CVE-2004-0042 in vsftpd
Summary
by MITRE
vsftpd 1.1.3 generates different error messages depending on whether or not a valid username exists, which allows remote attackers to identify valid usernames.
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Analysis
by VulDB Data Team • 06/17/2018
The vulnerability identified as CVE-2004-0042 affects vsftpd version 1.1.3 and represents a classic information disclosure flaw that undermines the security of FTP services. This issue stems from the server's inconsistent error message handling mechanism, where the service provides different responses to client requests based on whether the attempted username exists in the system's user database. Such behavior creates a clear attack surface that adversaries can exploit to perform user enumeration attacks against FTP servers running this vulnerable version. The flaw directly violates fundamental security principles by exposing system information through unintended channels, making it easier for threat actors to gather intelligence for subsequent attacks.
The technical implementation of this vulnerability resides in the server's authentication handling logic within vsftpd 1.1.3. When a client attempts to authenticate with the FTP server, the service processes the username and password combination through a validation routine that returns distinct error messages for different failure scenarios. Specifically, when a valid username is provided but an incorrect password is supplied, the server generates one error message type. However, when an invalid username is provided, regardless of the password entered, the server returns a different error message that clearly indicates the username does not exist. This differential response behavior enables attackers to systematically test usernames and observe the server's responses to determine which usernames are valid within the system's user database.
The operational impact of CVE-2004-0042 extends beyond simple information disclosure, creating a foundation for more sophisticated attack vectors that align with multiple tactics outlined in the MITRE ATT&CK framework. This vulnerability particularly supports the reconnaissance phase of attack chains, enabling adversaries to gather user account information that can subsequently be used for password spraying, brute force attacks, or privilege escalation attempts. The flaw maps directly to CWE-200, which addresses "Information Exposure," and represents a specific instance of information leakage through error handling mechanisms. Security professionals can categorize this as a passive reconnaissance technique that provides attackers with valuable intelligence about the target system's user base without requiring active exploitation of other vulnerabilities.
Mitigation strategies for this vulnerability center on immediate software updates and configuration modifications that address the root cause of the inconsistent error handling. The most effective solution involves upgrading to a newer version of vsftpd that implements consistent error messaging regardless of authentication outcome, ensuring that all authentication failures return identical error responses to prevent user enumeration. System administrators should also consider implementing additional security controls such as rate limiting to prevent automated enumeration attempts, configuring firewall rules to restrict FTP access to trusted networks, and deploying intrusion detection systems that can identify patterns of user enumeration attempts. Organizations should conduct comprehensive vulnerability assessments to identify all instances of this vulnerable software version and ensure proper patch management procedures are in place to prevent similar issues from arising in the future. The vulnerability serves as a reminder of the importance of consistent error handling practices in security-critical applications and demonstrates how seemingly minor implementation flaws can create significant security risks.