CVE-2019-9764 in Consul
Summary
by MITRE
HashiCorp Consul 1.4.3 lacks server hostname verification for agent-to-agent TLS communication. In other words, the product behaves as if verify_server_hostname were set to false, even when it is actually set to true. This is fixed in 1.4.4.
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Analysis
by VulDB Data Team • 08/08/2023
HashiCorp Consul version 1.4.3 contains a critical security flaw in its TLS communication implementation that undermines the integrity of agent-to-agent communications within distributed systems. This vulnerability specifically affects the server hostname verification mechanism that should enforce strict certificate validation during TLS handshakes between Consul agents. The flaw manifests as a configuration bypass where the system ignores the verify_server_hostname setting, effectively operating in a mode that disables hostname verification regardless of the administrator's explicit configuration choices. This behavior creates a significant attack surface where malicious actors can exploit man-in-the-middle scenarios to impersonate legitimate Consul servers without proper certificate validation. The vulnerability represents a direct violation of fundamental TLS security principles and demonstrates a failure in the cryptographic implementation that should ensure secure inter-agent communication.
The technical nature of this flaw stems from a software defect in the Consul agent's TLS configuration processing logic, where the verify_server_hostname parameter is not properly enforced during the TLS handshake process. This misconfiguration allows attackers to establish TLS connections with Consul agents using certificates that do not match the expected server hostnames, effectively creating a false sense of security for administrators who believe their systems are properly configured with strict hostname verification. The vulnerability exists at the application layer of the network stack and impacts all Consul agent communications, including service discovery, health checks, and configuration management operations. According to CWE classification, this represents a weakness in the cryptographic system related to improper certificate validation and hostname checking, specifically categorized under CWE-295 for improper certificate validation. The flaw enables attackers to perform certificate pinning bypasses and can facilitate unauthorized access to sensitive distributed system information.
The operational impact of this vulnerability extends beyond simple network security concerns to potentially compromise the entire Consul-based infrastructure. Attackers who can intercept or manipulate traffic between Consul agents can gain unauthorized access to service catalogs, health information, and configuration data that would normally be protected by proper TLS validation. This vulnerability particularly affects organizations using Consul for service discovery and configuration management in production environments where security is paramount. The attack surface is significant as it impacts all components that rely on Consul's agent-to-agent communication, including service mesh implementations, load balancing configurations, and distributed application deployments. The vulnerability's impact aligns with ATT&CK technique T1046 for network service scanning and T1566 for credential harvesting, as it enables attackers to establish unauthorized communication channels and potentially escalate privileges within the distributed system. Organizations using Consul in environments with strict security requirements face increased risk of data breaches and system compromise.
Organizations affected by this vulnerability should immediately upgrade to Consul version 1.4.4 or later, which properly implements the verify_server_hostname configuration parameter. The upgrade process should include thorough testing of all Consul agent configurations to ensure proper TLS certificate validation is functioning as intended. System administrators should conduct comprehensive audits of their Consul deployments to identify any configurations that may have been relying on the incorrect behavior of the vulnerability. Network monitoring should be enhanced to detect unusual communication patterns that might indicate exploitation attempts. Security teams should review certificate management practices to ensure all Consul agents are properly configured with valid certificates that match their hostnames. The vulnerability also highlights the importance of maintaining current security patches and implementing proper configuration management practices to prevent similar issues in other distributed systems. Organizations should consider implementing additional network-level security controls such as network segmentation and traffic filtering to limit the potential impact of any successful exploitation attempts. Regular security assessments of distributed system components should be conducted to identify and remediate similar configuration weaknesses that could compromise system integrity.