CVE-2019-20894 in Traefik
Summary
by MITRE
Traefik 2.x, in certain configurations, allows HTTPS sessions to proceed without mutual TLS verification in a situation where ERR_BAD_SSL_CLIENT_AUTH_CERT should have occurred.
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Analysis
by VulDB Data Team • 07/03/2020
Traefik 2.x represents a modern reverse proxy and load balancer widely adopted in cloud-native environments and microservices architectures. The vulnerability described in CVE-2019-20894 manifests in specific configuration scenarios where the system fails to properly enforce mutual TLS authentication during HTTPS sessions. This flaw occurs when the proxy is configured to require client certificate authentication but does not adequately validate the certificate presented by clients. The technical implementation defect allows unauthorized clients to establish secure connections without proper client certificate verification, effectively bypassing a critical security control designed to ensure only trusted entities can access protected resources.
The vulnerability stems from improper handling of TLS handshake validation within Traefik's configuration parsing and certificate verification logic. When mutual TLS is enabled through configuration parameters, the system should validate that client certificates are properly signed by trusted certificate authorities and that the certificates meet the required security policies. However, in certain edge cases involving specific configuration combinations, the proxy fails to perform this validation, allowing connections to proceed despite missing or invalid client certificates. This behavior violates fundamental security principles of certificate-based authentication and creates a pathway for unauthorized access to services protected by mutual TLS requirements.
The operational impact of this vulnerability is significant within environments where mutual TLS serves as a primary security control for protecting sensitive APIs, internal services, or microservices communication. Attackers could exploit this weakness to establish secure connections to backend services that should only accept connections from specifically authorized clients. This scenario particularly affects organizations implementing zero-trust security models where client authentication is mandatory for accessing internal resources. The vulnerability essentially creates a false sense of security, as administrators might believe mutual TLS is properly enforced when it is not, potentially leading to unauthorized data access, service disruption, or lateral movement within the network infrastructure.
From a cybersecurity perspective, this vulnerability aligns with CWE-295 which addresses improper certificate validation and CWE-310 which covers cryptographic issues related to authentication. The flaw also maps to ATT&CK technique T1566 which involves credential harvesting through phishing or other means, as the compromised authentication mechanism could allow attackers to impersonate legitimate clients. Organizations using Traefik 2.x in production environments should immediately review their mutual TLS configurations and ensure that all client certificate validation is properly enforced. The recommended mitigation involves updating to patched versions of Traefik, implementing comprehensive configuration testing, and conducting security audits to verify that mutual TLS requirements are properly enforced across all service endpoints. Additionally, organizations should consider implementing additional monitoring and alerting mechanisms to detect anomalous connection patterns that might indicate exploitation attempts.