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

Vulnerability summary

CVE-2026-40611 is a high (CVSS 8.8) Path Traversal vulnerability in go-acme lego. EPSS estimates a 0.55% chance of exploitation in the next 30 days, higher than 44.9% of scored vulnerabilities.

Timeline

1. 21-04-2026
   PublishedVulnerability first disclosed.
2. 15-07-2026
   Last modifiedVulnerability information updated.

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.

• CWE-22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.