---
id: "CVE-2026-88010"
kind: "vulnerability"
aliases: ["CVE-2026-88010", "GHSA-8fcf-v89g-xpg6", "GO-2026-6558"]
title: "Traefik: BasicAuth singleflight coalescing reintroduces an unauthenticated username-enumeration timing oracle"
severity: "medium"
cvss: 6.3
epss: 0.00691
exploited: false
status: "open"
category: "package"
products: [{"name": "github.com/traefik/traefik", "fixed": [], "affected": "all versions", "ecosystem": "go"}]
ecosystems: ["go"]
tags: ["basic-auth", "package", "self-hosted", "traefik"]
not_affected_if: ["digest-auth", "no-basicauth-middleware", "traefik-v2"]
published_at: "2026-09-22T16:18:06+00:00"
first_seen_at: "2026-10-23T19:13:08+00:00"
updated_at: "2026-10-23T21:46:08+00:00"
sources: [{"source": "govuln", "url": "https://pkg.go.dev/vuln/GO-2026-6558", "first_seen_at": "2026-10-23T19:13:08+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-88010", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-88010", "first_seen_at": "2026-10-23T19:35:23+00:00"}]
---

# Traefik: BasicAuth singleflight coalescing reintroduces an unauthenticated username-enumeration timing oracle

Traefik's BasicAuth middleware leaks whether a username exists through response timing when many requests run concurrently. An attacker can list valid usernames but cannot read passwords or bypass login.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- go:github.com/traefik/traefik affected all versions fixed ?

## How to check

Check the Traefik version with 'traefik version'; affected if it is 3.6.11 through 3.7.12 (Traefik v2 and Digest auth are not affected).
Look for basicAuth middleware in the Traefik dynamic configuration or labels.

## Fix

Upgrade Traefik to 3.7.13 or later.

## Timeline

- 2026-10-23T19:13:08+00:00 govuln https://pkg.go.dev/vuln/GO-2026-6558
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-88010
- 2026-10-23T19:35:23+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-88010

## From the sources

Traefik is an open source HTTP reverse proxy and load balancer. From 3.6.11 until 3.7.13, checkPassword in pkg/middlewares/auth/basic_auth.go constructs the BasicAuth singleflight key from the submitted password and stored secret. Concurrent requests for absent usernames therefore coalesce on one key while configured usernames perform separate password hashes, creating an unauthenticated timing discrepancy that reveals whether a username exists. Sequential checks remain cost-equalized, Traefik v2 and Digest authentication are not affected, and the issue does not disclose credentials or bypass authentication. This issue is fixed in version 3.7.13.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_