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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.

AI summary snake-internal / snake-material-v2

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
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 (7 steps)

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. 3
    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. 6
    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 3 で見つけた異常検知の続報チェックを含めること。

  4. 7
    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 3 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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