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 allows for local privilege escalation to root. Exploiting a buffer overflow in the passwd utility, attackers can gain complete control of the system, potentially leading to system compromise and data breaches.
Step 1: User Account Modification: The attacker initiates the process by attempting to modify their user account using the passwd utility.
Step 2: Payload Injection: The attacker provides a specially crafted, excessively long string for either the shell or GECOS field during the account modification process. This string contains the malicious payload (e.g., shellcode) and is designed to overflow the allocated buffer.
Step 3: Buffer Overflow: The passwd utility, lacking proper input validation, copies the oversized string into a fixed-size buffer. This overwrites adjacent memory locations.
Step 4: Control Hijack: The overflow corrupts critical data, such as the return address on the stack. The attacker's crafted input overwrites the return address with the address of their injected shellcode.
Step 5: Shellcode Execution: When the passwd utility attempts to return, it jumps to the attacker's shellcode, which executes with root privileges.
Step 6: Privilege Escalation: The shellcode grants the attacker root access, allowing them to execute arbitrary commands and control the system.
The vulnerability stems from a buffer overflow in the passwd utility, specifically within the handling of the shell or GECOS fields during user account modification. The passwd program, when processing user-supplied input for these fields, fails to properly validate the length of the input. This lack of bounds checking allows an attacker to provide an excessively long string, overflowing the allocated buffer. This overwrite corrupts adjacent memory, including potentially critical data structures like the stack or heap, allowing an attacker to overwrite the return address of the function. By controlling the return address, the attacker can redirect program execution to malicious code (e.g., a shellcode) injected into the overflowed buffer, granting root privileges. The root cause is the absence of input validation and length checks on user-supplied data, leading to a classic buffer overflow condition.