CVE-2021-26432 in Windows
Summary
by MITRE • 08/12/2021
Windows Services for NFS ONCRPC XDR Driver Remote Code Execution Vulnerability
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Analysis
by VulDB Data Team • 08/15/2021
The Windows Services for NFS ONCRPC XDR Driver Remote Code Execution Vulnerability represents a critical security flaw in Microsoft Windows operating systems that affects the Network File System implementation. This vulnerability specifically targets the ONCRPC XDR (External Data Representation) driver component within the NFS services, creating a pathway for remote attackers to execute arbitrary code on affected systems. The flaw exists within the kernel-mode driver responsible for handling network file system operations, making it particularly dangerous as it operates at the core level of system functionality. Attackers can exploit this vulnerability by sending specially crafted network requests that trigger improper input validation within the XDR processing routines, potentially leading to complete system compromise without requiring local access or authentication credentials.
The technical implementation of this vulnerability stems from inadequate validation of input parameters within the ONCRPC XDR driver's processing functions. When the driver receives malformed data structures during network file system operations, it fails to properly validate the incoming parameters before processing them, leading to memory corruption conditions. This memory corruption can be leveraged to overwrite critical memory locations, ultimately allowing attackers to execute malicious code with system-level privileges. The vulnerability falls under the Common Weakness Enumeration category of improper input validation and can be classified as a buffer overflow condition when the driver attempts to process oversized or malformed data structures. According to the ATT&CK framework, this vulnerability maps to the T1210 technique of Exploitation of Remote Services, where attackers target network services to gain initial access and establish persistence within target environments.
The operational impact of CVE-2021-26432 extends beyond simple remote code execution, as it provides attackers with complete control over affected systems. Once exploited, adversaries can perform privilege escalation, establish persistent backdoors, and access sensitive data stored on network file shares. The vulnerability affects multiple Windows versions including Windows 10, Windows Server 2016, and Windows Server 2019, making it particularly concerning for enterprise environments that rely heavily on NFS services. Organizations using Windows systems with NFS functionality enabled are at significant risk, as the attack surface includes any system that accepts network file system requests. The vulnerability's remote exploitability means that attackers can target systems from outside the network perimeter, potentially leading to widespread compromise across multiple network segments. Network traffic analysis reveals that exploitation typically occurs through standard NFS protocols on port 2049, making detection challenging as the malicious traffic can appear legitimate.
Mitigation strategies for this vulnerability require immediate patch deployment through Microsoft's regular security updates, as the company released patches for this specific flaw in their monthly security bulletin cycle. Organizations should prioritize patching all affected Windows systems, particularly those running NFS services or having NFS client functionality enabled. Network segmentation and firewall rules should be implemented to restrict access to NFS ports and services, limiting potential attack vectors. The principle of least privilege should be enforced by disabling NFS services on systems that do not require them, reducing the overall attack surface. Security monitoring should be enhanced to detect unusual NFS traffic patterns and potential exploitation attempts, with particular attention to malformed XDR requests. Additionally, implementing network intrusion detection systems and configuring proper logging for NFS services can help identify exploitation attempts. According to Microsoft security guidance, organizations should also consider implementing application whitelisting policies to prevent unauthorized code execution and maintain regular vulnerability assessments to identify similar weaknesses in other system components. The vulnerability's classification as a remote code execution flaw underscores the importance of maintaining up-to-date security patches and implementing comprehensive network security controls to prevent unauthorized access to critical system resources.