CVE-2007-6738 in pyftpdlib
Summary
by MITRE
pyftpdlib before 0.1.1 does not choose a random value for the port associated with the PASV command, which makes it easier for remote attackers to obtain potentially sensitive information about the number of in-progress data connections by reading the response to this command.
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Analysis
by VulDB Data Team • 02/07/2019
The vulnerability described in CVE-2007-6738 affects pyftpdlib versions prior to 0.1.1 and relates to the implementation of the PASV command within the FTP protocol. This flaw specifically impacts the passive mode operation of FTP servers where the server provides a port number to the client for establishing data connections. The vulnerability stems from the server's failure to generate cryptographically secure random values for the port numbers used in the PASV response, creating predictable patterns that can be exploited by remote attackers.
The technical implementation of this vulnerability lies in the predictable port selection mechanism within the pyftpdlib FTP server implementation. When a client issues a PASV command, the server responds with a port number that should be randomly selected to prevent attackers from easily determining system information. However, the vulnerable version of pyftpdlib uses a non-random selection process that can result in port numbers following predictable sequences or patterns. This predictable behavior directly violates security principles that require randomization for sensitive operations and creates opportunities for attackers to infer system state information.
The operational impact of this vulnerability extends beyond simple information disclosure, as it can enable more sophisticated attacks by providing attackers with insights into the server's connection management and potentially revealing information about the number of active data connections. This information can be leveraged to perform connection enumeration, identify server configurations, or even aid in subsequent exploitation attempts. The vulnerability aligns with CWE-330 Use of Insufficiently Random Values, which specifically addresses the use of predictable random numbers in security-sensitive contexts. Attackers can utilize this information to craft more targeted attacks against the FTP server infrastructure.
The vulnerability demonstrates a clear violation of the principle of least privilege and proper randomization in security-sensitive operations. The lack of cryptographic randomness in port selection creates a predictable attack surface that can be exploited to gather intelligence about the FTP server's operational state. This weakness can be categorized under the MITRE ATT&CK framework as part of the reconnaissance phase, where attackers gather information about the target system to plan subsequent attacks. The vulnerability also relates to the broader category of information disclosure attacks that can be used to establish a baseline for more complex exploitation techniques.
Mitigation strategies for this vulnerability require upgrading to pyftpdlib version 0.1.1 or later, which implements proper randomization for PASV port selection. System administrators should also consider implementing additional network security controls such as firewall rules to restrict FTP access, monitoring for suspicious PASV command usage, and employing intrusion detection systems that can identify patterns consistent with this type of information gathering. The fix addresses the core issue by ensuring that all port numbers used in the PASV response are generated using cryptographically secure random number generators, thereby eliminating the predictable patterns that made the vulnerability exploitable.