CVE-2009-1072 in Linux
Summary
by MITRE
nfsd in the Linux kernel before 2.6.28.9 does not drop the CAP_MKNOD capability before handling a user request in a thread, which allows local users to create device nodes, as demonstrated on a filesystem that has been exported with the root_squash option.
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Analysis
by VulDB Data Team • 08/31/2019
The vulnerability described in CVE-2009-1072 represents a critical capability exposure within the Linux kernel's Network File System daemon implementation. This flaw exists in nfsd versions prior to 2.6.28.9 where the kernel fails to properly drop the CAP_MKNOD capability when processing user requests in threaded contexts. The CAP_MKNOD capability grants the ability to create device nodes within the filesystem, which constitutes a significant security risk when combined with the root_squash export option commonly used in NFS configurations.
The technical nature of this vulnerability stems from improper privilege management within the kernel's NFS handling code. When a user makes a request to an NFS-exported filesystem, the nfsd process should operate with reduced privileges to prevent unauthorized system modifications. However, the flaw allows the process to retain the CAP_MKNOD capability, enabling malicious users to create device nodes that could potentially be exploited for privilege escalation or system compromise. This capability is particularly dangerous because device nodes can be used to access hardware directly, bypassing normal file system permissions and access controls.
The operational impact of this vulnerability extends beyond simple local privilege escalation, as it specifically targets NFS environments where the root_squash option is employed. Root_squash is designed to map the root user of remote clients to an unprivileged user account on the server, providing an additional layer of security. However, this vulnerability allows local users to circumvent this protection mechanism entirely, undermining the security model that NFS relies upon. The attack vector requires local access to the system but can be particularly damaging in multi-user environments where NFS services are actively used and properly configured with root_squash.
From a cybersecurity perspective, this vulnerability aligns with CWE-250, which deals with execution of code with improper privileges, and relates to the broader category of privilege escalation flaws. The ATT&CK framework would categorize this under privilege escalation techniques, specifically focusing on capabilities and access control mechanisms. The vulnerability demonstrates how improper capability management can create persistent security weaknesses, as the flaw exists in the kernel level code rather than application-specific implementations. Organizations should prioritize immediate patching of affected systems and review their NFS configurations to ensure that root_squash is properly implemented. Additionally, system administrators should monitor for unauthorized device node creation and implement proper access controls to prevent exploitation of this vulnerability. The incident highlights the importance of maintaining up-to-date kernel versions and conducting regular security assessments of core system components to prevent similar capability-based privilege escalation issues.