CVE-2007-6740 in pyftpdlib
Summary
by MITRE
The ftp_STOU function in FTPServer.py in pyftpdlib before 0.2.0 does not limit the number of attempts to discover a unique filename, which might allow remote authenticated users to cause a denial of service via a STOU command.
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Analysis
by VulDB Data Team • 02/07/2019
The vulnerability described in CVE-2007-6740 resides within the pyftpdlib FTP server implementation, specifically in the ftp_STOU function located in FTPServer.py. This flaw represents a classic denial of service vulnerability that exploits the server's inability to properly handle filename generation attempts during the STOU (Store Unique) command execution. The STOU command is designed to store files with unique names, typically by appending a counter or timestamp to prevent filename collisions. However, the implementation in pyftpdlib versions prior to 0.2.0 lacks proper bounds checking on the number of attempts made to generate a unique filename.
The technical nature of this vulnerability stems from the absence of a maximum attempt limit in the filename generation algorithm. When a remote authenticated user issues a STOU command, the server begins generating unique filenames to avoid conflicts with existing files. Without a mechanism to cap the number of attempts, an attacker can repeatedly issue STOU commands, causing the server to continuously attempt to generate unique filenames. This leads to excessive resource consumption as the server's CPU cycles and memory are consumed in the futile process of finding unique names, ultimately resulting in a denial of service condition that affects legitimate users attempting to access the FTP service.
From an operational impact perspective, this vulnerability allows authenticated attackers to consume server resources indefinitely, potentially leading to complete service unavailability for other users. The attack is relatively simple to execute since it requires only valid FTP credentials and does not necessitate complex exploitation techniques. The vulnerability affects any system running pyftpdlib versions earlier than 0.2.0, making it a significant concern for organizations that have not updated their FTP server implementations. The impact extends beyond simple resource exhaustion to potentially affecting system stability and availability, particularly in environments where FTP services are critical to business operations.
The vulnerability maps to CWE-400, which specifically addresses "Uncontrolled Resource Consumption" in software systems, and aligns with ATT&CK technique T1499.004, "Endpoint Denial of Service," which encompasses attacks that consume system resources to make services unavailable. Organizations should implement immediate mitigations including upgrading to pyftpdlib version 0.2.0 or later, where the issue has been resolved through proper implementation of attempt limiting mechanisms. Additional protective measures include implementing connection rate limiting, monitoring for unusual STOU command patterns, and configuring proper access controls to minimize the attack surface. System administrators should also consider implementing intrusion detection systems that can identify and alert on excessive STOU command usage patterns, providing early warning of potential exploitation attempts.