CVE-2019-19271 in ProFTPD
Summary
by MITRE
An issue was discovered in tls_verify_crl in ProFTPD before 1.3.6. A wrong iteration variable, used when checking a client certificate against CRL entries (installed by a system administrator), can cause some CRL entries to be ignored, and can allow clients whose certificates have been revoked to proceed with a connection to the server.
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Analysis
by VulDB Data Team • 02/28/2024
The vulnerability identified as CVE-2019-19271 represents a critical flaw in the ProFTPD server's certificate revocation list validation mechanism, specifically within the tls_verify_crl function. This issue affects ProFTPD versions prior to 1.3.6 and stems from a fundamental programming error that undermines the security of TLS certificate validation. The flaw manifests when the server attempts to verify client certificates against a certificate revocation list that has been configured by system administrators to maintain security compliance. The improper iteration variable usage creates a logical error that prevents complete validation of the certificate revocation list entries.
The technical root cause of this vulnerability lies in a software defect where the iteration variable within the certificate validation loop is incorrectly assigned or incremented, causing the system to skip certain entries in the certificate revocation list. This misconfiguration means that when a client attempts to establish a TLS connection, the server fails to properly check all revoked certificates against the CRL, allowing connections to proceed even when the client's certificate has been explicitly revoked. The vulnerability essentially creates a bypass mechanism that undermines the intended security controls designed to prevent access by compromised or unauthorized certificates.
From an operational security perspective, this vulnerability presents a significant risk to organizations relying on ProFTPD for file transfer services with TLS encryption. Attackers who have obtained revoked certificates or those who can exploit this flaw can potentially establish unauthorized connections to the FTP server, bypassing the certificate validation controls that should prevent such access. The impact extends beyond simple unauthorized access, as it undermines the entire certificate-based authentication system and can lead to data exfiltration, privilege escalation, and further lateral movement within network environments where the compromised FTP server resides. This vulnerability directly violates the principle of least privilege and certificate validation integrity that forms the foundation of secure TLS implementations.
The flaw aligns with CWE-691, which addresses insufficient control flow management in security-critical code sections, and demonstrates how improper loop variable handling can create security vulnerabilities. From an ATT&CK framework perspective, this vulnerability maps to T1566 (Phishing) and T1071.004 (Application Layer Protocol: DNS) when attackers leverage compromised certificates, and potentially to T1562.001 (Resource Hijacking) if the vulnerability enables unauthorized access to file systems. Organizations should immediately implement mitigation strategies including upgrading to ProFTPD version 1.3.6 or later, which contains the corrected tls_verify_crl function. Additionally, system administrators should conduct thorough audits of their certificate management practices and ensure that all revoked certificates are properly accounted for in their CRL configurations. Network monitoring should be enhanced to detect unusual authentication patterns that might indicate exploitation attempts, and the principle of defense in depth should be reinforced through multiple layers of certificate validation and access control mechanisms.