CVE-2013-4851 in FreeBSD
Summary
by MITRE
The vfs_hang_addrlist function in sys/kern/vfs_export.c in the NFS server implementation in the kernel in FreeBSD 8.3 and 9.x through 9.1-RELEASE-p5 controls authorization for host/subnet export entries on the basis of group information sent by the client, which allows remote attackers to bypass file permissions on NFS filesystems via crafted requests.
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Analysis
by VulDB Data Team • 05/21/2021
The vulnerability identified as CVE-2013-4851 represents a critical authorization flaw within the Network File System (NFS) server implementation of FreeBSD operating systems. This issue affects FreeBSD versions 8.3 and all 9.x releases through 9.1-RELEASE-p5, where the kernel's handling of NFS export permissions contains a fundamental security weakness that can be exploited by remote attackers to gain unauthorized access to file systems. The flaw specifically resides in the vfs_hang_addrlist function located within the sys/kern/vfs_export.c source file, which governs how the kernel processes host and subnet export entries for NFS shares.
The technical nature of this vulnerability stems from the improper validation of group information transmitted by NFS clients during authentication processes. When an NFS server receives a request from a client, it evaluates the client's group membership to determine appropriate access permissions for file operations. However, the implementation in affected FreeBSD versions fails to properly verify or validate the group information sent by the client, allowing malicious actors to manipulate this data in their crafted requests. This weakness creates a scenario where an attacker can construct specially formatted NFS requests that appear to originate from authorized hosts or networks while simultaneously presenting forged group information that grants elevated privileges.
The operational impact of this vulnerability extends beyond simple unauthorized file access, as it fundamentally undermines the security model of NFS exports by enabling privilege escalation through network-based attacks. Remote attackers who can reach the NFS server can exploit this flaw to bypass normal file permission controls and potentially access files that should be restricted to specific user groups or hosts. This vulnerability is particularly dangerous in environments where NFS servers handle sensitive data and where multiple users or systems rely on proper access controls to maintain data confidentiality and integrity. The attack can be executed without requiring local system access or authentication credentials, making it especially concerning for networked environments where NFS services are exposed to untrusted networks.
The security implications of CVE-2013-4851 align with CWE-284, which addresses improper access control in software implementations, and can be mapped to ATT&CK technique T1078 for valid accounts and T1046 for network service scanning. Organizations running affected FreeBSD systems face significant risks including data exfiltration, unauthorized modifications to critical files, and potential lateral movement within networked environments where NFS services are utilized. The vulnerability demonstrates how flaws in kernel-level network protocol implementations can create systemic security weaknesses that affect entire file systems and data repositories. Mitigation strategies should include immediate patching of affected FreeBSD systems to the latest stable releases, implementing network segmentation to limit exposure of NFS services, and monitoring for unusual NFS access patterns that might indicate exploitation attempts. Additionally, administrators should consider disabling unnecessary NFS exports and implementing additional network-level controls such as firewalls to restrict access to NFS server ports and services.
This vulnerability serves as a reminder of the critical importance of proper input validation in kernel space implementations, particularly for network services that handle authentication and authorization data. The flaw illustrates how seemingly minor implementation details in security-critical code paths can result in significant privilege escalation capabilities for remote attackers. Organizations should conduct comprehensive vulnerability assessments of their NFS implementations and review all kernel-level network service configurations to identify similar authorization bypass opportunities. The incident underscores the necessity of thorough security testing and code review processes for operating system components that handle sensitive authorization data, particularly in environments where network services are exposed to potentially hostile network environments.