dbcve.org
Path TraversalWeakness · CWE-22

CVE-2026-40611

HIGH · 8.8 CVSS v3.1 Published 2026-04-21
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
95/100
Remediation priority · Urgent
Remotely reachable No privileges

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The webroot HTTP-01 challenge provider in lego (Go ACME library) versions prior to 4.34.0 lacks proper path validation, allowing a malicious ACME server to send crafted challenge tokens containing '../' sequences. This enables path traversal attacks allowing arbitrary file write and potentially file deletion anywhere the lego process has write permissions.

MitigationUpgrade to lego version 4.34.0 or later which includes proper path validation. As a defense-in-depth measure, run lego processes with minimal file system permissions to limit impact of potential compromise.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine lego version
    Run 'lego --version' or check the installed package version using your package manager (e.g., 'apt list --installed | grep lego', 'rpm -q lego', or 'brew list lego')
    Affected if The installed version is earlier than 4.34.0 (the fixed version)
  2. Identify lego configuration files
    Look for lego configuration files in common locations: ~/.lego/config.yaml, /etc/lego/config.yaml, or a directory specified via LEGO_CONFIG_PATH environment variable
    Affected if A configuration file exists and contains a 'webroot' provider setting for HTTP-01 challenges
  3. Inspect the webroot path setting
    Open the lego configuration file and locate the 'webroot' or 'http01' provider configuration; check for a 'webroot' path directive that specifies the document root for ACME challenges
    Affected if A webroot path is explicitly configured, indicating the webroot HTTP-01 challenge provider is in use
  4. Review ACME server configuration
    In the lego configuration file, check the 'server' or 'acme' endpoint setting to identify which ACME server lego is communicating with
    Affected if The configured ACME server is untrusted, third-party, or not explicitly verified as the Let's Encrypt production/staging server

A user is affected if lego version is earlier than 4.34.0 AND the webroot HTTP-01 challenge provider is configured, regardless of ACME server trustworthiness.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade to lego version 4.34.0 or later which includes proper path validation. As a defense-in-depth measure, run lego processes with minimal file system permissions to limit impact of potential compromise.

Recommended fix High confidence

lego v4.34.0 or later

  1. 1. Identify the current lego version by running 'lego --version' or checking your dependency management system (e.g., go.mod)
  2. 2. If using Go modules, update the dependency to version 4.34.0 or later: 'go get github.com/go-acme/lego/[email protected]'
  3. 3. If using a binary distribution, download lego v4.34.0 or later from the official releases (github.com/go-acme/lego/releases)
  4. 4. Replace the existing lego binary with the new version
  5. 5. Verify the upgrade was successful by running 'lego --version' and confirming it shows 4.34.0 or later
  6. 6. Test that the webroot HTTP-01 challenge functionality works correctly
  7. 7. If using automated deployment, update your deployment scripts to reference the fixed version

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-40611 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
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