CVE-2026-77889 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 flaw within the Dynamic Host Configuration Protocol (DHCP) server implementation in Microsoft Windows, specifically characterized as a type confusion error. This class of defect arises when software incorrectly interprets or handles data types during execution, leading to operations being performed on objects that are not of the expected type. In the context of network services like DHCP, which is fundamental for assigning IP addresses and other configuration parameters to devices on an internet protocol (IP) network, such a flaw can have severe consequences for system stability and availability. The core technical issue lies in how the server processes incoming requests or packets that trigger this incompatible type access. When the parser encounters specific malformed or specially crafted input data, it fails to validate the expected structure correctly, allowing memory corruption or logical errors to occur during processing.
From a security architecture perspective, this vulnerability aligns with CWE-843, which defines Access of Resource Using Incompatible Type, and is closely related to CWE-120, Buffer Overflow, as type confusion often leads to out-of-bounds writes or reads if the memory layout assumptions are violated. The operational impact of exploiting this flaw is primarily a denial of service (DoS). An unauthorized attacker positioned on the same network segment can send maliciously crafted DHCP packets that trigger the type confusion error within the Windows DHCP Server process. This exploitation causes the server to crash, become unresponsive, or enter an unstable state where it cannot serve legitimate clients. Since DHCP is often a single point of failure for network connectivity in many enterprise environments, disrupting this service effectively isolates devices from obtaining necessary network configurations, thereby halting business operations and potentially causing cascading failures across dependent systems that rely on dynamic address assignment.
The attack vector for this vulnerability is remote but requires the attacker to be able to communicate with the DHCP server over a network protocol. This typically places it within the scope of ATT&CK technique T1498, Network Denial of Service, specifically under sub-techniques involving resource exhaustion or service disruption through application layer attacks. Because the flaw resides in the core networking stack component responsible for handling client requests, no user interaction is required to trigger the exploit once network access is established. This makes it particularly dangerous in internal networks where lateral movement might be facilitated by disrupting critical infrastructure services before attempting further exploitation against other targets.
Mitigation strategies must focus on both immediate remediation and long-term architectural hardening. The primary defense is applying the latest security updates provided by Microsoft, which patch the underlying code logic to properly validate data types and enforce strict boundaries during packet parsing. Organizations should ensure that their Windows DHCP servers are fully patched against known vulnerabilities in this component. Additionally, network segmentation plays a crucial role in limiting exposure; restricting access to DHCP services only from trusted subnets or implementing firewall rules that filter out malformed packets can reduce the attack surface. Monitoring for unusual spikes in DHCP traffic or repeated request patterns may also help detect attempted exploitation activities before they result in service disruption. Regular vulnerability assessments and penetration testing should include checks for type confusion flaws in network-facing applications to identify similar weaknesses proactively.