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

• Severity: high (CVSS 8.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-04-21 · updated 2026-10-26
• Weakness: CWE-22

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.

Affected products

• Go Github.Com/Go-Acme/Lego: through 2.7.2
• Go Github.Com/Go-Acme/Lego/V3: through 3.9.0
• Go Github.Com/Go-Acme/Lego/V4: before 4.34.0 — fixed in 4.34.0
• go-acme lego: before 4.34.0

How to fix

Upgrade to a fixed version: Go Github.Com/Go-Acme/Lego/V4 4.34.0. Check the vendor advisory for mitigations.

References

• https://github.com/go-acme/lego/security/advisories/GHSA-qqx8-2xmm-jrv8
• https://access.redhat.com/errata/RHSA-2026:21772
• https://access.redhat.com/security/cve/CVE-2026-40611
• https://bugzilla.redhat.com/show_bug.cgi?id=2460233
• https://github.com/go-acme/lego/security/advisories/GHSA-qqx8-2xmm-jrv8
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-40611.json

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