CVE-2026-83655 in Windows
Summary
by MITRE • 10/14/2026
Out-of-bounds read in Network Policy Server (NPS) allows an unauthorized attacker to deny service over a network.
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Analysis
by VulDB Data Team • 10/14/2026
The vulnerability identified as an out-of-bounds read within the Microsoft Network Policy Server, commonly known by its acronym NPS, represents a critical security flaw that compromises the availability of authentication and authorization services in Windows-based environments. This specific defect occurs when the server processes certain malformed or specially crafted network packets during the handling of Extensible Authentication Protocol (EAP) messages. The root cause lies in improper boundary checks within the memory management routines responsible for parsing these protocol structures, allowing an attacker to read data beyond the allocated buffer limits without triggering immediate application crashes that might alert security monitoring systems.
From a technical perspective, this flaw is classified under Common Weakness Enumeration as CWE-125, which denotes Out-of-bounds Read vulnerabilities. The exploitation mechanism typically involves sending specific RADIUS packets containing malformed EAP payloads to the NPS service listening on UDP port 1812 or 1813. When the server attempts to process these inputs, it accesses memory locations that are not intended for reading by the application logic. While out-of-bounds reads often lead to information disclosure in other contexts, in this specific implementation within Network Policy Server, the primary consequence manifests as a denial of service condition. The improper handling causes the NPS service or potentially the entire host system to become unresponsive, effectively halting all authentication requests processed by that server instance.
The operational impact of this vulnerability is severe for organizations relying on centralized identity management and network access control. Network Policy Server serves as the backend engine for Remote Authentication Dial-In User Service (RADIUS) in many enterprise deployments, managing access for wireless networks, virtual private connections, and wired port-based authentication via 802.1X. An attacker capable of exploiting this flaw can disrupt these critical infrastructure components by causing a service outage. This denial of service prevents legitimate users from authenticating to the network, leading to significant productivity losses and potential security gaps if fallback mechanisms are not in place. Furthermore, because NPS is often integrated with Active Directory, an outage here can cascade into broader authentication failures across the domain environment.
In terms of threat modeling, this vulnerability aligns with MITRE ATT&CK technique T1499, Endpoint Denial of Service, specifically under the sub-technique for resource exhaustion or service disruption through application layer attacks. It also relates to CWE-20, Improper Input Validation, as the server fails to adequately sanitize and validate the length fields within the incoming RADIUS/EAP packets before processing them. The attack vector is remote, requiring no prior authentication if the NPS service is exposed to untrusted networks or if an attacker has access to a compromised internal segment that can reach the NPS servers over the network protocol stack.
Mitigation strategies must focus on both immediate remediation and long-term architectural hardening. Microsoft has released security updates addressing this specific flaw, which should be applied immediately to all systems running vulnerable versions of Windows Server with the Network Policy Server role installed. Administrators are advised to verify that the latest cumulative security patches for their respective operating system version are deployed. In addition to patching, network segmentation plays a crucial role in limiting exposure. NPS servers should not be directly accessible from untrusted networks; instead, they should reside behind firewalls or load balancers that restrict access strictly to authorized authentication gateways such as wireless controllers or VPN concentrators. Implementing strict rate-limiting and deep packet inspection on these intermediary devices can also help detect and block malformed packets before they reach the vulnerable NPS service, providing an additional layer of defense against exploitation attempts.