CVE-2009-3623 in Linux
Summary
by MITRE
The lookup_cb_cred function in fs/nfsd/nfs4callback.c in the nfsd4 subsystem in the Linux kernel before 2.6.31.2 attempts to access a credentials cache even when a client specifies the AUTH_NULL authentication flavor, which allows remote attackers to cause a denial of service (NULL pointer dereference and system crash) via an NFSv4 mount request.
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Analysis
by VulDB Data Team • 03/27/2025
The vulnerability described in CVE-2009-3623 represents a critical flaw in the Linux kernel's Network File System version 4 implementation that specifically affects the nfsd4 subsystem. This issue manifests within the lookup_cb_cred function located in fs/nfsd/nfs4callback.c, where the kernel fails to properly validate authentication credentials before attempting to access cached credential information. The flaw occurs during NFSv4 mount requests when a client specifies the AUTH_NULL authentication flavor, which is a legitimate but unauthenticated authentication method that should not require credential caching. This particular authentication method is designed to allow clients to connect without providing credentials, typically used for testing or in specific network configurations where authentication is handled at a different layer. However, the kernel implementation does not account for this scenario properly, leading to a fundamental design oversight in the credential handling process.
The technical exploitation of this vulnerability results in a NULL pointer dereference condition that ultimately causes a system crash and denial of service. When the nfsd4 subsystem receives an NFSv4 mount request with AUTH_NULL authentication, the lookup_cb_cred function attempts to access a credentials cache entry that has not been properly initialized or validated for this specific authentication type. This improper handling creates a scenario where the kernel's memory management system encounters a null reference during the credential lookup process, leading to an immediate system panic and complete service disruption. The vulnerability is particularly concerning because it can be triggered remotely without requiring any special privileges or authentication, making it an attractive target for denial of service attacks against NFS servers. The flaw essentially bypasses normal authentication validation procedures and directly impacts the kernel's credential management subsystem, which is fundamental to maintaining system security and access control.
The operational impact of CVE-2009-3623 extends beyond simple service disruption to potentially compromise the stability and availability of networked systems that rely on NFSv4 services. Organizations running Linux servers that provide NFSv4 file services are particularly vulnerable to this attack, as it can be exploited by any remote attacker who can establish an NFSv4 connection to the target system. The vulnerability affects systems running kernel versions prior to 2.6.31.2, which were widely deployed in enterprise environments, making the potential attack surface substantial. This flaw aligns with CWE-476, which describes NULL pointer dereference conditions in software implementations, and represents a classic example of inadequate input validation and error handling in kernel space code. From an operational security perspective, this vulnerability undermines the reliability of NFS services and can be leveraged to perform coordinated denial of service attacks against critical infrastructure components. The attack vector is particularly dangerous because it requires minimal prerequisites and can be executed against any system running vulnerable kernel versions, potentially affecting data availability for multiple users and applications simultaneously.
Organizations affected by CVE-2009-3623 should implement immediate mitigation strategies including kernel updates to versions 2.6.31.2 or later, which contain the necessary patches to address the credential validation issue. System administrators should also consider implementing network-level restrictions to limit NFSv4 access to trusted networks and monitor for unusual NFS mount activity that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper authentication validation in kernel space code and highlights the need for comprehensive testing of authentication mechanisms, particularly those that handle edge cases like unauthenticated connections. From a defensive standpoint, this vulnerability emphasizes the critical role of kernel security patches in maintaining system integrity and the necessity of implementing robust monitoring for system crashes or unexpected behavior. Additionally, organizations should consider implementing intrusion detection systems that can identify patterns consistent with NFSv4 attack vectors and ensure that all network services are regularly updated to protect against known vulnerabilities. The ATT&CK framework categorizes this type of vulnerability under privilege escalation and denial of service tactics, where the attack leverages a kernel-level flaw to disrupt system availability without requiring elevated privileges, making it a particularly effective vector for system compromise.