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CVE-2026-40611 high CVSS 8.8

Path Traversal in github.com/go-acme/lego/v4 (CVE-2026-40611)

Summary

Path traversal in github.com/go-acme/lego/v4 (CVE-2026-40611). Successful exploitation can lead to full system takeover. Mitigation: upgrade to 4.34.0 or later.

Original description

The description as published by the source.

Let's Encrypt client and ACME library written in Go (Lego). Prior to 4.34.0, the webroot HTTP-01 challenge provider in lego is vulnerable to arbitrary file write and deletion via path traversal. A malicious ACME server can supply a crafted challenge token containing ../ sequences, causing lego to write attacker-influenced content to any path writable by the lego process. This vulnerability is fixed in 4.34.0.

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-2026-40611 has been found in github.com/go-acme/lego/v4.
Successful exploitation can lead to full system takeover. CVSS score: 8.8/10.
What to do: upgrade github.com/go-acme/lego/v4 to 4.34.0 or later.
If unsure, ask your IT team or search "github.com/go-acme/lego/v4 CVE-2026-40611" on the vendor's site.

CVE-2026-40611 (github.com/go-acme/lego/v4) — CWE-22 / CVSS v3 8.8
Attack vector: remote (network-reachable) / unauthenticated
Patched: 4.34.0 — 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/go-acme/lego/v4.

📍 Affected scope

Check the vendor advisory for the affected versions of github.com/go-acme/lego/v4.

🔥 Severity

Severity: High (CVSS 8.8/10). Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to 4.34.0.

🛡️ 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 request patterns matching this CVE's known IOCs. Run grep -r 'github.com/go-acme/lego/v4' . 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/go-acme/lego/v4' . | 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/go-acme/lego/v4 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
    Apply patch patch 
    Upgrade github.com/go-acme/lego/v4 to 4.34.0

    ステージング環境で 4.34.0 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を含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    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。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

Go github.com/go-acme/lego/v4
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"4.34.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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