Source: cve@mitre.org
Certain configurations of wu-ftp FTP server 2.4 use a _PATH_EXECPATH setting to a directory with dangerous commands, such as /bin, which allows remote authenticated users to gain root access via the "site exec" command.
Remote attackers can gain root access on vulnerable systems running wu-ftpd 2.4 by exploiting a misconfiguration that allows the execution of arbitrary commands. This vulnerability leverages the site exec command and a flawed setting of _PATH_EXECPATH to execute commands with elevated privileges, leading to a complete system compromise.
Step 1: Authentication: The attacker authenticates to the vulnerable FTP server using valid credentials (or potentially exploiting a separate vulnerability to bypass authentication).
Step 2: Configuration Review: The attacker probes the server to determine the value of _PATH_EXECPATH. This might involve attempting to execute commands and observing the results or reviewing publicly available information about the server configuration.
Step 3: Command Injection: The attacker crafts a site exec command, specifying a command to be executed with elevated privileges (e.g., site exec /bin/sh -c 'command').
Step 4: Command Execution: The FTP server, due to the misconfigured _PATH_EXECPATH, executes the attacker-supplied command within the context of the configured path.
Step 5: Privilege Escalation: The attacker's command, often a shell command, is executed with the privileges of the FTP server process, which, in many configurations, runs with root privileges.
Step 6: System Compromise: The attacker gains root access and can then execute arbitrary commands, potentially installing backdoors, stealing data, or taking complete control of the system.
The vulnerability stems from a design flaw in wu-ftpd 2.4 where the site exec command, intended for executing commands on the server, is vulnerable to path manipulation. Specifically, the _PATH_EXECPATH configuration variable, intended to restrict the execution path, can be set to a directory containing dangerous commands like /bin. The FTP server then executes commands specified by the attacker within this context, effectively bypassing intended security controls. The root cause is a lack of proper input validation and sanitization of the command arguments passed to the operating system's shell, allowing for command injection. This is not a buffer overflow or race condition but a command injection vulnerability.
This vulnerability is a classic example of a simple but effective attack vector. While no specific APT groups are exclusively known to target this, it's a common tactic used by various threat actors due to its ease of exploitation and high impact. It's a low-hanging fruit for attackers. CISA KEV status is unlikely due to the age of the vulnerability, but it aligns with the types of vulnerabilities that CISA would advise against.
Monitor FTP server logs for suspicious site exec commands, especially those attempting to execute shell commands (e.g., /bin/sh, /bin/bash).
Analyze network traffic for FTP sessions containing site exec commands with unusual arguments.
Implement file integrity monitoring to detect changes to critical system files (e.g., /etc/passwd, /etc/shadow).
Use intrusion detection systems (IDS) with signatures specifically designed to detect exploitation attempts against wu-ftpd's site exec command.
Review FTP server configuration files for the _PATH_EXECPATH setting and ensure it is configured securely (e.g., not pointing to a directory containing dangerous commands).
Upgrade to a patched version of wu-ftpd or, preferably, a more modern and secure FTP server implementation (e.g., vsftpd, ProFTPD).
If upgrading is not immediately possible, disable the site exec command entirely if it is not required for legitimate use.
Carefully review and restrict the _PATH_EXECPATH setting. Ensure it points to a safe directory and that the server's user has minimal privileges within that directory.
Implement strong authentication mechanisms, including multi-factor authentication (MFA), to prevent unauthorized access.
Regularly audit and monitor FTP server configurations for any unauthorized changes.
Apply the principle of least privilege: the FTP server process should run with the minimum necessary privileges.