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medium advisory

CVE-2026-40414: Windows TCP/IP Null Pointer Dereference Denial-of-Service

A null pointer dereference vulnerability exists in Windows TCP/IP, allowing an unauthorized attacker on an adjacent network to cause a denial-of-service condition.

CVE-2026-40414 describes a null pointer dereference vulnerability in the Windows TCP/IP stack. An attacker on an adjacent network can exploit this flaw to trigger a denial-of-service (DoS) condition. The vulnerability resides within the handling of specific TCP/IP packets, where a malformed or unexpected packet structure can lead to a null pointer dereference, crashing the system or rendering it unresponsive. This issue was reported to Microsoft and assigned a CVSS v3.1 score of 7.4. The vulnerability affects systems where the TCP/IP stack is exposed to adjacent networks, increasing the attack surface.

Attack Chain

  1. Attacker identifies a target Windows system on an adjacent network.
  2. Attacker crafts a malicious TCP/IP packet designed to trigger the null pointer dereference.
  3. The attacker sends the crafted packet to the target system.
  4. The Windows TCP/IP stack receives and processes the packet.
  5. Due to the malformed structure of the packet, the TCP/IP stack attempts to dereference a null pointer.
  6. This null pointer dereference causes the system to crash or become unresponsive.
  7. The target system experiences a denial-of-service condition, impacting network connectivity and application availability.

Impact

Successful exploitation of CVE-2026-40414 results in a denial-of-service condition on the target Windows system. This can disrupt network communications, prevent access to critical services, and potentially lead to data loss or corruption if applications are abruptly terminated. The vulnerability is rated as HIGH severity with a CVSS base score of 7.4. The impact is limited to systems on adjacent networks, but could be significant for organizations relying on those systems for essential operations.

Recommendation

  • Apply the security update provided by Microsoft to patch CVE-2026-40414 as soon as possible, referenced in the Microsoft advisory.
  • Monitor network traffic for suspicious TCP/IP packets originating from adjacent networks using the Sigma rule.
  • Consider implementing network segmentation to limit the exposure of critical systems to adjacent networks.
  • Enable logging for network connections and analyze logs for unexpected patterns, as this could be indicative of exploitation attempts.

Detection coverage 2

Detect Potential CVE-2026-40414 Exploitation - Anomalous TCP Flags

low

Detects CVE-2026-40414 exploitation attempts by identifying unusual TCP flag combinations that might indicate a malformed packet crafted to trigger the vulnerability.

sigma tactics: availability techniques: T1499.004 sources: network_connection, windows

Detect Potential CVE-2026-40414 Exploitation - High Volume of TCP Resets from Single Host

medium

Detects a potential DoS attack related to CVE-2026-40414 by identifying a high volume of TCP reset (RST) packets originating from a single host within a short time frame, which could indicate a crash or malfunction caused by the vulnerability.

sigma tactics: availability techniques: T1499.004 sources: network_connection, windows

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