Source: cve@mitre.org
Buffer overflow in passwd in BSD based operating systems 4.3 and earlier allows local users to gain root privileges by specifying a long shell or GECOS field.
Critical vulnerability discovered in legacy BSD-based operating systems, allowing local users to achieve root privilege escalation through a buffer overflow in the passwd utility. This vulnerability, dating back to the late 1980s, poses a significant risk to any systems still running these outdated operating systems, potentially enabling complete system compromise.
Step 1: User Interaction: A local user attempts to change their password using the passwd utility.
Step 2: Input Provision: The user is prompted for various information, including the shell and GECOS fields.
Step 3: Malicious Payload: The attacker provides a long string (the malicious payload) for either the shell or GECOS field, exceeding the allocated buffer size.
Step 4: Buffer Overflow: The passwd utility, lacking proper bounds checking, copies the oversized input into the buffer, overwriting adjacent memory locations.
Step 5: Control Hijack: The overflow overwrites the return address on the stack. When passwd returns, it jumps to the attacker-controlled address.
Step 6: Privilege Escalation: The attacker's shellcode (or a redirection to a known location) executes with root privileges, granting the attacker full control over the system.
The vulnerability stems from a buffer overflow within the passwd utility, specifically when handling the user's shell or GECOS field during password updates. The passwd program, when processing user input for these fields, fails to properly validate the length of the provided data. This lack of bounds checking allows an attacker to supply an excessively long string, overflowing a fixed-size buffer allocated for these fields. This overflow overwrites adjacent memory, potentially overwriting critical data like the return address or function pointers. By carefully crafting the overflow, an attacker can overwrite the return address with the address of malicious code (shellcode) or redirect execution to a controlled location, ultimately gaining root privileges. The root cause is the absence of input validation and the use of unsafe string manipulation functions (e.g., strcpy without bounds checking) in the passwd program.
Due to the age of the vulnerability, it is unlikely to be directly targeted by modern APTs. However, it could be a component of a larger attack chain targeting legacy systems. This vulnerability is not listed on the CISA KEV catalog due to its age and the rarity of vulnerable systems in modern networks.
Monitor system logs for unusually long shell or GECOS field entries in the passwd utility logs (e.g., /var/log/auth.log or similar).
Analyze core dumps or memory snapshots of the passwd process for evidence of buffer overflows.
Network-based intrusion detection systems (IDS) can be configured to detect attempts to exploit this vulnerability by monitoring for excessively long strings in network traffic related to user authentication or password changes, although this is less effective.
File integrity monitoring (FIM) to detect any changes to the passwd binary.
The primary remediation is to upgrade to a modern, supported operating system. This is the most effective solution.
If upgrading is not possible, apply any available security patches for the specific BSD variant. However, patches for such old systems are unlikely to be available.
Implement strong access controls to limit local user access to the system.
Regularly audit the system for any unauthorized accounts or modifications.
Consider using a network-based intrusion detection system (IDS) to monitor for suspicious activity related to password changes.