CVE-2026-40399 - Windows TCP/IP Stack-Based Buffer Overflow Local Privilege Escalation

CVE-2026-40399 is a stack-based buffer overflow vulnerability within the Windows TCP/IP protocol stack. This vulnerability allows an attacker with local access and valid credentials to elevate their privileges on the targeted system. Exploitation requires an attacker to be authenticated on the system, reducing the attack surface compared to unauthenticated remote code execution vulnerabilities. Microsoft has assigned this CVE a CVSS v3.1 score of 7.8, indicating a high severity. Successful exploitation results in a complete compromise of the affected system due to the privilege escalation achieved.

Attack Chain

1. An attacker gains authenticated local access to a vulnerable Windows system.
2. The attacker crafts a malicious TCP/IP packet or series of packets.
3. The attacker sends the crafted packets to the local system via a socket.
4. The Windows TCP/IP stack processes the malicious packet(s), triggering the buffer overflow.
5. The overflow overwrites adjacent stack memory, including return addresses and function pointers.
6. The attacker redirects control flow to attacker-controlled code or ROP gadgets.
7. The attacker’s code executes with elevated privileges (SYSTEM).
8. The attacker performs privileged actions, such as installing malware, modifying system settings, or accessing sensitive data.

Impact

Successful exploitation of CVE-2026-40399 allows a local attacker to escalate their privileges to SYSTEM. This results in complete control over the compromised system. The impact includes unauthorized data access, modification of system configuration, installation of malicious software, and lateral movement within the network if the compromised system has network connectivity. The vulnerability affects any system where a user has local access.

Recommendation

• Apply the security update released by Microsoft to patch CVE-2026-40399 as soon as possible (reference: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40399).
• Monitor for unusual network activity originating from processes running with elevated privileges, using process_creation and network_connection logs.
• Implement the Sigma rule Detect Suspicious Network Activity from Elevated Processes to identify potential exploitation attempts.
• Enable process creation logging to detect unusual processes being spawned by the network service (logsource: process_creation).