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CVE-2026-40912 high CVSS 8.2

Path Traversal in github.com/traefik/traefik/v3 (CVE-2026-40912)

Summary

Path traversal in github.com/traefik/traefik/v3 (CVE-2026-40912). Confidential information can be exposed externally. Exploitable via GET /api. Mitigation: upgrade to 3.6.14 or later.

Original description

The description as published by the source.

Summary

There is a high severity authentication bypass vulnerability in Traefik's StripPrefixRegex middleware when used in combination with ForwardAuth, BasicAuth, or DigestAuth.

The middleware matches the regex against the decoded URL path but uses the resulting byte length to slice the percent-encoded raw path. When a dot (or multiple dots) appears in the prefix portion of the URL, the raw path after stripping becomes a dot-segment (e.g. /./admin/secret).

ForwardAuth receives this dot-segment path in X-Forwarded-Uri, which does not match the protected path patterns and therefore allows the request through. The backend then normalizes the dot-segment to the real path per RFC 3986 and serves the protected content

An unauthenticated attacker can exploit this against any backend that performs dot-segment normalization.

Patches

For more information

If there are any questions or comments about this advisory, please open an issue.

Original Description

Summary

StripPrefixRegex uses the byte length of a decoded Path match to slice the encoded RawPath. When percent-encoded characters are in the prefix region, this produces a wrong RawPath. ForwardAuth then
receives this wrong path in X-Forwarded-Uri, sees a path that does…

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-2026-40912 has been found in github.com/traefik/traefik/v3.
Attackers can target a specific entry point like GET /api over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 8.2/10.
What to do: upgrade github.com/traefik/traefik/v3 to 3.6.14 or later.
If unsure, ask your IT team or search "github.com/traefik/traefik/v3 CVE-2026-40912" on the vendor's site.

CVE-2026-40912 (github.com/traefik/traefik/v3) — CWE-22 / CVSS v3 8.2
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: GET /api / GET /api/admin/secret / StripPrefixRegex / ForwardAuth
Patched: 3.6.14 — 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

Path traversal (CWE-22) exists in github.com/traefik/traefik/v3. Attackers reach the vulnerable code path via GET /api without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of github.com/traefik/traefik/v3. Attack surface: GET /api / GET /api/admin/secret / StripPrefixRegex / ForwardAuth.

🔥 Severity

Severity: High (CVSS 8.2/10). Confidential information can be exposed externally.

🔧 How to fix

Update to 3.6.14.

🛡️ 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 /api requests with malformed payloads or SQL meta-characters. Run grep -r 'github.com/traefik/traefik/v3' . 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/v3' . | 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/v3 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 /api' /var/log/nginx/access.log | grep -E '(unusual_payload|sqli_pattern)'

    アクセスログで GET /api 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/v3 to 3.6.14

    ステージング環境で 3.6.14 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 /api 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/v3
[{"type":"SEMVER","events":[{"introduced":"3.0.0-beta1"},{"fixed":"3.6.14"}]}]
go github.com/traefik/traefik/v2
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"2.11.43"}]}]
go github.com/traefik/traefik
[{"type":"SEMVER","events":[{"introduced":"0"},{"last_affected":"1.7.34"}]}]

References

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