CVE-2019-0199 in Instantis EnterpriseTrack
Summary
by MITRE
The HTTP/2 implementation in Apache Tomcat 9.0.0.M1 to 9.0.14 and 8.5.0 to 8.5.37 accepted streams with excessive numbers of SETTINGS frames and also permitted clients to keep streams open without reading/writing request/response data. By keeping streams open for requests that utilised the Servlet API's blocking I/O, clients were able to cause server-side threads to block eventually leading to thread exhaustion and a DoS.
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Analysis
by VulDB Data Team • 08/13/2024
The vulnerability identified as CVE-2019-0199 represents a critical denial of service weakness within Apache Tomcat's HTTP/2 protocol implementation. This flaw affects versions ranging from 9.0.0.M1 through 9.0.14 and 8.5.0 through 8.5.37, making it a widespread issue across multiple major release lines. The vulnerability stems from inadequate handling of HTTP/2 stream management and resource allocation, creating conditions where malicious clients can exploit the server's thread management mechanisms. The flaw specifically targets the server's ability to process concurrent connections and maintain proper resource utilization, fundamentally undermining the application server's operational integrity.
The technical implementation of this vulnerability exploits the HTTP/2 protocol's SETTINGS frame mechanism and stream lifecycle management within Tomcat's server architecture. Attackers can send excessive numbers of SETTINGS frames to individual streams while simultaneously maintaining those streams open without actively transmitting or processing request/response data. This behavior creates a resource exhaustion scenario where the server's thread pool becomes saturated with blocked threads. The vulnerability particularly leverages the Servlet API's blocking I/O operations, which are designed to wait for data transmission before proceeding. When clients maintain these streams open indefinitely, server threads remain occupied in waiting states, unable to process legitimate requests. This mechanism directly violates the expected behavior of HTTP/2 stream management and creates a predictable path to resource exhaustion.
The operational impact of CVE-2019-0199 extends beyond simple service disruption to represent a significant threat to application server availability and performance. Server-side thread exhaustion occurs when the configured thread pool reaches maximum capacity, preventing new legitimate requests from being processed. This creates cascading effects where the application server becomes unresponsive to all incoming traffic, effectively rendering services unavailable to authorized users. The vulnerability's exploitation requires minimal resources from the attacker, making it particularly dangerous as a low-effort denial of service vector. Organizations running affected Tomcat versions face potential business disruption, increased operational overhead from monitoring and response activities, and possible financial losses due to service unavailability. The attack can be executed from a single client connection, making detection and mitigation more challenging as it may appear as normal traffic patterns.
Mitigation strategies for CVE-2019-0199 focus on both immediate patching and temporary workaround implementations. The primary solution involves upgrading to Apache Tomcat versions 9.0.15 or 8.5.38, which contain the necessary code modifications to properly handle excessive SETTINGS frames and enforce proper stream lifecycle management. Organizations should implement comprehensive patch management procedures to ensure all affected systems receive updates promptly. Temporary mitigations include configuring connection limits, implementing rate limiting for SETTINGS frames, and adjusting thread pool configurations to reduce the impact of potential attacks. Network-level protections such as firewalls can be configured to limit concurrent connections and implement connection rate limiting. System administrators should also monitor thread pool utilization and implement alerting mechanisms to detect unusual resource consumption patterns that may indicate exploitation attempts. The vulnerability aligns with CWE-400, which addresses unchecked resource consumption, and maps to ATT&CK technique T1499.004 for resource exhaustion attacks, highlighting the need for both defensive and monitoring capabilities to address this threat effectively.