Source: cve@mitre.org
Vulnerability in rexec daemon (rexecd) in AT&T TCP/IP 4.0 for various SVR4 systems allows remote attackers to execute arbitrary commands.
Critical vulnerability in the rexecd daemon, a legacy remote execution service, allows unauthenticated remote command execution. Exploitation grants attackers complete control over vulnerable systems, potentially leading to data breaches and system compromise.
Step 1: Connection Establishment: The attacker initiates a connection to the rexecd daemon on the target system, typically on TCP port 512.
Step 2: Authentication Bypass: The attacker crafts a malicious request designed to bypass the authentication process. This often involves providing incorrect credentials or exploiting a flaw in the authentication logic.
Step 3: Command Injection: The attacker sends a specially crafted command string, designed to be longer than the allocated buffer within rexecd.
Step 4: Buffer Overflow: The oversized command string overflows the buffer, overwriting adjacent memory regions, including the return address on the stack.
Step 5: Code Execution: The attacker's crafted command string includes malicious code (e.g., shellcode) that is injected into the program's memory space. When rexecd attempts to return from the vulnerable function, the overwritten return address points to the attacker's shellcode.
Step 6: Shell Access: The attacker's shellcode executes, granting the attacker a shell with the privileges of the rexecd daemon (typically root or a highly privileged user).
The vulnerability stems from a flaw in how rexecd handles user authentication and command execution. Specifically, the daemon fails to properly validate the length or content of the command string provided by the client. This leads to a buffer overflow condition where a crafted command, exceeding the allocated buffer size, overwrites adjacent memory regions. This memory corruption allows an attacker to overwrite critical data structures, such as the return address, and redirect program execution to arbitrary code, effectively achieving remote code execution. The root cause is a lack of input validation and boundary checks on the command string received from the client. The vulnerability is exacerbated by the daemon's privileged execution context.
Due to the age of the vulnerability, specific APT groups are unlikely to be actively targeting this. However, any attacker seeking to gain initial access to a legacy system could leverage this. No CISA KEV status.
Network traffic analysis: Look for unusual connections to TCP port 512 (rexec).
Log analysis: Examine system logs for suspicious activity related to rexecd, such as failed login attempts or unexpected command executions.
File integrity monitoring: Monitor critical system files for unauthorized modifications.
Intrusion Detection Systems (IDS): Deploy IDS rules to detect known exploit patterns or anomalous network traffic associated with rexecd exploitation.
Forensic analysis: Examine system memory and disk images for evidence of shellcode execution or modified system binaries.
Disable rexecd: The most effective remediation is to disable the rexecd service if it is not required. This eliminates the attack surface entirely.
Patching: If disabling is not possible, apply security patches from the vendor. However, given the age of the vulnerability, patches may not be available.
Network Segmentation: Isolate systems running rexecd from the rest of the network to limit the impact of a successful exploit.
Implement Strong Authentication: Enforce strong authentication mechanisms, such as multi-factor authentication (MFA), to prevent unauthorized access.
Regular Security Audits: Conduct regular security audits and penetration testing to identify and address vulnerabilities.
Update System Software: Ensure that the operating system and all installed software are up-to-date with the latest security patches.