CVE-2016-2094 in HTTPS NIO Connector
Summary
by MITRE
The HTTPS NIO Connector allows remote attackers to cause a denial of service (thread consumption) by opening a socket and not sending an SSL handshake, aka a read-timeout vulnerability.
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Analysis
by VulDB Data Team • 02/03/2019
The vulnerability identified as CVE-2016-2094 represents a significant denial of service weakness within the HTTPS NIO Connector implementation of Apache Tomcat. This flaw specifically targets the connector's handling of SSL/TLS connections, creating a scenario where remote attackers can consume system resources through improper socket management. The vulnerability stems from the connector's inability to efficiently handle connections that initiate socket establishment but fail to complete the required SSL handshake process. When an attacker opens a socket connection without proceeding to send the necessary SSL handshake data, the system maintains the connection in a waiting state indefinitely, leading to thread exhaustion and subsequent service disruption.
From a technical perspective, this vulnerability operates at the network protocol level within the NIO (Non-blocking I/O) connector architecture, which is designed to handle multiple concurrent connections efficiently. The flaw manifests when the connector receives a socket connection but does not receive the required SSL handshake within the configured read timeout period. The system continues to maintain the connection state and associated threads, effectively creating a resource leak that can be exploited to exhaust available threads. This behavior aligns with CWE-400, which categorizes the vulnerability as an Uncontrolled Resource Consumption, specifically targeting thread resources in a multi-threaded environment. The root cause lies in the connector's lack of proper timeout handling and connection state management for incomplete SSL handshakes, creating a persistent resource drain that can be amplified through coordinated attacks.
The operational impact of CVE-2016-2094 extends beyond simple service disruption to encompass broader system stability and availability concerns. When exploited, the vulnerability allows attackers to consume all available threads in the connector's thread pool, rendering the web server unable to accept new connections or process legitimate requests. This creates a cascading effect that can impact not only the targeted application but potentially the entire server infrastructure. The attack requires minimal resources from the attacker's perspective, making it particularly dangerous as it can be executed through automated tools to overwhelm systems with relatively few concurrent connections. Organizations utilizing Apache Tomcat with HTTPS NIO Connector configurations become vulnerable to this attack vector, with the severity increasing based on the number of available threads and the system's overall connection handling capacity. The vulnerability also maps to ATT&CK technique T1499.004, which describes resource exhaustion attacks targeting network services, and specifically addresses the broader category of denial of service attacks that compromise system availability.
Mitigation strategies for CVE-2016-2094 primarily focus on implementing proper timeout configurations and connection management within the Apache Tomcat environment. The most effective immediate solution involves configuring appropriate read timeout values in the connector settings to ensure that incomplete SSL handshakes are automatically terminated after a predefined period. Administrators should also consider implementing connection limits and rate limiting mechanisms to prevent abuse of the vulnerable connector. Additionally, upgrading to patched versions of Apache Tomcat that address this specific vulnerability represents the most comprehensive long-term solution. Security hardening measures should include monitoring for unusual connection patterns and implementing intrusion detection systems that can identify potential exploitation attempts. Organizations should also consider implementing network-level controls such as firewall rules that limit the number of concurrent connections from individual IP addresses, providing an additional layer of defense against resource exhaustion attacks. The implementation of these mitigations aligns with security best practices outlined in the OWASP Top Ten and NIST cybersecurity frameworks, emphasizing the importance of proper resource management and connection handling in web application security.