CVE-1999-1140

Step 1: User Account Modification: An attacker gains access to a local user account (or creates one).

Step 2: GECOS Field Manipulation: The attacker modifies the GECOS field associated with their user account. This is typically done using tools like chfn or directly editing the /etc/passwd file (though this is less common now).

Step 3: Triggering CrackLib: The attacker triggers CrackLib's password checking functionality. This could be done by attempting to change their password or by using a tool that interacts with CrackLib.

Step 4: Buffer Overflow: CrackLib attempts to process the GECOS field, which is now excessively long. The long input overflows the allocated buffer.

Step 5: Control Hijack: The overflow overwrites the return address on the stack. The attacker's crafted input includes a new return address, pointing to shellcode or a system call.

Step 6: Privilege Escalation: When CrackLib returns, it jumps to the attacker-controlled address, executing the attacker's code with the privileges of the process (potentially root).

The vulnerability lies within CrackLib's processing of the GECOS field, typically found in user account information. Specifically, the code responsible for handling the GECOS field does not adequately validate the length of the input. When a user's GECOS field is excessively long, it overflows a fixed-size buffer allocated on the stack. This buffer overflow overwrites adjacent memory, potentially including critical data like the return address. By carefully crafting the overflowing data, an attacker can overwrite the return address with the address of malicious code (shellcode) or a system call, leading to arbitrary code execution with elevated privileges. The root cause is a lack of bounds checking on the input data, allowing for the overflow.