, 3.6.14, 2.11.43); upgrading removes the vulnerable code path.

Kodem intelligence

• https://github.com/traefik/traefik/releases/tag/v2.11.43
• https://github.com/traefik/traefik/releases/tag/v3.6.14
• https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2

1. What is CVE-2026-41263? CVE-2026-41263 is a medium-severity security vulnerability in github.com/traefik/traefik/v3 (go), affecting versions >= 3.7.0-ea.1, < 3.7.0-rc.2. It is fixed in 3.7.0-rc.2, 3.6.14, 2.11.43.
2. How severe is CVE-2026-41263? CVE-2026-41263 has a CVSS score of 3.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-41263?
   • github.com/traefik/traefik/v3 (go) (versions >= 3.7.0-ea.1, < 3.7.0-rc.2)
   • github.com/traefik/traefik/v2 (go) (versions < 2.11.43)
   • github.com/traefik/traefik (go) (versions <= 1.7.34)
4. Is there a fix for CVE-2026-41263? Yes. CVE-2026-41263 is fixed in 3.7.0-rc.2, 3.6.14, 2.11.43. Upgrade to this version or later.
5. Is CVE-2026-41263 exploitable, and should I be worried? Whether CVE-2026-41263 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-41263 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-41263?
   • Upgrade github.com/traefik/traefik/v3 to 3.7.0-rc.2 or later
   • Upgrade github.com/traefik/traefik/v3 to 3.6.14 or later
   • Upgrade github.com/traefik/traefik/v2 to 2.11.43 or later