CVE-2026-77499 in Windows
Summary
by MITRE • 09/09/2026
Access of resource using incompatible type ('type confusion') in Windows DHCP Server allows an unauthorized attacker to deny service over a network.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability described involves a critical logic error within the Dynamic Host Configuration Protocol server component of Microsoft Windows, specifically manifesting as a type confusion flaw. This class of defect occurs when software incorrectly assumes or handles data types during processing, leading to operations being performed on incompatible objects. In this specific instance, the DHCP service fails to properly validate the structure and expected format of incoming network packets before attempting to process them. When an attacker crafts a malicious packet that violates these implicit type assumptions, the server's internal state machine enters an undefined or unstable condition. This misalignment between the expected data structure and the actual received payload disrupts the normal execution flow of the DHCP service logic.
The operational impact of this vulnerability is primarily centered on availability rather than confidentiality or integrity. Because the flaw triggers a failure in the core processing routine, it results in a denial of service scenario where the Windows DHCP server becomes unresponsive or crashes entirely. An unauthorized attacker positioned on the same network segment can exploit this condition by sending specifically crafted requests that trigger the type confusion error repeatedly or sufficiently to cause a system halt. This effectively removes the ability of other clients on the network to obtain IP addresses and related configuration parameters, thereby disrupting local network connectivity and potentially causing broader service outages for dependent applications relying on dynamic address assignment.
From a technical classification perspective, this issue aligns with CWE-843, which defines access of resource using incompatible type. The exploitation vector is remote over the network without requiring prior authentication, placing it within MITRE ATT&CK technique T1499, Endpoint Denial of Service. The attacker leverages the inherent trust placed in incoming protocol messages by the DHCP service to execute this denial-of-service attack. Unlike memory corruption vulnerabilities that might allow for code execution, this specific flaw is exploited strictly to degrade or halt system functionality through resource exhaustion or state corruption within the application layer.
Mitigation strategies should focus on both immediate remediation and long-term architectural controls. The primary defense is the timely installation of security updates provided by Microsoft that address the input validation logic in the DHCP server component. Administrators must ensure that all Windows servers running the DHCP role are patched to versions where this type confusion has been resolved. Additionally, network-level defenses such as Access Control Lists on routers and switches can be configured to restrict UDP port 67 access only from trusted subnets or specific client ranges, thereby limiting the attack surface available to potential adversaries. Implementing rate-limiting policies for DHCP requests can also help mitigate the impact of volumetric attempts to trigger this vulnerability by filtering out anomalous traffic patterns before they reach the vulnerable service logic.