CVE-2026-42915: Windows TCP/IP DOS Vulnerability

CVE-2026-42915 Overview

CVE-2026-42915 is a denial-of-service vulnerability in the Windows TCP/IP networking stack. The flaw stems from incorrect calculation of buffer size [CWE-131] when processing network traffic. An authenticated attacker positioned on an adjacent network can trigger the condition to deny service to affected systems.

Microsoft published an advisory through the Microsoft Security Response Center (MSRC). The vulnerability requires low privileges and no user interaction, but the attacker must have adjacency to the target network segment.

Critical Impact

An authorized attacker on an adjacent network can disrupt availability of Windows systems by exploiting a buffer size miscalculation in the TCP/IP stack.

Affected Products

• Windows TCP/IP networking stack (per Microsoft advisory)
• Specific Windows versions and builds: refer to the Microsoft CVE-2026-42915 Advisory

Discovery Timeline

• 2026-06-09 - CVE-2026-42915 published to NVD
• 2026-06-09 - Last updated in NVD database

Technical Details for CVE-2026-42915

Vulnerability Analysis

The vulnerability is classified under [CWE-131] Incorrect Calculation of Buffer Size. The Windows TCP/IP stack computes a buffer length value incorrectly while handling network input. The miscalculation leads to a condition that exhausts or corrupts kernel networking resources, resulting in denial of service.

Because the issue resides in the kernel-mode networking stack, successful exploitation impacts overall system availability rather than a single user-mode process. The CWE-131 pattern typically arises from arithmetic errors, off-by-one mistakes, or incorrect handling of size fields in protocol headers.

Microsoft characterizes the impact as availability-only. There is no indication that the vulnerability permits information disclosure or integrity violation.

Root Cause

The root cause is an arithmetic or sizing error in code paths that allocate or index buffers for TCP/IP packet processing. When attacker-influenced inputs reach the affected calculation, the resulting buffer size does not match the actual data layout, triggering the denial-of-service condition.

Attack Vector

The attack vector is Adjacent Network. The attacker must reside on the same logical or physical network segment as the target, such as the same VLAN, Wi-Fi network, or broadcast domain. The attacker also requires low-level authorized access. No user interaction is needed on the target. Exploitation involves sending crafted network traffic that drives the affected TCP/IP code path into the miscalculated buffer condition.

No public proof-of-concept exploit is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. See the Microsoft CVE-2026-42915 Advisory for technical details.

Detection Methods for CVE-2026-42915

Indicators of Compromise

• Unexpected kernel stop errors or bug checks referencing tcpip.sys on Windows hosts.
• Sudden loss of network connectivity on multiple hosts sharing the same broadcast domain or VLAN.
• Repeated reboots or service restarts correlated with anomalous traffic from a single adjacent source.

Detection Strategies

• Monitor Windows Event Logs for System events related to TCP/IP driver failures, bug checks, and unexpected reboots.
• Inspect network telemetry for malformed or oversized packets originating from internal subnets that precede availability incidents.
• Correlate endpoint availability loss with NetFlow or packet capture data on adjacent segments to identify the source host.

Monitoring Recommendations

• Alert on clusters of Windows hosts losing network connectivity within the same VLAN in a short time window.
• Track patch state for affected Windows builds and flag unpatched systems exposed to multi-tenant or guest networks.
• Continuously baseline traffic patterns on internal segments so anomalous protocol behavior triggers investigation.

How to Mitigate CVE-2026-42915

Immediate Actions Required

• Apply the Microsoft security update referenced in the Microsoft CVE-2026-42915 Advisory as soon as it is available for your Windows build.
• Prioritize patching for hosts on shared, guest, or otherwise untrusted adjacent networks where authenticated users may reside.
• Verify update deployment status across managed endpoints and servers using your patch management system.

Patch Information

Microsoft addresses CVE-2026-42915 through the standard Windows security update channel. Refer to the Microsoft CVE-2026-42915 Advisory for the specific KB articles and updated build numbers that correspond to each supported Windows version.

Workarounds

• Restrict adjacent network access by segmenting sensitive Windows hosts away from general-purpose user, guest, and IoT VLANs.
• Enforce 802.1X or equivalent network access control so only authorized devices can join segments containing critical Windows infrastructure.
• Apply host-based firewall rules to limit inbound TCP/IP traffic to required sources and protocols until the patch is deployed.