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CVE-2026-39858 critical CVSS 10.0

Vulnerability in github.com/traefik/traefik (CVE-2026-39858)

Summary

Vulnerability in github.com/traefik/traefik (CVE-2026-39858). Confidential information can be exposed externally. Exploitable via GET /protected. Mitigation: upgrade to 2.11.43 or later.

Original description

The description as published by the source.

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 high severity authentication bypass vulnerability in Traefik's ForwardAuth and snippet-based authentication middleware. Traefik's forwarded-header sanitization logic targets only canonical header names (e.g., X-Forwarded-Proto) and does not strip or normalize alias variants that use underscores instead of dashes (e.g., X_Forwarded_Proto). These unsanitized alias headers are forwarded intact to the authentication backend. When the backend normalizes underscore and dash header forms equivalently, an attacker can inject spoofed trust context — such as a trusted scheme or host — through the alias headers and bypass authentication on protected routes without valid credentials. This issue has been patched in versions 2.11.43, 3.6.14, and 3.7.0-rc.2.

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-39858 has been found in github.com/traefik/traefik.
Attackers can target a specific entry point like GET /protected over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 10.0/10.
What to do: upgrade github.com/traefik/traefik to 2.11.43 or later.
If unsure, ask your IT team or search "github.com/traefik/traefik CVE-2026-39858" on the vendor's site.

CVE-2026-39858 (github.com/traefik/traefik) — CWE-290 / CVSS v3 10.0
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: GET /protected / ForwardAuth / X_Forwarded_Proto
Patched: 2.11.43 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-290) exists in github.com/traefik/traefik. Attackers reach the vulnerable code path via GET /protected without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of github.com/traefik/traefik. Attack surface: GET /protected / ForwardAuth / X_Forwarded_Proto.

🔥 Severity

Severity: Critical (CVSS 10.0/10). Confidential information can be exposed externally.

🔧 How to fix

Update to 2.11.43.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual GET /protected requests with malformed payloads or SQL meta-characters. Run grep -r 'github.com/traefik/traefik' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'github.com/traefik/traefik' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で github.com/traefik/traefik を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Hunt for indicators of compromise detect 
    grep 'GET /protected' /var/log/nginx/access.log | grep -E '(unusual_payload|sqli_pattern)'

    アクセスログで GET /protected への異常なリクエスト (不正な認証ヘッダ・SQLメタ文字)を過去 30〜90日分捜索。WAF/SIEM があれば該当パスのアラート発火履歴を確認。

  3. 3
    Apply patch patch 
    Upgrade github.com/traefik/traefik to 2.11.43

    ステージング環境で 2.11.43 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスと、Step 2 で見つけた異常検知の続報チェックを含めること。

  4. 4
    Post-deployment verification verify 
    Replay attack against GET /protected on staging to confirm patch closes the vector

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番では Step 2 と同じログクエリでアラート再発が無いか継続監視。

Affected packages

Go github.com/traefik/traefik
[{"type":"SEMVER","events":[{"introduced":"0"},{"last_affected":"1.7.34"}]}]
Go github.com/traefik/traefik/v2
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"2.11.43"}]}]
Go github.com/traefik/traefik/v3
[{"type":"SEMVER","events":[{"introduced":"3.0.0-beta1"},{"fixed":"3.6.14"}]}]

References

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