CVE-2002-2393 in FTP Server
Summary
by MITRE
Serv-U FTP server 3.0, 3.1 and 4.0.0.4 does not accept new connections while validating user folder access rights, which allows remote attackers to cause a denial of service (no new connections) via a series of MKD commands.
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Analysis
by VulDB Data Team • 08/26/2025
The vulnerability identified as CVE-2002-2393 affects Serv-U FTP server versions 3.0, 3.1, and 4.0.0.4, presenting a significant denial of service risk that impacts system availability and operational continuity. This flaw manifests when the FTP server processes user folder access validation, creating a scenario where the server becomes unresponsive to new connection attempts during this critical validation phase. The vulnerability specifically exploits the server's handling of MKD (make directory) commands, which when executed in sequence, trigger the problematic validation behavior that blocks all subsequent connection attempts.
The technical root cause of this vulnerability lies in the server's synchronous processing approach during user folder access validation. When a user attempts to create a directory through MKD commands, the Serv-U server performs access rights validation before accepting new connections. This validation process creates a race condition where the server's connection handling mechanism becomes blocked or starved, preventing it from accepting any new incoming connections. The flaw represents a classic denial of service vulnerability that operates through resource exhaustion or process blocking, making it particularly dangerous for environments where continuous FTP service availability is critical.
From an operational impact perspective, this vulnerability can severely disrupt business operations by rendering the FTP service completely inaccessible to legitimate users and applications. The denial of service condition affects all new connection attempts, effectively making the server unreachable for any FTP activities including file transfers, directory listings, and user authentication processes. Attackers can exploit this vulnerability with minimal resources by simply sending a series of MKD commands, making the attack vector both simple to execute and highly effective in causing service disruption. The vulnerability affects organizations that rely on FTP services for data exchange, file management, and remote access operations.
The vulnerability aligns with CWE-400, which categorizes it as a resource management error where the system fails to properly handle resource allocation during concurrent operations. This weakness creates a condition where the server's connection handling resources become unavailable or blocked, preventing normal service operation. From the MITRE ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique related to network denial of service, where adversaries exploit service weaknesses to prevent legitimate use of network resources. The attack requires no special privileges and can be executed from any remote location, making it particularly dangerous for publicly accessible FTP servers.
Organizations should implement immediate mitigations including updating to patched versions of Serv-U FTP server, implementing connection rate limiting, and deploying network-level firewalls to restrict access to critical FTP services. The recommended solution involves applying vendor patches that address the synchronization issues in the folder access validation process, ensuring that validation operations do not block new connection handling. Additional defensive measures include implementing monitoring for unusual MKD command patterns and establishing backup FTP services to maintain operational continuity during potential exploitation attempts. The vulnerability demonstrates the importance of proper resource management and concurrent access control in server applications, particularly those handling network connections and user authentication processes.