Buffer overflow in FreeBSD libmytinfo library allows local users to execute commands via a long TERMCAP environmental variable.
FreeBSD systems are vulnerable to a local privilege escalation attack due to a buffer overflow in the libmytinfo library. By setting a maliciously crafted TERMCAP environment variable, an attacker can overwrite memory and potentially execute arbitrary code, gaining unauthorized access to the system and compromising its integrity.
Step 1: Environment Variable Setup: The attacker sets the TERMCAP environment variable to a string that is significantly longer than the allocated buffer within libmytinfo.
Step 2: Program Execution: A program that uses libmytinfo (e.g., vi, emacs, or any program that uses terminal capabilities) is executed by the attacker or a user they control.
Step 3: Library Initialization: The vulnerable libmytinfo library is loaded and initialized by the program.
Step 4: TERMCAP Processing: The library attempts to read and process the TERMCAP environment variable.
Step 5: Buffer Overflow: The long TERMCAP string is copied into a fixed-size buffer without proper bounds checking, causing a buffer overflow.
Step 6: Memory Corruption: The overflow overwrites adjacent memory, potentially including the return address on the stack or other critical data.
Step 7: Code Redirection: The attacker's crafted TERMCAP string includes data that overwrites the return address, redirecting program execution to the attacker's shellcode.
Step 8: Payload Execution: The attacker's shellcode is executed with the privileges of the user running the program, granting the attacker a shell or other desired access.
The vulnerability lies within the libmytinfo library, specifically in how it handles the TERMCAP environment variable. The library fails to properly validate the size of the TERMCAP string before copying it into a fixed-size buffer. This leads to a buffer overflow when a long TERMCAP string is provided, overwriting adjacent memory regions. The attacker can then overwrite critical data structures, such as the return address on the stack, to redirect program execution to a malicious payload, typically shellcode, allowing them to execute arbitrary commands with the privileges of the user running the vulnerable program. The root cause is a lack of bounds checking during the string copy operation, a classic example of a memory management error.