NFTP client Buffer Overflow (CVE-2019-25361)

Overview

A critical security flaw has been identified in the Ayukov NFTP client version 1.71. This vulnerability allows a remote attacker to take complete control of a user’s system by exploiting a weakness in how the client processes a specific FTP command.

Vulnerability Explanation

In simple terms, the NFTP client has a programming error in its handling of the SYST command, which is a standard FTP command used to query a server’s operating system. The client does not properly check the size of the data it receives in response to this command. A malicious FTP server can send a specially crafted, oversized SYST response that overflows a memory buffer in the client software. This overflow can be manipulated to inject and execute malicious code on the victim’s machine.

Impact and Risk

The impact of this vulnerability is severe. By convincing a user to connect to a malicious FTP server (which could be disguised as a legitimate file repository), an attacker can exploit this flaw to execute arbitrary code on the victim’s computer with the same privileges as the user running the NFTP client. In documented cases, this exploit has been used to install a “bind shell,” opening a backdoor network port (5150) that gives the attacker persistent, remote command-line access to the compromised system. This can lead to data theft, installation of ransomware or other malware, and the system being used as a foothold for further attacks within a network.

Remediation and Mitigation

Immediate action is required to protect affected systems.

Primary Remediation:

• Upgrade or Replace: The NFTP client version 1.71 is vulnerable. Users must immediately upgrade to a patched version if one is available from the vendor. If a patch is not available, discontinue use of Ayukov NFTP 1.71 and switch to a different, maintained FTP client.

Mitigation Steps:

• Network Controls: Implement egress firewall rules at the network perimeter to block outbound TCP port 5150. This can prevent the specific bind shell from establishing a connection back to the attacker, though it does not fix the underlying vulnerability.
• User Awareness: Advise users to exercise extreme caution when connecting to untrusted or unfamiliar FTP servers. The exploit requires a connection to a malicious server to be triggered.
• Principle of Least Privilege: Ensure users do not run the NFTP client with administrative privileges. This can limit the scope of damage if an exploit occurs.
• Monitor for Compromise: Monitor network traffic for unexpected connections on port 5150 and inspect systems for suspicious processes or files, as exploitation may have already occurred.