CVE-2020-35513 in Linux
Summary
by MITRE • 01/26/2021
A flaw incorrect umask during file or directory modification in the Linux kernel NFS (network file system) functionality was found in the way user create and delete object using NFSv4.2 or newer if both simultaneously accessing the NFS by the other process that is not using new NFSv4.2. A user with access to the NFS could use this flaw to starve the resources causing denial of service.
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Analysis
by VulDB Data Team • 10/12/2024
The vulnerability identified as CVE-2020-35513 represents a critical flaw in the Linux kernel's Network File System implementation that specifically affects NFSv4.2 and newer versions. This issue manifests when users create or delete file system objects through NFSv4.2 protocols while other processes simultaneously access the same NFS share using older NFSv4.1 or earlier versions. The root cause lies in how the kernel handles umask values during file and directory modifications, creating a scenario where improper permission handling can occur. The flaw is particularly dangerous because it allows for resource exhaustion through deliberate exploitation, making it a significant concern for system administrators managing networked file systems.
The technical implementation of this vulnerability stems from the inconsistent handling of umask values across different NFS protocol versions during concurrent file operations. When NFSv4.2 clients interact with the kernel's file system layer, the umask setting may not be properly applied or maintained during object creation and deletion processes. This misconfiguration creates opportunities for malicious users to manipulate file permissions and access controls in ways that were not intended by the system design. The vulnerability specifically affects the kernel's file system call processing where the umask value is either not correctly inherited or applied during the modification operations. This issue is classified under CWE-276 which deals with incorrect permissions and access control mechanisms, and it directly relates to improper handling of file system attributes and access controls within networked environments.
The operational impact of CVE-2020-35513 extends beyond simple permission misconfigurations to potentially enable complete denial of service conditions. An attacker with access to the affected NFS share can exploit this flaw to consume system resources through resource starvation attacks, where multiple concurrent operations exhaust available file handles, memory, or other critical system resources. This vulnerability affects the availability aspect of the CIA triad by allowing malicious users to disrupt normal file system operations and potentially prevent legitimate users from accessing shared resources. The attack vector is particularly concerning because it requires minimal privileges to exploit, making it accessible to any user with access to the NFS share, and the impact can cascade across multiple system components that rely on proper file system operations.
Mitigation strategies for CVE-2020-35513 should focus on both immediate patching and operational controls to prevent exploitation. The primary solution involves applying the latest kernel updates from the vendor that address the specific umask handling issue in NFSv4.2 implementations. System administrators should also consider implementing strict access controls and monitoring for concurrent NFS operations that might indicate exploitation attempts. Network segmentation and limiting the number of concurrent NFS connections can help reduce the attack surface. Additionally, implementing proper resource limits and quotas on NFS shares can help prevent resource exhaustion attacks. Organizations should also consider using network monitoring tools to detect unusual patterns of file creation and deletion that might indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under privilege escalation and denial of service tactics, with the exploitation potentially leading to broader system compromise through resource exhaustion attacks that can impact system stability and availability.