CVE-2008-6534 in NULL FTP
Summary
by MITRE
Incomplete blacklist vulnerability in NULL FTP Server Free and Pro 1.1.0.7 allows remote authenticated users to execute arbitrary commands via a custom SITE command containing shell metacharacters such as "&" (ampersand) in the middle of an argument.
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Analysis
by VulDB Data Team • 05/24/2025
The CVE-2008-6534 vulnerability represents a critical command injection flaw in the NULL FTP Server Free and Pro versions 1.1.0.7, demonstrating a classic incomplete input validation issue that enables remote command execution. This vulnerability specifically targets the server's handling of custom SITE commands, which are extension commands used in ftp protocol implementations to perform server-specific operations. The flaw occurs when the server fails to properly sanitize user input passed through these custom commands, creating a pathway for malicious actors to inject shell metacharacters that can be interpreted by the underlying operating system.
The technical implementation of this vulnerability stems from the server's inadequate filtering of input parameters within the SITE command processing logic. When an authenticated user submits a custom SITE command containing shell metacharacters such as the ampersand character, the server processes this input without proper sanitization or validation. The ampersand symbol serves as a command separator in many shell environments, allowing multiple commands to be executed sequentially. This incomplete blacklist approach means that while the server may block certain known dangerous characters, it fails to account for all possible injection vectors, particularly those that can be embedded within argument strings rather than at command boundaries.
The operational impact of this vulnerability extends beyond simple command execution, as it allows authenticated attackers to gain arbitrary code execution capabilities on the affected system. This presents a significant risk to organizations relying on legacy ftp server implementations, as the vulnerability can be exploited by users who have legitimate access credentials to the ftp service. Attackers can leverage this privilege escalation to execute system commands, potentially leading to complete system compromise, data exfiltration, or further network infiltration. The vulnerability's classification aligns with CWE-77 and CWE-94, which address improper input validation and code injection respectively, while also mapping to ATT&CK techniques such as T1059 for command and scripting interpreter and T1068 for exploit for privilege escalation.
Mitigation strategies for this vulnerability require immediate implementation of proper input validation and sanitization measures within the ftp server's command processing pipeline. Organizations should implement comprehensive whitelisting approaches rather than relying on incomplete blacklists, ensuring that all input parameters are strictly validated against expected formats and character sets. System administrators should also consider applying vendor-specific patches or upgrading to more recent ftp server implementations that properly address input validation. Additional defensive measures include restricting ftp service access to trusted networks, implementing network segmentation, and monitoring for suspicious SITE command usage patterns. The vulnerability highlights the importance of following secure coding practices and implementing defense-in-depth strategies to prevent similar injection vulnerabilities in network service implementations.