Source: cve@mitre.org
Buffer overflow in ppp program in FreeBSD 2.1 and earlier allows local users to gain privileges via a long HOME environment variable.
FreeBSD 2.1 and earlier systems are vulnerable to a local privilege escalation attack due to a buffer overflow in the ppp program. An attacker can leverage a crafted HOME environment variable to overwrite memory and execute arbitrary code, gaining root access on the compromised system.
Step 1: Environment Variable Setup: The attacker sets a long, malicious HOME environment variable. This variable contains a payload designed to overwrite the stack and execute arbitrary code. The payload typically includes shellcode to gain a privileged shell.
Step 2: Program Execution: The attacker executes the ppp program, either directly or indirectly through another program that calls ppp.
Step 3: Buffer Overflow Trigger: The ppp program attempts to use the HOME environment variable. Due to the lack of bounds checking, the long HOME variable overflows the allocated buffer.
Step 4: Memory Overwrite: The overflow overwrites critical data on the stack, including the return address.
Step 5: Code Execution: When ppp attempts to return from a function, it jumps to the attacker's shellcode, which then executes with the privileges of the ppp process (often root).
The vulnerability lies within the ppp program, specifically in how it handles the HOME environment variable. The program fails to properly validate the size of the HOME variable before copying its contents into a fixed-size buffer. This leads to a buffer overflow when a long HOME variable is provided. By crafting a malicious HOME variable containing shellcode, an attacker can overwrite adjacent memory regions, including the return address on the stack. When ppp attempts to return, control is transferred to the attacker's shellcode, allowing them to execute arbitrary commands with the privileges of the ppp process, typically root.
Due to the age of the vulnerability, it is unlikely to be actively targeted by sophisticated APTs. However, it could be used in opportunistic attacks or as part of a larger chain. This vulnerability is not listed on the CISA KEV.
Monitor system logs for unusual activity related to the ppp program, such as unexpected crashes or errors.
Analyze process execution logs for instances where ppp is run with unusually long environment variables, especially the HOME variable.
Examine core dumps or memory snapshots for signs of stack corruption or overwritten return addresses.
Network traffic analysis may reveal attempts to exploit the vulnerability if the program is accessible remotely (unlikely in this case, as it's a local exploit).
Upgrade to a patched version of FreeBSD. This is the primary and most effective remediation.
If upgrading is not immediately possible, restrict access to the ppp program. Limit who can execute it.
Implement environment variable filtering to prevent excessively long or malicious environment variables from being passed to the ppp program. This is a partial mitigation.
Apply the principle of least privilege. Ensure users do not have unnecessary privileges on the system.
Regularly audit system configurations and security settings.