CVE-2016-6817 in Tomcat
Summary
by MITRE
The HTTP/2 header parser in Apache Tomcat 9.0.0.M1 to 9.0.0.M11 and 8.5.0 to 8.5.6 entered an infinite loop if a header was received that was larger than the available buffer. This made a denial of service attack possible.
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Analysis
by VulDB Data Team • 10/15/2024
The vulnerability identified as CVE-2016-6817 represents a critical denial of service weakness in Apache Tomcat's HTTP/2 implementation that affects versions from 9.0.0.M1 through 9.0.0.M11 and 8.5.0 through 8.5.6. This flaw specifically manifests within the header parsing mechanism of the HTTP/2 protocol handler, where the system fails to properly manage buffer boundaries when processing incoming headers. The vulnerability stems from inadequate input validation and buffer management logic that allows maliciously crafted HTTP/2 headers to trigger unexpected behavior in the processing pipeline.
The technical root cause of this vulnerability lies in the improper handling of header data that exceeds the allocated buffer capacity during HTTP/2 parsing operations. When a header field surpasses the available buffer space, the parser enters an infinite loop condition rather than gracefully handling the overflow scenario. This occurs because the parsing algorithm lacks proper boundary checking and recovery mechanisms, causing the system to continuously attempt to process the oversized header without advancing the parsing state. The flaw operates at the application layer and specifically targets the HTTP/2 protocol implementation within the Tomcat server, making it particularly dangerous in environments where HTTP/2 is actively utilized.
The operational impact of this vulnerability is severe and directly translates to a denial of service condition that can effectively render the affected Tomcat server unavailable to legitimate users. An attacker can exploit this weakness by sending specially crafted HTTP/2 requests containing oversized headers, which causes the server to consume excessive CPU resources in the infinite loop scenario. The sustained resource consumption can lead to complete service unavailability, system performance degradation, and potential system crashes depending on the server configuration and resource constraints. This vulnerability affects organizations relying on HTTP/2 functionality and can be particularly devastating in high-traffic environments where service availability is critical.
Mitigation strategies for CVE-2016-6817 primarily involve upgrading to patched versions of Apache Tomcat where the HTTP/2 header parsing logic has been corrected to properly handle oversized headers and prevent infinite loop conditions. Organizations should also implement network-level protections such as rate limiting and header size restrictions at load balancers or reverse proxies to prevent exploitation. Additionally, monitoring systems should be configured to detect unusual CPU utilization patterns that may indicate exploitation attempts. The vulnerability aligns with CWE-835 which addresses infinite loops and the ATT&CK technique T1499.004 for network denial of service attacks. Organizations should also consider implementing web application firewalls and configuring proper input validation at multiple layers of their network infrastructure to provide defense in depth against similar vulnerabilities.