CVE-1999-1568 in FTP Server
Summary
by MITRE
Off-by-one error in NcFTPd FTP server before 2.4.1 allows a remote attacker to cause a denial of service (crash) via a long PORT command.
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Analysis
by VulDB Data Team • 04/18/2026
The vulnerability described in CVE-1999-1568 represents a classic off-by-one error in the NcFTPd FTP server implementation that existed prior to version 2.4.1. This type of programming flaw falls under the CWE-129 category of Improper Validation of Array Index, where the software fails to properly validate input data before using it as an array index or buffer size parameter. The specific nature of this vulnerability stems from how the FTP server processes the PORT command, which is used by clients to specify the IP address and port number for data connections in FTP protocol implementations.
The technical flaw manifests when a remote attacker sends a specially crafted PORT command containing an excessively long argument string that exceeds the allocated buffer space. In the context of the FTP protocol, the PORT command is used to establish data connections between client and server by specifying the IP address and port number of the data connection endpoint. When the NcFTPd server processes this command without proper bounds checking, it attempts to store the command arguments in a fixed-size buffer that cannot accommodate the oversized input. This leads to a buffer overflow condition that corrupts adjacent memory locations and ultimately causes the application to crash or terminate unexpectedly.
The operational impact of this vulnerability extends beyond simple denial of service as it can be exploited by remote attackers without authentication requirements, making it particularly dangerous in networked environments. The vulnerability affects the availability of the FTP service, potentially disrupting legitimate file transfer operations for authorized users. From an attacker perspective, this represents a low-effort, high-impact method of service disruption that can be executed from anywhere on the internet. The vulnerability aligns with ATT&CK technique T1499.004 for Network Denial of Service, where attackers leverage software flaws to cause service interruption. The crash occurs during normal FTP protocol operation, making it difficult for administrators to distinguish between legitimate connection issues and malicious exploitation attempts.
Mitigation strategies for this vulnerability involve immediate patching of the NcFTPd server to version 2.4.1 or later, which contains the necessary bounds checking fixes for the PORT command processing. Organizations should also implement network-level protections such as firewall rules that limit access to FTP ports from trusted networks only, and consider deploying intrusion detection systems that can identify suspicious PORT command patterns. The fix typically involves implementing proper input validation routines that check the length of command arguments before processing them, ensuring that buffer allocation matches the actual input size. Additionally, administrators should conduct regular security assessments of their FTP server configurations and monitor for unusual connection patterns that might indicate exploitation attempts. This vulnerability demonstrates the importance of proper input validation and buffer management in network services, aligning with security best practices outlined in various industry standards including those from the Open Web Application Security Project and the Center for Internet Security benchmarks.