CVE-2019-9517 in Enterprise Manager Ops Center
Summary
by MITRE • 01/25/2023
Some HTTP/2 implementations are vulnerable to unconstrained interal data buffering, potentially leading to a denial of service. The attacker opens the HTTP/2 window so the peer can send without constraint; however, they leave the TCP window closed so the peer cannot actually write (many of) the bytes on the wire. The attacker then sends a stream of requests for a large response object. Depending on how the servers queue the responses, this can consume excess memory, CPU, or both.
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Analysis
by VulDB Data Team • 01/15/2025
The vulnerability described in CVE-2019-9517 represents a critical denial of service weakness affecting HTTP/2 implementations across various software systems. This flaw exploits the fundamental mechanics of HTTP/2 flow control mechanisms, specifically targeting the disconnect between HTTP/2 level window management and underlying TCP window sizing. The vulnerability stems from how servers handle internal buffering of response data when processing concurrent requests, creating a scenario where attackers can manipulate resource consumption patterns to exhaustion.
The technical implementation of this vulnerability relies on manipulating the HTTP/2 flow control window sizes to create a specific attack vector. Attackers establish HTTP/2 windows that permit unlimited data transmission from the peer, effectively opening the floodgates for data flow at the application layer. However, they simultaneously maintain TCP windows in a closed state, preventing actual network transmission of data packets. This creates a scenario where the peer's internal buffers fill up with data that cannot be transmitted over the network connection, leading to resource exhaustion. The attack specifically targets the server's response queuing mechanisms, where incoming requests for large objects trigger substantial memory allocation and processing overhead.
The operational impact of CVE-2019-9517 extends beyond simple service disruption to encompass significant resource consumption patterns that can affect system stability and availability. When exploited, this vulnerability can cause servers to consume excessive memory resources as they queue responses for transmission, potentially leading to memory exhaustion and system crashes. Additionally, the CPU utilization spikes as servers process numerous concurrent requests and manage their internal buffering mechanisms. The vulnerability affects a broad range of HTTP/2 implementations including popular web servers, proxies, and application frameworks that handle HTTP/2 traffic. This makes it particularly dangerous as it can impact critical infrastructure components and applications that rely on HTTP/2 for performance optimization.
The underlying flaw aligns with CWE-400, which addresses unspecified resource exhaustion vulnerabilities, and demonstrates characteristics consistent with the attack pattern described in the MITRE ATT&CK framework under T1499.1 for network denial of service attacks. This vulnerability particularly affects systems that implement HTTP/2 without proper bounds checking on internal buffering mechanisms and insufficient monitoring of resource consumption patterns. The attack vector is particularly effective against implementations that do not properly validate the relationship between HTTP/2 window sizes and underlying TCP window states, creating a mismatch that allows for resource manipulation.
Mitigation strategies for CVE-2019-9517 require implementing proper flow control boundaries and resource monitoring within HTTP/2 implementations. System administrators should ensure that HTTP/2 servers enforce maximum buffer sizes for queued responses and implement connection-level resource limits to prevent excessive memory consumption. Network security teams should deploy monitoring solutions that track HTTP/2 flow control anomalies and unusual buffering patterns. Updates to affected software implementations should address the specific flow control window management issues, ensuring that HTTP/2 window adjustments are properly coordinated with underlying TCP window states. Organizations should also implement rate limiting mechanisms for HTTP/2 connections and establish automated alerts for resource consumption thresholds that could indicate exploitation attempts.