CVE-2024-0736: Easy File Sharing FTP Server DoS Flaw

CVE-2024-0736 Overview

CVE-2024-0736 is a denial of service vulnerability affecting EFS Easy File Sharing FTP Server 3.6. The flaw resides in the Login component, where manipulation of the password argument causes the service to enter an unavailable state. An unauthenticated remote attacker can trigger the condition over the network without user interaction. The vulnerability is classified under CWE-404: Improper Resource Shutdown or Release. Public disclosure occurred through VulDB under identifier VDB-251559, and exploit details have been published, increasing the likelihood of opportunistic abuse against exposed FTP servers.

Critical Impact
A remote, unauthenticated attacker can crash the FTP service by sending a crafted password value during login, disrupting file sharing availability.

Affected Products

EFS Easy File Sharing FTP Server 3.6
Deployments exposing the FTP login interface to untrusted networks
Legacy Windows hosts running the unmaintained EFS FTP product

Discovery Timeline

2024-01-19 - CVE-2024-0736 published to the National Vulnerability Database
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2024-0736

Vulnerability Analysis

The vulnerability exists in the authentication handler of Easy File Sharing FTP Server 3.6. When a client submits a login request, the server processes the password argument without adequately bounding or validating the input before passing it to internal resource handling routines. A malformed or oversized password value triggers an improper resource shutdown, terminating the FTP service.

The issue is categorized as [CWE-404: Improper Resource Shutdown or Release]. Because authentication is not required to reach the vulnerable code path, any attacker capable of reaching TCP port 21 on the host can disrupt service. The impact is limited to availability. There is no confirmed path to code execution, information disclosure, or integrity compromise through this specific flaw.

Root Cause

The root cause is the server's failure to validate the password input before consuming it during the login workflow. The handler does not gracefully release sockets, buffers, or session structures when malformed input is encountered, causing the service process to terminate or hang.

Attack Vector

Exploitation occurs over the network against the FTP listener. The attacker establishes a TCP session to the FTP port, issues a USER command followed by a PASS command containing the malicious argument, and observes the service becoming unresponsive. No credentials, prior access, or user interaction are required. A working proof of concept has been published on 0day.today as exploit #39249, lowering the technical barrier for abuse.

Detection Methods for CVE-2024-0736

Indicators of Compromise

Unexpected termination or restart of the fsftp.exe (or equivalent) Easy File Sharing FTP Server process
FTP control channel sessions originating from unknown sources that disconnect immediately after a PASS command
Abnormally long or malformed strings observed in the password field of FTP authentication logs
Repeated TCP connections to port 21 followed by service crash entries in the Windows Event Log

Detection Strategies

Inspect FTP server logs for PASS commands containing unusually long payloads or non-printable characters preceding service failures
Correlate Windows service crash events with inbound TCP/21 connection records to identify exploitation attempts
Deploy network IDS signatures that flag FTP PASS arguments exceeding expected length thresholds
Monitor for repeated short-lived FTP sessions from a single source IP, which can indicate denial of service probing

Monitoring Recommendations

Enable verbose FTP authentication logging and forward logs to a centralized analytics platform for retention and search
Track service uptime and restart counts for the Easy File Sharing FTP Server process and alert on anomalies
Watch external attack surface management feeds for exposed instances of Easy File Sharing FTP Server 3.6 on owned IP ranges

How to Mitigate CVE-2024-0736

Immediate Actions Required

Restrict inbound TCP/21 access to Easy File Sharing FTP Server 3.6 hosts using host or perimeter firewall rules
Decommission or replace Easy File Sharing FTP Server 3.6, which is an unmaintained product without a vendor patch for this issue
Migrate file transfer workloads to a supported, actively maintained FTP/SFTP platform with current security updates
Inventory and isolate any internet-facing instances and treat them as candidates for immediate removal

Patch Information

No vendor patch is referenced in the advisory data for CVE-2024-0736. The Easy File Sharing FTP Server product line has not received active maintenance, and no fixed version has been published. Organizations should plan migration to a supported alternative rather than wait for a fix.

Workarounds

Place the FTP service behind a VPN or zero trust network access gateway to remove unauthenticated exposure
Apply network access control lists permitting only known client IP ranges to reach the FTP listener
Configure intrusion prevention rules to drop FTP PASS commands with arguments exceeding a defined length
Implement watchdog scripts that automatically restart the service after a crash to reduce downtime while migration is planned

bash

# Example Windows firewall rule restricting FTP access to a trusted subnet
netsh advfirewall firewall add rule name="Restrict EFS FTP" ^
  dir=in action=block protocol=TCP localport=21

netsh advfirewall firewall add rule name="Allow EFS FTP Trusted" ^
  dir=in action=allow protocol=TCP localport=21 ^
  remoteip=10.0.0.0/24