CVE-2018-6332 in HHVM
Summary
by MITRE
A potential denial-of-service issue in the Proxygen handling of invalid HTTP2 settings which can cause the server to spend disproportionate resources. This affects all supported versions of HHVM (3.24.3 and 3.21.7 and below) when using the proxygen server to handle HTTP2 requests.
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Analysis
by VulDB Data Team • 05/06/2025
The vulnerability identified as CVE-2018-6332 represents a critical denial-of-service weakness within the Proxygen HTTP/2 implementation used by HHVM servers. This flaw specifically targets the handling of malformed HTTP/2 settings frames, creating a scenario where legitimate server resources become consumed disproportionately during request processing. The vulnerability affects all supported versions of HHVM including 3.24.3 and versions 3.21.7 and below, making it a widespread concern for organizations utilizing this server-side technology stack. The issue manifests when the proxygen server component encounters invalid HTTP/2 settings, leading to resource exhaustion that can ultimately result in service disruption.
The technical root cause of this vulnerability lies in the insufficient validation and error handling mechanisms within Proxygen's HTTP/2 protocol implementation. When processing HTTP/2 settings frames containing malformed or invalid parameters, the server fails to properly terminate or limit resource consumption during the parsing process. This improper handling creates a condition where the server continues to allocate memory and processing cycles in response to malformed input, potentially leading to memory exhaustion or excessive cpu utilization. The vulnerability operates at the protocol level, specifically targeting HTTP/2 settings frame processing within the proxygen component that serves as the HTTP/2 server implementation for HHVM. This represents a classic example of inadequate input validation that can be exploited to consume server resources without requiring authentication or specialized privileges.
The operational impact of CVE-2018-6332 extends beyond simple service disruption to encompass broader system stability and resource management concerns. Attackers can exploit this vulnerability by sending carefully crafted HTTP/2 requests containing invalid settings frames, causing the affected HHVM servers to consume excessive resources in processing these malformed requests. The resource consumption can escalate rapidly, potentially leading to complete service unavailability for legitimate users. This vulnerability particularly affects web applications and services that rely on HHVM's proxygen server for handling HTTP/2 traffic, making it a significant concern for high-traffic websites and applications that depend on efficient resource utilization. Organizations may experience cascading effects including increased latency, reduced throughput, and potential system crashes that can impact business continuity and user experience.
Mitigation strategies for CVE-2018-6332 primarily focus on updating affected HHVM installations to versions that contain patches addressing the HTTP/2 settings handling flaw. Organizations should prioritize immediate deployment of security updates provided by HHVM maintainers to resolve the vulnerability. Additionally, implementing network-level protections such as rate limiting and request filtering can help reduce the impact of exploitation attempts by limiting the volume of HTTP/2 requests that reach vulnerable servers. Monitoring systems should be enhanced to detect unusual resource consumption patterns that may indicate exploitation attempts. The vulnerability aligns with CWE-400, which addresses improper handling of resource consumption, and relates to ATT&CK technique T1499.001 for resource exhaustion attacks. Organizations should also consider implementing web application firewalls and HTTP/2 protocol validation mechanisms to provide additional layers of protection against malformed HTTP/2 traffic that could exploit this vulnerability.