CVE-2025-8671 in Fastly H20 (MadeYouReset)
Summary
by MITRE • 08/13/2025
A mismatch caused by client-triggered server-sent stream resets between HTTP/2 specifications and the internal architectures of some HTTP/2 implementations may result in excessive server resource consumption leading to denial-of-service (DoS). By opening streams and then rapidly triggering the server to reset them—using malformed frames or flow control errors—an attacker can exploit incorrect stream accounting. Streams reset by the server are considered closed at the protocol level, even though backend processing continues. This allows a client to cause the server to handle an unbounded number of concurrent streams on a single connection. This CVE will be updated as affected product details are released.
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Analysis
by VulDB Data Team • 01/21/2026
This vulnerability represents a critical design flaw in HTTP/2 implementation architectures that stems from a fundamental mismatch between protocol specifications and internal processing mechanisms. The issue manifests when clients deliberately trigger server-sent stream resets through malformed frames or flow control errors, creating a scenario where the server's internal stream accounting becomes inconsistent with the actual protocol state. According to the CWE catalog, this maps to CWE-400: Uncontrolled Resource Consumption, specifically involving improper handling of stream lifecycle management within HTTP/2 servers. The vulnerability exploits the discrepancy between the HTTP/2 RFC 7540 specification which defines stream states and reset behavior, and how various implementations internally track and manage these streams.
The operational impact of this vulnerability is severe as it enables a single client connection to potentially exhaust server resources through a carefully crafted sequence of stream operations. When servers reset streams, they should ideally decrement their internal stream counters and release associated resources, but in affected implementations, the reset mechanism fails to properly account for backend processing that continues even after protocol-level closure. This creates a state where the server maintains an artificially inflated count of active streams while simultaneously continuing to process the underlying work, leading to exponential resource consumption. The attack vector specifically targets flow control mechanisms and frame validation, making it particularly insidious as it can be executed with minimal network overhead while maximizing computational impact.
The exploitation pattern involves opening multiple streams and rapidly triggering resets through malformed HTTP/2 frames or by manipulating flow control windows to force server-side resets. This creates a condition where the server's internal stream tracking system becomes desynchronized from actual resource usage, allowing an attacker to maintain an unbounded number of concurrent streams on a single connection. The vulnerability is particularly dangerous because it can be executed with relatively simple client-side code and does not require elevated privileges or specialized tools. Security frameworks such as MITRE ATT&CK categorize this under T1499.004: Endpoint Denial of Service, specifically targeting resource exhaustion through protocol manipulation. The DoS condition occurs because the server's resource allocation mechanisms are bypassed through the stream accounting discrepancy, causing memory and CPU resources to be consumed without proper cleanup.
Mitigation strategies should focus on implementing robust stream accounting mechanisms that properly synchronize protocol-level stream states with internal processing states. Servers should enforce stricter validation of incoming frames and implement rate limiting for stream reset operations to prevent rapid succession of reset triggers. Additionally, implementing proper resource cleanup procedures that occur immediately upon protocol-level stream closure, regardless of backend processing status, will prevent the accumulation of phantom streams. Network administrators should consider implementing connection-level rate limiting and monitoring for unusual stream reset patterns. The fix typically involves updating HTTP/2 implementations to properly handle stream accounting during reset operations and ensuring that all internal resource tracking aligns with protocol specification requirements. Organizations should prioritize patching affected implementations as soon as vendor updates become available, as this vulnerability can be exploited remotely without authentication and can cause significant service disruption.