CVE-2005-0851 in Filezilla Server
Summary
by MITRE
FileZilla FTP server before 0.9.6, when using MODE Z (zlib compression), allows remote attackers to cause a denial of service (infinite loop) via certain file uploads or directory listings.
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Analysis
by VulDB Data Team • 07/01/2021
The vulnerability identified as CVE-2005-0851 affects FileZilla FTP server versions prior to 0.9.6 and specifically targets the implementation of MODE Z compression functionality. This flaw manifests when the server processes file uploads or directory listings that utilize zlib compression, creating a condition that can be exploited by remote attackers to trigger an infinite loop in the server process. The vulnerability represents a classic denial of service weakness that can severely impact the availability of the FTP service.
The technical implementation of this vulnerability stems from inadequate input validation and error handling within the MODE Z compression processing module of the FileZilla server. When the server receives certain file transfer requests or directory listing commands that incorporate compressed data, the decompression routine fails to properly validate the compressed data structure. This validation failure allows maliciously crafted compressed data to cause the decompression algorithm to enter an infinite loop, consuming excessive CPU resources and rendering the FTP service unavailable to legitimate users. The flaw operates at the application layer and can be triggered through standard FTP client commands without requiring authentication.
The operational impact of this vulnerability extends beyond simple service disruption as it can be exploited by attackers to consume significant system resources and potentially cause server crashes or restarts. The infinite loop condition affects the server's ability to process legitimate requests, effectively creating a denial of service condition that impacts all users of the FTP service. This vulnerability is particularly concerning in environments where FTP services are critical for business operations, as it can be exploited remotely without requiring any special privileges or credentials.
Security practitioners should note that this vulnerability aligns with CWE-835, which describes the weakness of an infinite loop or infinite recursion in software implementations. The flaw also relates to ATT&CK technique T1499.004, which covers the use of denial of service attacks against network services. Organizations should prioritize immediate patching of FileZilla servers to version 0.9.6 or later, as this represents the most effective mitigation strategy. Additionally, network administrators should implement monitoring to detect unusual CPU utilization patterns that might indicate exploitation attempts. The vulnerability highlights the importance of proper input validation and error handling in network service implementations, particularly when dealing with compression algorithms that can be manipulated to cause resource exhaustion.