CVE-2026-41263: Traefik: BasicAuth middleware: timing side-channel vulnerability

## 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 - 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 ## For more information If there are any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> # 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 </details> ---