CVE-2020-11996 in Tomcat
Summary
by MITRE
A specially crafted sequence of HTTP/2 requests sent to Apache Tomcat 10.0.0-M1 to 10.0.0-M5, 9.0.0.M1 to 9.0.35 and 8.5.0 to 8.5.55 could trigger high CPU usage for several seconds. If a sufficient number of such requests were made on concurrent HTTP/2 connections, the server could become unresponsive.
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Analysis
by VulDB Data Team • 10/27/2020
This vulnerability in Apache Tomcat represents a significant denial of service risk that exploits weaknesses in the HTTP/2 protocol implementation. The flaw manifests when maliciously crafted sequences of HTTP/2 requests are transmitted to affected versions of the web server, specifically targeting releases from 10.0.0-M1 through 10.0.0-M5, 9.0.0.M1 through 9.0.35, and 8.5.0 through 8.5.55. The vulnerability operates through a specific pattern of request handling that causes the server to consume excessive computational resources, leading to elevated cpu utilization that can persist for several seconds. This behavior creates a cascading effect where multiple concurrent connections can overwhelm the server's processing capabilities, ultimately resulting in service unavailability.
The technical mechanism behind this vulnerability involves the improper handling of HTTP/2 frames and stream management within Tomcat's protocol implementation. When the server receives these specially crafted requests, it enters a processing loop that consumes disproportionate cpu cycles compared to normal request handling operations. The vulnerability is particularly dangerous because it requires minimal resources to execute, allowing attackers to create substantial load with relatively simple request sequences. The impact is amplified when multiple concurrent connections are utilized, as each connection contributes to the overall cpu consumption, potentially causing the server to become completely unresponsive to legitimate requests. This behavior aligns with common patterns identified in the CWE database under weakness category 400, which deals with resource exhaustion vulnerabilities, and specifically relates to improper handling of concurrent connections and resource management.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the availability of critical web applications hosted on affected Tomcat servers. Organizations relying on these vulnerable versions face significant risk of operational downtime, especially during peak traffic periods when multiple concurrent connections are common. The vulnerability affects the fundamental server responsiveness, making it difficult for legitimate users to access services while attackers can maintain the denial of service condition with minimal effort. This scenario creates a particularly challenging security landscape where the impact is not limited to a single application but can affect entire server infrastructure. The vulnerability also demonstrates the importance of proper protocol implementation and resource management, as outlined in various security frameworks and guidelines that emphasize the need for robust handling of concurrent operations and proper resource allocation.
Mitigation strategies for this vulnerability require immediate patching of affected Tomcat versions to the latest stable releases that contain the necessary fixes. Organizations should prioritize updating their Tomcat installations to versions that have addressed the HTTP/2 processing flaws, typically found in releases following the affected versions. Network-level protections such as rate limiting and connection throttling can provide temporary mitigation while patches are deployed, though these measures are not permanent solutions. Security monitoring should be enhanced to detect unusual cpu utilization patterns that may indicate exploitation attempts, and baseline performance metrics should be established to quickly identify when the server is being overwhelmed. Implementation of proper access controls and authentication mechanisms can also help reduce the attack surface, as the vulnerability requires direct access to the web server to exploit. Organizations should also consider implementing intrusion detection systems that can identify and alert on suspicious HTTP/2 request patterns, as this vulnerability specifically targets the HTTP/2 protocol implementation within the server stack. The remediation process should include comprehensive testing to ensure that the patch does not introduce compatibility issues with existing applications, and that normal server operations return to expected performance levels after the update is complete.