CVE-2016-9499 in FTP Server
Summary
by MITRE
Accellion FTP server prior to version FTA_9_12_220 only returns the username in the server response if the username is invalid. An attacker may use this information to determine valid user accounts and enumerate them.
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Analysis
by VulDB Data Team • 12/27/2024
The Accellion FTP server vulnerability CVE-2016-9499 represents a classic account enumeration flaw that exposes critical information about user authentication mechanisms. This vulnerability affects versions prior to FTA_9_12_220 and demonstrates a fundamental security weakness in how the system handles authentication responses. The flaw operates by selectively revealing authentication state information based on username validity, creating an information disclosure vector that directly enables automated account enumeration attacks.
The technical implementation of this vulnerability stems from the server's inconsistent response handling during authentication attempts. When an attacker submits a username, the system only provides username feedback when the username is invalid, while returning no explicit indication of username validity for valid accounts. This behavior creates a predictable pattern that attackers can exploit through automated tools to systematically determine which usernames exist within the system. The vulnerability specifically relates to improper error handling and information disclosure in authentication protocols, falling under CWE-200 Information Exposure and CWE-305 Authentication Bypass.
The operational impact of this vulnerability extends beyond simple account enumeration, creating a foundation for more sophisticated attacks including credential stuffing, brute force attempts, and social engineering campaigns. Attackers can leverage the enumerated user accounts to target specific individuals with phishing campaigns or to focus their authentication attempts on valid accounts, significantly reducing the attack surface required for successful exploitation. This vulnerability directly maps to ATT&CK technique T1078 Valid Accounts, as it enables adversaries to acquire legitimate user credentials through information gathering rather than direct compromise.
Mitigation strategies for CVE-2016-9499 require immediate patching of the Accellion FTP server to version FTA_9_12_220 or later, which addresses the inconsistent authentication response behavior. Organizations should also implement account lockout mechanisms, rate limiting for authentication attempts, and consider implementing multi-factor authentication to reduce the impact of successful enumeration. Network-level protections including firewall rules and intrusion detection systems can help monitor for automated enumeration attempts, while security monitoring should focus on detecting unusual authentication patterns that may indicate account enumeration activity. The vulnerability highlights the importance of consistent error handling in authentication systems and demonstrates how seemingly minor implementation flaws can create significant security risks.