CVE-2005-0850 in Filezilla Server
Summary
by MITRE
FileZilla FTP server before 0.9.6 allows remote attackers to cause a denial of service via a request for a filename containing an MS-DOS device name such as CON, NUL, COM1, LPT1, and others.
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Analysis
by VulDB Data Team • 06/06/2019
The vulnerability identified as CVE-2005-0850 represents a significant denial of service weakness in FileZilla FTP server versions prior to 0.9.6. This flaw exploits the server's handling of filenames that contain Microsoft DOS device names, which are reserved system identifiers used by the Windows operating system. The vulnerability stems from the server's inadequate validation of file names during the file listing and retrieval processes, creating an opportunity for remote attackers to disrupt service availability.
The technical implementation of this vulnerability involves the exploitation of legacy Windows file system conventions where device names such as CON, NUL, COM1, LPT1, PRN, AUX, and others are reserved for system use. When the FileZilla server processes requests for files with these names, it fails to properly sanitize the input, causing the server to attempt operations that either crash the service or consume excessive system resources. The flaw occurs at the application layer where the FTP server's file name parsing logic does not adequately distinguish between legitimate file names and system device identifiers, leading to unexpected behavior during file operations.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be exploited by remote attackers without authentication requirements, making it particularly dangerous in public or unsecured network environments. Attackers can trigger the denial of service condition by simply requesting files with these specific device names, causing the FTP server to either terminate unexpectedly or become unresponsive to legitimate requests. This vulnerability affects the availability aspect of the CIA triad and can be classified under CWE-20 as "Improper Input Validation" within the context of the Common Weakness Enumeration framework.
From an adversarial perspective, this vulnerability aligns with ATT&CK technique T1499.004 for "Endpoint Denial of Service" and represents a classic example of how legacy system conventions can create security weaknesses in modern applications. The exploitability of this vulnerability is high due to the simple nature of the attack vector and the fact that it requires no specialized tools or authentication. Network defenders should note that this vulnerability can be particularly problematic in environments where FileZilla servers are exposed to untrusted networks or where automated scanning tools might probe for such weaknesses. The impact is particularly severe in mission-critical environments where continuous FTP service availability is essential for business operations.
The recommended mitigation strategy involves upgrading to FileZilla server version 0.9.6 or later, which includes proper input validation for filenames containing device names. Additionally, administrators should implement network segmentation to limit exposure of FTP servers to untrusted networks, deploy intrusion detection systems to monitor for suspicious filename patterns, and consider implementing application-level firewalls that can filter out requests containing known problematic device names. Regular security assessments and vulnerability scanning should include checks for this specific weakness, particularly in legacy systems where upgrading might not be immediately feasible. Organizations should also consider implementing logging and monitoring solutions that can detect and alert on unusual filename access patterns that might indicate exploitation attempts.