Bug 2460233 (CVE-2026-40611)

Summary: CVE-2026-40611 github.com/go-acme/lego: Lego: Arbitrary file write and deletion via path traversal from a malicious ACME server
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
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Priority: high    
Version: unspecifiedCC: bbrownin, fdeutsch, oramraz, sdawley, smullick, stirabos, thason
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in lego, the Let's Encrypt client and ACME library written in Go. A malicious ACME (Automated Certificate Management Environment) server can exploit a path traversal vulnerability in the webroot HTTP-01 challenge provider. By supplying a specially crafted challenge token containing directory traversal sequences, the server can cause lego to write or delete files in arbitrary locations on the system where lego is running, potentially leading to system compromise.
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Bug Depends On: 2461423    
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Description OSIDB Bzimport 2026-04-21 20:02:20 UTC 
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.