CVE-2007-6741 in pyftpdlib
Summary
by MITRE
The ftp_PORT function in FTPServer.py in pyftpdlib before 0.2.0 does not prevent TCP connections to privileged ports if the destination IP address matches the source IP address of the connection from the FTP client, which might allow remote authenticated users to conduct FTP bounce attacks via crafted FTP data, as demonstrated by an FTP bounce attack against a NAT server, a related issue to CVE-1999-0017.
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Analysis
by VulDB Data Team • 02/07/2019
The vulnerability described in CVE-2007-6741 represents a critical security flaw in the pyftpdlib FTP server implementation that enables authenticated users to perform FTP bounce attacks. This issue specifically affects versions of pyftpdlib prior to 0.2.0 and stems from a fundamental design flaw in how the ftp_PORT function handles TCP connection requests. The vulnerability operates by exploiting the lack of proper validation when processing FTP PORT commands, allowing malicious users to manipulate the connection process in ways that bypass normal security restrictions.
The technical implementation of this flaw occurs within the FTPServer.py file where the ftp_PORT function fails to properly validate destination port numbers against privileged port ranges. When an FTP client establishes a connection and sends a PORT command, the function should verify that the destination IP address and port combination adheres to security policies preventing connections to privileged ports below 1024. However, the vulnerable implementation only checks if the destination IP matches the source IP address of the original FTP client connection, without enforcing port validation rules that would prevent access to privileged ports. This oversight creates a pathway for attackers to use the FTP server as an intermediary to probe or attack other systems, particularly those behind NAT configurations.
The operational impact of this vulnerability extends beyond simple port scanning attacks to enable sophisticated network reconnaissance and exploitation activities. Attackers can leverage this flaw to conduct FTP bounce attacks against NAT servers, effectively using the compromised FTP server as a proxy to access internal network resources that would otherwise be protected by network segmentation. This capability represents a significant escalation from the original CVE-1999-0017 issue, which was a similar problem in older FTP implementations, but demonstrates how such vulnerabilities can persist and evolve in modern software implementations. The vulnerability particularly affects environments where FTP servers are deployed in network architectures that rely on NAT for security boundaries, as the bounce attack can bypass these protective mechanisms.
Security professionals should recognize this vulnerability as a variant of CWE-284, which deals with inadequate access control, and it aligns with ATT&CK technique T1071.004 for application layer protocol tunneling. The flaw fundamentally compromises the server's ability to enforce proper network access controls and can lead to unauthorized network scanning, port probing, and potentially more serious attacks against internal systems. Organizations using pyftpdlib versions prior to 0.2.0 should immediately implement mitigation strategies including upgrading to patched versions, implementing proper network segmentation, and configuring firewall rules to restrict FTP server access to privileged ports. Additionally, administrators should consider implementing network monitoring to detect anomalous FTP traffic patterns that might indicate bounce attack attempts. The vulnerability highlights the importance of proper input validation and access control enforcement in network service implementations, particularly when dealing with protocols that inherently require complex network interaction patterns.