Source: cve@mitre.org
NFS on SunOS 4.1 through 4.1.2 ignores the high order 16 bits in a 32 bit UID, which allows a local user to gain root access if the lower 16 bits are set to 0, as fixed by the NFS jumbo patch upgrade.
SunOS NFS, versions 4.1 through 4.1.2, is vulnerable to a local privilege escalation. This vulnerability allows a local user to gain root access by manipulating their user ID, bypassing security checks due to a flawed implementation in how the NFS server handles user ID mapping.
Step 1: User Account Creation: An attacker creates a local user account on the vulnerable SunOS system.
Step 2: UID Manipulation: The attacker modifies the user's UID to a value where the lower 16 bits are 0 (e.g., 65536, 131072, etc.). This can be done by editing the /etc/passwd file or using tools that allow UID modification.
Step 3: NFS Access: The attacker attempts to access an NFS share mounted on the system.
Step 4: UID Truncation: The NFS server, upon receiving the attacker's request, truncates the 32-bit UID, effectively treating the attacker's UID as 0 (root).
Step 5: Privilege Escalation: Because the server now believes the attacker is root, it grants the attacker root-level access to the NFS share, allowing them to read, write, and execute files with root privileges.
The vulnerability stems from an integer overflow/truncation issue within the NFS server's UID handling. The server, when processing user ID information, only considers the lower 16 bits of a 32-bit UID. A local user can craft a UID where the lower 16 bits are 0 (e.g., UID 0, which is root), and the upper 16 bits are non-zero. The NFS server, due to its flawed implementation, truncates the UID, effectively treating the crafted UID as root (0). This allows the attacker to bypass access controls and perform actions with root privileges. The root cause is a lack of proper validation and sanitization of the UID value before it is used for access control decisions. This is a classic example of an integer overflow leading to a privilege escalation.
While no specific APTs are known to exclusively target this vulnerability, it's a prime candidate for inclusion in a broader attack chain. The age of the vulnerability makes it a low-hanging fruit for attackers. Not listed on CISA KEV due to its age and the rarity of the affected systems.
Monitor system logs (e.g., /var/log/syslog, /var/adm/messages) for suspicious activity related to NFS access, especially from local users.
Analyze network traffic for NFS requests. Look for unusual UID values or attempts to access restricted files.
Use file integrity monitoring tools (e.g., Tripwire) to detect unauthorized modifications to system files.
Check the /etc/passwd file for any unexpected UID changes, particularly those targeting UID 0.
Implement host-based intrusion detection systems (HIDS) to identify anomalous system calls and process behavior.
Upgrade to a patched version of SunOS (e.g., 4.1.3 or later) that includes the NFS jumbo patch or equivalent fix.
If upgrading is not possible, apply the NFS jumbo patch or a vendor-provided security patch.
Restrict NFS access to trusted hosts and networks. Implement strong network segmentation.
Regularly audit user accounts and permissions. Enforce least privilege principles.
Monitor system logs and network traffic for suspicious activity.
Implement a host-based intrusion detection system (HIDS) to detect anomalous system calls and process behavior.