CVE-2020-36964: YATinyWinFTP DOS Vulnerability

CVE-2020-36964 Overview

CVE-2020-36964 is a denial of service vulnerability affecting YATinyWinFTP, a lightweight FTP server for Windows. The vulnerability allows remote attackers to crash the FTP service by sending a specially crafted 272-byte buffer with a trailing space character. This triggers a buffer overflow condition that results in an immediate service crash, disrupting FTP availability for all connected users.

Critical Impact

Remote attackers can exploit this vulnerability without authentication to crash the YATinyWinFTP service, causing complete denial of service for all FTP operations.

Affected Products

• YATinyWinFTP (all versions)

Discovery Timeline

• 2026-01-28 - CVE CVE-2020-36964 published to NVD
• 2026-01-29 - Last updated in NVD database

Technical Details for CVE-2020-36964

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-bounds Write), indicating that the application writes data past the boundaries of allocated memory buffers. The flaw exists in how YATinyWinFTP processes incoming FTP commands. When the server receives a malformed command consisting of a 272-byte buffer followed by a trailing space character, it fails to properly validate the input length before copying data into a fixed-size buffer.

The attack is network-accessible, meaning any remote attacker who can establish a TCP connection to the FTP service port can exploit this vulnerability. No authentication credentials are required, and no user interaction is needed—simply connecting and sending the malformed payload is sufficient to trigger the crash. The impact is limited to availability; there is no evidence that this vulnerability allows data exfiltration or arbitrary code execution.

Root Cause

The root cause is improper input validation in the command parsing routine. YATinyWinFTP allocates a fixed-size buffer for incoming FTP commands but does not enforce proper bounds checking when copying user-supplied input. The specific trigger—a 272-byte payload with a trailing space—suggests the application uses a boundary condition related to command parsing that fails to account for edge cases involving whitespace handling.

Attack Vector

The attack is executed over the network against the FTP service port (typically TCP port 21). An attacker establishes a connection to the vulnerable YATinyWinFTP server and transmits a crafted payload consisting of exactly 272 bytes followed by a space character. Upon receiving this input, the server's command parser attempts to process the data, triggering an out-of-bounds write that corrupts memory and causes the service to crash.

Proof-of-concept code for this vulnerability has been published to Exploit-DB #49127. The exploit demonstrates how connecting to the FTP server and sending the malformed buffer immediately crashes the service. Additional technical details are available in the VulnCheck Advisory and the GitHub PoC Repository.

Detection Methods for CVE-2020-36964

Indicators of Compromise

• FTP service crashes or unexpected terminations following network connections
• Repeated TCP connections to the FTP port followed by immediate disconnections
• Windows Event Log entries indicating application crashes for YATinyWinFTP process
• Network traffic containing 272-byte payloads directed at FTP service ports

Detection Strategies

• Deploy network intrusion detection rules to identify oversized or malformed FTP commands
• Monitor for repeated FTP service restarts or crash events in system logs
• Implement connection rate limiting to detect potential denial of service attempts
• Use SentinelOne's behavioral AI to identify exploitation attempts and service anomalies

Monitoring Recommendations

• Enable detailed logging for the FTP service to capture connection attempts and command sequences
• Configure alerts for unexpected process terminations of the YATinyWinFTP executable
• Monitor network traffic patterns for connections that transmit data and disconnect rapidly
• Implement file integrity monitoring on FTP server configuration and binary files

How to Mitigate CVE-2020-36964

Immediate Actions Required

• Restrict network access to the FTP service using firewall rules to trusted IP ranges only
• Consider replacing YATinyWinFTP with a more actively maintained and secure FTP server solution
• Implement network-level rate limiting to reduce the impact of repeated exploitation attempts
• Enable service monitoring with automatic restart capabilities to minimize downtime during attacks

Patch Information

No official vendor patch is currently available for this vulnerability. YATinyWinFTP appears to be an unmaintained project, and organizations relying on this software should evaluate migration to alternative FTP solutions. Security details and proof-of-concept information can be found in the referenced VulnCheck Advisory.

Workarounds

• Deploy a reverse proxy or application-layer firewall in front of the FTP service to filter malicious payloads
• Use network segmentation to isolate the FTP server from critical infrastructure
• Implement IP allowlisting to restrict FTP access to known, trusted clients only
• Consider migrating to alternative FTP server software with active security support

# Windows Firewall configuration to restrict FTP access to trusted networks
netsh advfirewall firewall add rule name="Restrict FTP Access" dir=in action=allow protocol=tcp localport=21 remoteip=192.168.1.0/24
netsh advfirewall firewall add rule name="Block FTP Public" dir=in action=block protocol=tcp localport=21