CVE-2009-4105 in FTP Server
Summary
by MITRE
TYPSoft FTP Server 1.10 allows remote authenticated users to cause a denial of service (crash) by sending an APPE (append) command immediately followed by a DELE (delete) command without sending file data in between these two commands.
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Analysis
by VulDB Data Team • 10/10/2025
The vulnerability identified as CVE-2009-4105 affects TYPSoft FTP Server version 1.10 and represents a classic race condition flaw in command processing that leads to denial of service conditions. This issue manifests when authenticated remote users exploit a specific sequence of FTP commands that the server fails to handle properly, resulting in application instability and potential system crashes. The vulnerability demonstrates a fundamental flaw in the server's state management and command execution logic, where the sequential processing of file operations creates an exploitable gap in the protocol handling mechanism.
The technical exploitation involves a precise timing attack pattern where an authenticated user sends an APPE command followed immediately by a DELE command without transmitting any actual file data between these operations. This specific command sequence triggers an internal state inconsistency within the FTP server's memory management and file handle processing routines. The server's inability to properly validate or handle this particular command ordering results in memory corruption or invalid pointer dereferences that ultimately cause the application to terminate unexpectedly. This type of vulnerability falls under CWE-362, which describes race conditions in software systems, and specifically aligns with the category of improper handling of concurrent operations that can lead to system instability and denial of service conditions.
The operational impact of this vulnerability extends beyond simple service disruption as it provides attackers with a method to systematically destabilize the FTP server without requiring elevated privileges or complex exploitation techniques. The fact that authentication is required limits the scope of potential attackers to those who already have legitimate access credentials, but this still represents a significant security concern for organizations that maintain FTP services with multiple user accounts. The vulnerability can be exploited repeatedly and systematically, making it particularly dangerous in environments where FTP services are critical for business operations. Attackers can leverage this flaw to repeatedly crash the server, causing service unavailability that impacts legitimate users and potentially leading to data access interruptions.
Organizations should implement immediate mitigations including upgrading to patched versions of TYPSoft FTP Server or applying vendor-provided security patches that address the command processing logic. Network segmentation and access control measures should be enhanced to limit exposure of vulnerable FTP services to unnecessary network traffic. Monitoring systems should be configured to detect unusual command sequences that might indicate exploitation attempts, particularly patterns involving rapid APPE followed by DELE commands. The vulnerability also highlights the importance of proper input validation and state management in network services, as similar issues could potentially exist in other FTP implementations or similar network protocols. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation and denial of service tactics, with the exploitation requiring minimal technical sophistication while providing significant operational impact.