CVE-2019-9518 in HTTP2
Summary
by MITRE
Some HTTP/2 implementations are vulnerable to a flood of empty frames, potentially leading to a denial of service. The attacker sends a stream of frames with an empty payload and without the end-of-stream flag. These frames can be DATA, HEADERS, CONTINUATION and/or PUSH_PROMISE. The peer spends time processing each frame disproportionate to attack bandwidth. This can consume excess CPU.
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Analysis
by VulDB Data Team • 01/12/2026
The vulnerability identified as CVE-2019-9518 represents a critical denial of service weakness in HTTP/2 protocol implementations that affects the core processing mechanisms of web servers and clients. This flaw exploits the fundamental design of HTTP/2's frame-based communication model where each frame must be individually processed regardless of its payload content. The vulnerability specifically targets the handling of empty frames that lack meaningful data but still require processing overhead, creating a scenario where malicious actors can consume disproportionate system resources through seemingly innocuous network traffic patterns. The issue manifests when implementations fail to properly optimize frame processing for empty payloads, leading to inefficient resource utilization during legitimate network operations.
The technical exploitation of this vulnerability occurs through the deliberate construction and transmission of multiple consecutive frames containing empty payloads without the end-of-stream flag being set. These frames can take various forms including DATA frames that carry no actual data, HEADERS frames with empty header blocks, CONTINUATION frames that continue header blocks, and PUSH_PROMISE frames that initiate server push operations without meaningful content. Each frame, regardless of payload size, requires the receiving implementation to perform validation, parsing, and processing operations that consume CPU cycles and memory resources. The protocol's design does not distinguish between frames with empty payloads and those with substantial data, creating an inherent inefficiency that attackers can exploit to create resource exhaustion conditions.
The operational impact of CVE-2019-9518 extends beyond simple service disruption to encompass significant performance degradation and resource consumption that can affect entire network infrastructure. When exploited, this vulnerability allows attackers to consume CPU resources at rates that far exceed the bandwidth of the attack traffic itself, creating a scenario where a relatively small amount of malicious traffic can generate enormous processing overhead. This characteristic makes the vulnerability particularly dangerous in environments where HTTP/2 is heavily utilized, such as modern web applications, content delivery networks, and enterprise infrastructure. The resource consumption patterns align with attack techniques documented in the attack matrix under the category of resource exhaustion attacks, specifically targeting the computational resources of network endpoints rather than network bandwidth.
Implementation-specific mitigations for this vulnerability typically involve introducing rate limiting mechanisms, frame processing optimizations, and buffer management strategies that prevent the accumulation of empty frames that would otherwise consume processing resources. Security practitioners should implement frame size limits, connection-level flow control adjustments, and processing timeouts that prevent indefinite frame handling. The vulnerability demonstrates the importance of proper protocol implementation practices and adherence to security guidelines established by organizations such as the Internet Engineering Task Force. This issue represents a failure in protocol compliance related to the proper handling of edge cases in HTTP/2 implementations, and aligns with common weaknesses identified in CWE-400 which covers unchecked resource consumption in network protocols. Organizations should prioritize updating their HTTP/2 implementations to versions that include proper frame processing optimizations and implement network monitoring to detect anomalous frame patterns that may indicate exploitation attempts.
The broader implications of this vulnerability highlight the challenges inherent in implementing complex protocols that must balance performance optimization with security considerations. HTTP/2's frame-based architecture, while providing significant performance benefits, creates opportunities for resource exhaustion attacks when implementations fail to account for edge cases in frame handling. The vulnerability serves as a reminder that protocol implementations must consider not just the typical usage patterns but also potential malicious exploitation vectors that could leverage protocol design characteristics to create security issues. This particular weakness demonstrates how seemingly benign protocol features can become security liabilities when not properly secured through defensive programming practices and comprehensive testing of edge cases.