CVE-2001-0187 in wu-ftpd
Summary
by MITRE
Format string vulnerability in wu-ftp 2.6.1 and earlier, when running with debug mode enabled, allows remote attackers to execute arbitrary commands via a malformed argument that is recorded in a PASV port assignment.
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Analysis
by VulDB Data Team • 10/08/2025
The vulnerability identified as CVE-2001-0187 represents a critical format string flaw within the wu-ftp server version 2.6.1 and earlier implementations. This security weakness specifically manifests when the FTP daemon operates in debug mode, creating an exploitable condition that can be leveraged by remote attackers to gain unauthorized system access. The vulnerability stems from improper input validation and handling of user-supplied data within the server's logging mechanisms, particularly during PASV port assignment operations. When debug mode is enabled, the server records various arguments and parameters to log files, but fails to properly sanitize these inputs before using them in format string operations, creating a classic buffer overflow scenario that can be manipulated for code execution.
The technical exploitation of this vulnerability relies on the attacker crafting a malformed argument that gets processed during the PASV command execution sequence. The wu-ftp server's debug logging functionality treats user-provided data as format specifiers rather than literal strings, allowing attackers to inject format string directives that can read from memory locations or write to arbitrary addresses. This particular flaw aligns with CWE-134, which specifically addresses format string vulnerabilities where format strings are constructed using data from untrusted sources. The attack vector is particularly dangerous because it requires no authentication, as the vulnerability exists in the server's response to unauthenticated PASV commands, making it accessible to any remote attacker. The operational impact extends beyond simple command execution to potentially allow full system compromise, as successful exploitation can enable attackers to execute arbitrary code with the privileges of the ftp daemon process, often running with elevated system permissions.
The implications of this vulnerability extend significantly within enterprise network security contexts where FTP services remain operational, particularly in legacy systems that have not been properly updated or patched. The debug mode activation often occurs inadvertently during system troubleshooting or maintenance, creating a window of opportunity for exploitation that may persist for extended periods without detection. Security professionals should note that this vulnerability demonstrates the importance of proper input sanitization and the dangerous consequences of enabling debug functionality in production environments. The ATT&CK framework categorizes this vulnerability under T1059.007 for command and scripting interpreter, as exploitation results in arbitrary command execution, while also mapping to T1566 for social engineering through malicious file execution. Organizations must implement comprehensive patch management processes to address such vulnerabilities, as the window of opportunity for exploitation is extended by the presence of debug modes that should never be enabled in production environments. The remediation strategy involves immediate patching of affected wu-ftp versions, disabling debug mode in all production deployments, and implementing network monitoring to detect anomalous PASV command usage patterns that may indicate exploitation attempts.
The broader security implications highlight the critical need for secure coding practices in network services, particularly regarding format string handling and input validation. This vulnerability exemplifies how seemingly benign debugging features can create significant security risks when not properly secured, emphasizing the principle that debug functionality should never be exposed in production environments. The flaw also demonstrates the importance of proper privilege separation in service implementations, as the ftp daemon typically runs with elevated privileges, making successful exploitation potentially catastrophic for system integrity. Organizations implementing security controls should consider this vulnerability when assessing their network service attack surface, particularly focusing on legacy FTP implementations that may still be in use. The vulnerability serves as a reminder that even older software versions can harbor critical flaws that require immediate attention, especially when debug modes are enabled and accessible to remote users.