CVE-1999-0627

Step 1: Reconnaissance: The attacker identifies systems running the rexd service, typically by port scanning (port 512, 513, 514). Step 2: Trust Relationship Assessment: The attacker determines if the target system trusts the attacker's system or if the attacker can spoof a trusted IP address. This may involve examining .rhosts or /etc/hosts.equiv files. Step 3: Authentication Bypass: The attacker exploits the weak authentication by either spoofing their IP address to match a trusted host or by leveraging compromised credentials if available. Step 4: Command Injection: The attacker crafts a command to be executed on the target system. This command is sent to the rexd service. Step 5: Command Execution: The rexd service, trusting the source, executes the attacker's command with the privileges of the user running rexd (often root). Step 6: Post-Exploitation: The attacker can then perform actions such as installing backdoors, exfiltrating data, or further compromising the system.

The root cause of CVE-1999-0627 lies in the fundamental design of the rexd service. It relies on a trust-based authentication mechanism, typically using the .rhosts or /etc/hosts.equiv files. This mechanism inherently trusts hosts and users listed in these files, without requiring strong authentication. The service accepts commands from trusted hosts and executes them with the privileges of the user running rexd. The lack of proper authentication and authorization allows an attacker to spoof their source IP address or leverage compromised credentials to gain unauthorized access and execute arbitrary commands. The vulnerability is not a specific code flaw like a buffer overflow or race condition, but rather a design flaw in the authentication model.