CVE-2026-77895 in Windows
Summary
by MITRE • 09/09/2026
Out-of-bounds read 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 identified as an out-of-bounds read within the Microsoft Dynamic Host Configuration Protocol (DHCP) server represents a critical memory safety flaw that compromises the availability of network infrastructure services. This specific defect occurs when the DHCP server processes specially crafted packets, leading to an attempt to access memory locations beyond the intended buffer boundaries. Because this is classified as an out-of-bounds read rather than a write, it does not typically allow for arbitrary code execution or privilege escalation in its direct form. Instead, the primary consequence of accessing invalid memory addresses is often the corruption of internal state variables or the triggering of hardware exceptions that force the application to terminate abruptly. This behavior directly facilitates a denial of service scenario where an attacker can disrupt network connectivity by causing the DHCP server process to crash repeatedly upon receiving maliciously formatted requests.
From a technical perspective, this flaw aligns with Common Weakness Enumeration (CWE) category CWE-125, which describes out-of-bounds read vulnerabilities. These weaknesses typically arise from insufficient validation of input data lengths or indices before accessing array elements or memory buffers. In the context of network-facing services like DHCP, such errors are particularly dangerous because they can be triggered remotely without requiring authentication. The attacker exploits this by sending packets that contain malformed option fields or header structures that exceed expected limits. When the server attempts to parse these inputs, it fails to properly check bounds before reading from memory, resulting in an access violation. This type of error is often a result of complex parsing logic where edge cases are not adequately handled during the decoding of DHCP message components.
The operational impact of this vulnerability centers on service availability rather than confidentiality or integrity. Since the DHCP server is responsible for dynamically assigning IP addresses and other network configuration parameters to clients, its unavailability can halt new device connections and disrupt existing leases depending on lease renewal timing. An unauthorized attacker positioned within the same broadcast domain or with routed access to the DHCP server port (UDP 67/68) can exploit this flaw by sending a single crafted packet or through a sustained flood of malformed requests. This leads to intermittent or complete service disruption, effectively denying legitimate users their ability to obtain network connectivity. The impact is severe for organizations relying on automated IP address management, as manual intervention may be required to restore service stability after the server crashes.
Mitigation strategies primarily involve applying vendor-provided security updates that patch the underlying memory handling logic within the DHCP server component. Administrators should ensure that all systems running Windows Server with the DHCP role installed are updated to the latest cumulative patches released by Microsoft. In environments where immediate patching is not feasible, network segmentation can provide a layer of defense by restricting access to UDP ports 67 and 68 from untrusted networks. Additionally, implementing ingress filtering on routers or firewalls to drop malformed packets before they reach the DHCP server can reduce exposure. Security monitoring tools should be configured to detect anomalies in DHCP traffic patterns, such as an unusual volume of requests with invalid option codes, which may indicate exploitation attempts against this vulnerability.
This incident is categorized under MITRE ATT&CK technique T1499, specifically Endpoint Denial of Service via resource exhaustion or service disruption. The attack vector relies on the network layer to deliver malicious payloads that trigger application-level crashes. Understanding the mechanics of out-of-bounds reads helps security teams recognize similar patterns in other network services and prioritize hardening efforts around input validation mechanisms. By addressing these fundamental coding errors, organizations can significantly reduce their risk profile against remote denial of service attacks targeting critical infrastructure components like DHCP servers.