CVE-2019-15523 in csync2
Summary
by MITRE • 12/31/2020
An issue was discovered in LINBIT csync2 through 2.0. It does not correctly check for the return value GNUTLS_E_WARNING_ALERT_RECEIVED of the gnutls_handshake() function. It neglects to call this function again, as required by the design of the API.
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Analysis
by VulDB Data Team • 12/31/2020
The vulnerability in LINBIT csync2 versions up to 2.0 represents a critical security flaw in the cryptographic handshake process that undermines the integrity of secure communications. This issue stems from improper handling of the gnutls_handshake() function return value, specifically the GNUTLS_E_WARNING_ALERT_RECEIVED code which indicates that a warning alert was received during the TLS handshake process. The csync2 application fails to properly validate this specific return code and does not re-execute the handshake function as mandated by the gnutls library API design, creating a potential security bypass mechanism.
The technical flaw manifests in the application's failure to adhere to the GNU TLS library's documented API behavior where warning alerts must be handled through re-attempts of the handshake operation. When gnutls_handshake() returns GNUTLS_E_WARNING_ALERT_RECEIVED, the library design requires that the application call the function again to properly process the warning and determine whether to continue or abort the connection. This oversight allows malicious actors to potentially inject warning alerts that the application fails to properly evaluate, creating opportunities for man-in-the-middle attacks or authentication bypass scenarios.
This vulnerability directly impacts the operational security of systems relying on csync2 for file synchronization across networked environments, particularly those requiring secure communication channels. The improper handshake handling could enable attackers to downgrade security protocols or inject malicious content without proper authentication, as the application fails to validate the complete security handshake process. The implications extend to data integrity and confidentiality, as the synchronization process may proceed with compromised security guarantees, potentially allowing unauthorized access to synchronized files and system configurations.
The vulnerability aligns with CWE-295 which addresses improper certificate validation and CWE-310 which covers cryptographic issues in security protocols. From an ATT&CK framework perspective, this weakness maps to T1046 Network Service Scanning and T1566 Impersonation of Services, as it enables attackers to potentially establish unauthorized communication channels. The flaw also relates to T1552 Credential Access and T1071 Application Layer Protocol, since it undermines the cryptographic protections that should secure data transmission between synchronized systems. Organizations using csync2 should implement immediate mitigations including upgrading to patched versions, implementing additional network monitoring for anomalous handshake patterns, and ensuring proper certificate validation procedures are in place.
Mitigation strategies should prioritize immediate deployment of patched csync2 versions that properly handle the gnutls_handshake() return values according to the library specifications. System administrators should also implement comprehensive monitoring of TLS handshake operations and establish alerting mechanisms for warning alert occurrences. Additional security measures include verifying certificate pinning configurations, implementing network segmentation to limit exposure, and conducting thorough security audits of all synchronization processes. The vulnerability demonstrates the critical importance of proper API usage in cryptographic applications and highlights the need for comprehensive testing of security-critical code paths. Organizations should also review their incident response procedures to ensure rapid detection and remediation of similar issues in other cryptographic implementations.