CVE-2021-37533 is a medium-severity vulnerability in the Apache Commons Net library, specifically affecting the FTP client implementation prior to version 3.9.0. The root cause is improper input validation (CWE-20) related to the handling of the PASV (passive mode) FTP response. In FTP passive mode, the server responds with an IP address and port number for the client to connect to for data transfer. Prior to version 3.9.0, the Apache Commons Net FTP client trusted the IP address provided by the server in the PASV response without verification. This trust allows a malicious FTP server to redirect the client to connect to an arbitrary host, potentially on the client’s private network or another internal resource. The vulnerability requires the client to initiate a connection to a malicious FTP server first, which then returns a crafted PASV response containing a different host IP. This can lead to information leakage about internal services running on the client’s private network, as the client may attempt to connect to these internal hosts, revealing their existence or behavior. The vulnerability does not allow direct code execution or integrity compromise but poses a confidentiality risk. Starting with Apache Commons Net 3.9.0, the default behavior was changed to ignore the host provided in the PASV response, aligning with cURL’s approach, thereby mitigating this risk. The CVSS 3.1 base score is 6.5 (medium), reflecting network attack vector, low attack complexity, no privileges required, but requiring user interaction (the user initiating the FTP connection). The impact is primarily on confidentiality, with no impact on integrity or availability. There are no known exploits in the wild, and no patches are explicitly linked here, but upgrading to version 3.9.0 or later is the recommended remediation. This vulnerability is relevant for any application or service that uses Apache Commons Net FTP client versions prior to 3.9.0 to connect to untrusted FTP servers or servers that could be compromised or malicious.

For European organizations, the primary impact of this vulnerability is the potential leakage of sensitive internal network information when connecting to malicious or compromised FTP servers using vulnerable Apache Commons Net versions. This can expose internal service details, network topology, or presence of critical infrastructure components, which could be leveraged in subsequent targeted attacks or lateral movement by adversaries. Organizations relying on FTP for file transfers, especially in sectors like finance, manufacturing, healthcare, and government, may inadvertently expose internal network details if their software stack includes vulnerable Apache Commons Net versions. While the vulnerability does not directly allow data modification or denial of service, the confidentiality breach could facilitate espionage, reconnaissance, or preparation for more severe attacks. Given that FTP is still used in legacy systems and some industrial environments, the risk is non-negligible. The requirement for user interaction (initiating FTP connection) limits automated exploitation but does not eliminate risk, especially in environments where users connect to external FTP servers regularly. The lack of known exploits in the wild reduces immediate threat but does not preclude future exploitation. Overall, the vulnerability could undermine trust in internal network security and increase the attack surface for European organizations that have not updated their Apache Commons Net dependencies.

1. Upgrade Apache Commons Net to version 3.9.0 or later, where the default behavior ignores the host provided in the PASV response, effectively mitigating the vulnerability. 2. Audit all applications and services within the organization that use Apache Commons Net FTP client to identify vulnerable versions. 3. Where upgrading is not immediately feasible, implement network-level controls to restrict FTP connections to trusted servers only, using firewall rules or proxy filtering. 4. Employ network segmentation and monitoring to detect unusual FTP data connections initiated to unexpected internal hosts, which could indicate exploitation attempts. 5. Educate users and administrators about the risks of connecting to untrusted FTP servers, emphasizing the importance of verifying server authenticity. 6. Consider replacing FTP with more secure protocols such as SFTP or FTPS, which provide encryption and stronger authentication mechanisms. 7. Monitor vendor advisories and security bulletins for any updates or patches related to Apache Commons Net and apply them promptly. 8. Implement application-level logging to detect and analyze PASV responses that specify unexpected hosts, enabling early detection of potential attacks.