TCP/IP — CraftedSignal Threat Feed

CVE-2026-40405 - Windows TCP/IP Null Pointer Dereference DoS

hello@craftedsignal.io — Tue, 12 May 2026 18:47:30 +0000

CVE-2026-40405 is a denial-of-service vulnerability affecting the Windows TCP/IP stack. An unauthenticated, remote attacker can exploit this vulnerability to cause a null pointer dereference, leading to a denial-of-service condition on the affected system. The vulnerability resides within the Windows TCP/IP implementation and can be triggered by sending a specially crafted network packet. Microsoft has released a security update to address this vulnerability; defenders should apply the relevant patch as soon as feasible. The CVSS v3.1 base score for this vulnerability is 7.5, indicating a high severity.

Attack Chain

The attacker identifies a vulnerable Windows system exposed on the network.
The attacker crafts a malicious TCP/IP packet specifically designed to trigger the null pointer dereference.
The attacker sends the crafted packet to the target system over the network (port 80 or 443).
The Windows TCP/IP stack processes the packet.
The malicious packet triggers a null pointer dereference within the TCP/IP stack’s code.
The null pointer dereference causes the system to crash or become unresponsive.
The target system experiences a denial-of-service condition, disrupting network services.

Impact

Successful exploitation of CVE-2026-40405 results in a denial-of-service condition on the targeted Windows system. This can disrupt network services, prevent legitimate users from accessing resources, and potentially cause data loss. Given the nature of the vulnerability, any Windows system using TCP/IP may be affected until patched, impacting both servers and workstations in various sectors.

Recommendation

Apply the security update released by Microsoft to patch CVE-2026-40405. See the advisory at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40405.
Monitor network traffic for suspicious or malformed TCP/IP packets that may be indicative of exploitation attempts using the provided Sigma rule.
Consider using network intrusion detection systems (NIDS) to identify and block malicious traffic targeting CVE-2026-40405.

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

hello@craftedsignal.io — Tue, 12 May 2026 18:46:38 +0000

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

An attacker gains authenticated local access to a vulnerable Windows system.
The attacker crafts a malicious TCP/IP packet or series of packets.
The attacker sends the crafted packets to the local system via a socket.
The Windows TCP/IP stack processes the malicious packet(s), triggering the buffer overflow.
The overflow overwrites adjacent stack memory, including return addresses and function pointers.
The attacker redirects control flow to attacker-controlled code or ROP gadgets.
The attacker’s code executes with elevated privileges (SYSTEM).
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).

CVE-2026-33837 - Windows TCP/IP Heap-Based Buffer Overflow for Privilege Escalation

hello@craftedsignal.io — Tue, 12 May 2026 18:20:05 +0000

CVE-2026-33837 describes a heap-based buffer overflow vulnerability present in the TCP/IP stack of Microsoft Windows. An attacker who has already gained local access to a system can exploit this flaw to escalate their privileges. The vulnerability stems from improper memory management within the TCP/IP driver when handling network packets. Successful exploitation allows an attacker to execute arbitrary code with elevated privileges. This poses a significant risk to system integrity and confidentiality, potentially granting attackers full control over the compromised machine.

Attack Chain

Attacker gains initial local access to a Windows system through legitimate means or exploiting another vulnerability.
Attacker crafts a malicious network packet designed to trigger the heap-based buffer overflow in the TCP/IP stack.
The attacker sends the crafted packet to the vulnerable system, targeting a specific TCP/IP port.
The Windows TCP/IP driver receives the packet and attempts to process it, allocating memory on the heap.
Due to the vulnerability, the allocated buffer is too small to hold the incoming data, resulting in a heap-based buffer overflow.
The overflow allows the attacker to overwrite adjacent memory regions on the heap, including critical system data structures.
The attacker overwrites function pointers or other sensitive data to redirect program execution to attacker-controlled code.
The attacker’s code executes with elevated privileges, granting them complete control over the system.

Impact

Successful exploitation of CVE-2026-33837 allows a local attacker to escalate privileges to SYSTEM level. This can lead to a complete compromise of the affected system, including data theft, installation of malware, and lateral movement within the network. Given the widespread use of Windows, this vulnerability poses a significant risk to organizations.

Recommendation

Apply the security update released by Microsoft to patch CVE-2026-33837 (reference: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-33837).
Monitor process creation events for unexpected processes spawned by the System account (see example Sigma rule below).
Deploy the Sigma rules in this brief to your SIEM to detect potential exploitation attempts.