CVE‑2026‑45839 – Remote Code Execution in Windows 10/Server 2022

Impact

• Severity: CVSS v3.1 score 9.8 (Critical)
• Affected products: Windows 10 (all releases up to 22H2) and Windows Server 2022
• Exploit vector: Network‑based, no user interaction required
• Potential damage: Full system compromise, privilege escalation, data exfiltration

Technical Details

CVE‑2026‑45839 is a stack buffer overflow in the Windows TCP/IP stack. An attacker sends a specially crafted packet to the vulnerable system’s TCP/IP driver. The driver incorrectly validates the length field of the packet, allowing an attacker to overwrite the return address on the stack. Once the return address is overwritten, the attacker can redirect execution to arbitrary code, achieving remote code execution with SYSTEM privileges.

The flaw exists in the Tcpip.sys driver, version 10.0.22621.1 and earlier. The vulnerability is triggered when the TCPIP service parses an IP packet with a malformed TCP header. The driver performs a bounds check that fails to account for packets with a length field larger than the actual payload. This oversight leads to a classic stack overflow.

Mitigation Steps

1. Check current version – Run winver or systeminfo to confirm you are on Windows 10 22H2 or Windows Server 2022.
2. Download the patch – Apply the cumulative update KB100123 from the Microsoft Update Catalog or via WSUS.
   • Direct link: KB100123 – Windows 10/Server 2022 Update
3. Reboot the system after installation. The patch replaces the vulnerable Tcpip.sys with a hardened version.
4. Verify – After reboot, run sfc /scannow to ensure integrity.
5. Monitor – Enable Windows Defender Advanced Threat Protection (ATP) to detect any anomalous network activity.

Timeline

• March 12, 2026 – Microsoft Security Response Center (MSRC) identifies the flaw during internal testing.
• March 20, 2026 – Public advisory released, CVE‑2026‑45839 assigned.
• April 01, 2026 – Patch KB100123 released to Windows Update and Microsoft Update Catalog.
• April 10, 2026 – Advisory updated with detailed mitigation steps.
• April 15, 2026 – Mandatory patch deadline announced for critical infrastructure.

What to Do Now

• Immediate action: Apply KB100123 to all affected machines.
• If unable to patch immediately: Block inbound traffic on ports 80, 443, and 3389 from untrusted networks using Windows Firewall or a perimeter firewall.
• Long‑term: Review network segmentation and implement micro‑segmentation policies to limit exposure.

Further Resources

• Microsoft Security Advisory: CVE‑2026‑45839
• Detailed technical write‑up: Microsoft Docs – TCP/IP Stack Vulnerabilities
• Community discussion: GitHub Issue – CVE‑2026‑45839 analysis

Act now. Failure to patch exposes your environment to immediate compromise. Update, reboot, and verify.