CVE-2007-6737 in pyftpdlib
Summary
by MITRE
FTPServer.py in pyftpdlib before 0.2.0 does not increment the attempted_logins count for a USER command that specifies an invalid username, which makes it easier for remote attackers to obtain access via a brute-force attack.
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Analysis
by VulDB Data Team • 02/07/2019
The vulnerability identified as CVE-2007-6737 resides within the FTPServer.py component of pyftpdlib versions prior to 0.2.0, representing a significant security weakness that directly impacts authentication mechanisms. This flaw specifically affects the handling of USER commands during the FTP authentication process, creating a condition where the attempted_logins counter fails to increment when an invalid username is provided. The issue stems from improper state management within the authentication flow, where the system does not adequately track failed authentication attempts that involve non-existent user accounts. This oversight creates a fundamental gap in the server's ability to detect and respond to brute-force attack patterns, as the login attempt counter remains static regardless of whether a valid or invalid username is submitted. The vulnerability directly relates to CWE-305 authentication bypass weakness, where the system fails to properly implement account lockout mechanisms or authentication attempt tracking, making it easier for malicious actors to systematically test credentials without detection. The flaw operates at the application layer of the network stack, specifically within the FTP protocol implementation, where user authentication occurs before any password validation takes place.
The technical implementation of this vulnerability demonstrates a clear failure in access control enforcement and authentication monitoring. When a remote attacker sends a USER command with an invalid username, the system should increment the attempted_logins counter to maintain accurate authentication attempt tracking. However, due to the flawed code logic, the counter remains unchanged, effectively masking the actual number of authentication attempts being made. This behavior creates a false sense of security for the system administrators, as the login attempt monitoring mechanisms fail to register these brute-force attempts. The vulnerability enables attackers to conduct systematic username enumeration and password guessing attacks without the system's awareness, as the attempted_logins counter does not reflect the true number of login attempts. This specific weakness can be categorized under ATT&CK technique T1110.001 for Brute Force: Password Guessing, where the lack of proper authentication attempt tracking removes an important defense mechanism against automated attack tools. The flaw essentially provides attackers with a stealthy method to conduct credential stuffing or password spraying attacks without triggering any automated security responses.
The operational impact of this vulnerability extends beyond simple authentication bypass, creating a substantial risk to overall system security and availability. Attackers can leverage this weakness to conduct prolonged brute-force attacks against FTP servers without detection, potentially leading to unauthorized system access and data compromise. The vulnerability particularly affects systems where FTP services are exposed to the internet, as the lack of proper authentication attempt monitoring removes an important barrier to automated attacks. Organizations running pyftpdlib versions prior to 0.2.0 face increased risk of successful credential compromise, especially when combined with other vulnerabilities or attack vectors. The flaw also impacts security monitoring and incident response capabilities, as security systems relying on authentication attempt logs will not accurately reflect the true threat landscape. This vulnerability can be exploited by attackers using automated tools to systematically test common username and password combinations, with the system failing to detect the repeated attempts. The impact is particularly severe in environments where FTP services are used for file transfers, as successful exploitation could lead to data theft, system compromise, or disruption of services. The vulnerability also affects compliance requirements for security monitoring and logging, as the system fails to maintain accurate audit trails for authentication events.
Mitigation strategies for this vulnerability require immediate action to upgrade to pyftpdlib version 0.2.0 or later, where the authentication attempt tracking has been properly implemented. System administrators should also implement additional security measures including rate limiting for FTP login attempts, account lockout policies, and monitoring of authentication logs for suspicious activity patterns. Network-level protections such as firewall rules that limit FTP access to trusted IP addresses and intrusion detection systems that monitor for repeated failed authentication attempts should be deployed. Organizations should also consider implementing multi-factor authentication for FTP services when possible, and regularly review authentication logs for signs of brute-force attacks. The implementation of proper logging mechanisms and alerting systems can help detect when authentication attempts exceed normal thresholds, compensating for the lack of automatic login attempt counting. Security monitoring tools should be configured to detect unusual patterns in FTP authentication attempts, including rapid succession of login failures from the same source. Additionally, organizations should conduct regular vulnerability assessments and penetration testing to identify similar authentication-related weaknesses in their FTP implementations. The fix for this vulnerability aligns with security best practices outlined in NIST SP 800-53 and ISO 27001 standards, which emphasize the importance of proper access control and authentication monitoring in protecting information systems.