CVE-2019-25361: Ayukov NFTP Buffer Overflow Vulnerability

CVE-2019-25361 Overview

CVE-2019-25361 is a stack-based buffer overflow vulnerability affecting Ayukov NFTP client version 1.71. The vulnerability exists in the SYST command handling functionality, which fails to properly validate the length of server responses before copying them into a fixed-size buffer. This allows remote attackers to execute arbitrary code on systems running the vulnerable FTP client by sending specially crafted responses from a malicious FTP server.

Critical Impact

Remote code execution via buffer overflow allows attackers to gain full control of affected systems, potentially establishing a bind shell on port 5150 for persistent access.

Affected Products

• Ayukov NFTP client version 1.71

Discovery Timeline

• 2026-02-18 - CVE-2019-25361 published to NVD
• 2026-02-19 - Last updated in NVD database

Technical Details for CVE-2019-25361

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow). The NFTP client fails to perform adequate bounds checking when processing responses to the SYST command from an FTP server. When a user connects to a malicious FTP server, the server can respond with an oversized payload that exceeds the allocated buffer space on the stack, corrupting adjacent memory and allowing the attacker to overwrite the return address.

The network-accessible nature of this vulnerability means that attackers can set up rogue FTP servers or perform man-in-the-middle attacks to intercept legitimate FTP connections and inject malicious responses. User interaction is required, as the victim must initiate a connection to the attacker-controlled server.

Root Cause

The root cause is improper input validation in the SYST command response handler. The vulnerable code copies server-provided data into a fixed-size stack buffer without verifying that the input length does not exceed the buffer's capacity. This is a classic buffer overflow pattern where untrusted external input is processed without proper bounds checking.

Attack Vector

The attack requires network access and user interaction. An attacker can exploit this vulnerability by:

1. Setting up a malicious FTP server or compromising an existing FTP server
2. Waiting for or enticing victims to connect using the vulnerable NFTP client
3. Responding to the client's SYST command with a specially crafted oversized payload
4. The payload overwrites the stack, including the return address, redirecting execution to attacker-controlled shellcode
5. The shellcode establishes a bind shell on port 5150, providing persistent remote access to the compromised system

For detailed technical analysis and proof-of-concept information, refer to the Exploit-DB entry #47576 and the VulnCheck Advisory.

Detection Methods for CVE-2019-25361

Indicators of Compromise

• Unexpected outbound connections from FTP client processes to unusual ports, particularly port 5150
• Anomalous network traffic patterns during FTP sessions, including oversized server responses
• Crash dumps or memory access violations in the NFTP client process
• Presence of bind shells or unauthorized listening services on port 5150

Detection Strategies

• Monitor for FTP server responses exceeding normal expected lengths, particularly for SYST command responses
• Deploy network intrusion detection signatures to identify exploit payloads targeting NFTP buffer overflows
• Implement endpoint detection rules to alert on unexpected child processes spawned by nftp.exe or similar FTP client binaries
• Use memory protection mechanisms like DEP and ASLR to detect and prevent exploitation attempts

Monitoring Recommendations

• Enable detailed logging for FTP client connections and server responses
• Monitor network traffic for connections to untrusted or suspicious FTP servers
• Track process behavior for the NFTP client, alerting on unusual memory access patterns or process spawning
• Configure firewall rules to alert on unexpected listening services, particularly on port 5150

How to Mitigate CVE-2019-25361

Immediate Actions Required

• Discontinue use of Ayukov NFTP client version 1.71 until a patched version is available
• Use alternative, actively maintained FTP clients with proper input validation
• Implement network segmentation to limit exposure of systems running vulnerable software
• Enable exploit mitigation technologies (DEP, ASLR, CFG) on systems where the vulnerable client must be used temporarily

Patch Information

No vendor patch information is currently available in the NVD database. Users should check the Ayukov NFTP official page for updates and security advisories. Given the age of this software (version 1.71), users are strongly encouraged to migrate to a modern, actively maintained FTP client solution.

Workarounds

• Avoid connecting to untrusted or unknown FTP servers with the vulnerable client
• Use a VPN or secure tunnel when FTP access is required to reduce exposure to man-in-the-middle attacks
• Deploy application whitelisting to prevent execution of unauthorized code spawned by exploitation
• Configure host-based firewalls to block inbound connections on commonly targeted ports like 5150

# Block potential bind shell port used by this exploit
# Windows Firewall
netsh advfirewall firewall add rule name="Block NFTP Exploit Port" dir=in action=block protocol=tcp localport=5150

# Linux iptables
iptables -A INPUT -p tcp --dport 5150 -j DROP