CVE-2009-3639 in ProFTPD
Summary
by MITRE
The mod_tls module in ProFTPD before 1.3.2b, and 1.3.3 before 1.3.3rc2, when the dNSNameRequired TLS option is enabled, does not properly handle a \0 character in a domain name in the Subject Alternative Name field of an X.509 client certificate, which allows remote attackers to bypass intended client-hostname restrictions via a crafted certificate issued by a legitimate Certification Authority, a related issue to CVE-2009-2408.
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Analysis
by VulDB Data Team • 02/16/2025
The vulnerability described in CVE-2009-3639 represents a critical flaw in the mod_tls module of ProFTPD software that affects versions prior to 1.3.2b and 1.3.3rc2. This issue specifically targets the TLS certificate validation mechanism when the dNSNameRequired option is enabled, creating a security bypass condition that undermines the intended hostname verification process. The flaw exploits how the system processes domain names containing null characters within the Subject Alternative Name field of X.509 client certificates, allowing malicious actors to craft certificates that appear valid to the server while actually circumventing security controls.
The technical implementation of this vulnerability stems from improper input validation within the certificate parsing logic of ProFTPD's mod_tls module. When a certificate contains a null character within a domain name field in the Subject Alternative Name section, the validation routine fails to properly sanitize or reject such malformed entries. This parsing error occurs during the certificate verification phase when the server attempts to match the presented certificate against the expected hostname constraints. The null character effectively disrupts the string comparison operations, causing the system to accept certificates that should be rejected based on hostname mismatch criteria.
The operational impact of this vulnerability is significant as it enables remote attackers to bypass client-hostname restrictions that are fundamental to secure TLS communications. An attacker with access to a legitimate Certification Authority's certificate chain can craft a malicious certificate containing a null character in the domain name field, thereby circumventing the dNSNameRequired validation that should enforce strict hostname matching. This creates a scenario where a certificate issued by a trusted authority can be used to authenticate as any hostname, effectively breaking the trust model that TLS is designed to maintain. The vulnerability operates at the application layer and can be exploited without requiring elevated privileges or special access conditions.
This vulnerability aligns with CWE-20, which describes improper input validation, and represents a specific instance of how malformed data can be used to bypass security controls. The issue also maps to ATT&CK technique T1552.001, which covers unsecured credentials through credential access, as the flaw enables unauthorized certificate validation bypass that can lead to authentication impersonation. The vulnerability is particularly concerning in environments where strict certificate validation is enforced as a security control, as it undermines the entire certificate-based authentication mechanism. Organizations using ProFTPD with TLS enabled should immediately implement patches to address this vulnerability, as the impact extends beyond simple authentication bypass to potentially enable more sophisticated attacks including man-in-the-middle scenarios and unauthorized access to sensitive file systems. The related CVE-2009-2408 highlights similar issues in certificate handling that underscore the broader category of TLS certificate validation flaws affecting FTP servers and other network services.