CVE-2026-93568 in AMQ Broker
Summary
by MITRE • 09/18/2026
HTTP/2 and HTTP/3 Extended CONNECT requests are downgraded as regular CONNECT requests
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability described involves a protocol downgrade attack affecting the handling of extended CONNECT methods in HTTP/2 and HTTP/3 environments. In standard web architecture, the CONNECT method is traditionally used to establish tunnels for secure communications such as HTTPS or WebSocket connections through an intermediary proxy server. However, modern extensions to these protocols allow for more complex request structures that can carry additional metadata or utilize different framing mechanisms than traditional HTTP requests. When a client sends an extended CONNECT request intended to leverage specific features of HTTP/2 or HTTP/3, the vulnerable system fails to recognize the full scope of this extension and instead processes it as a legacy-style regular CONNECT request. This misclassification represents a significant deviation from expected protocol behavior because it strips away the enhanced capabilities that define the newer versions of these protocols.
From a technical perspective, this flaw stems from insufficient validation or parsing logic within the server-side implementation when interpreting incoming connection requests. The system likely relies on heuristic checks rather than strict adherence to RFC specifications for HTTP/2 and HTTP/3 extended methods. By downgrading the request type, the server ignores any additional parameters, headers, or framing instructions embedded in the extended CONNECT payload. This behavior effectively forces the communication channel into a more restrictive mode that may not support features like multiplexing efficiency optimizations, header compression variations, or specific tunneling requirements defined by newer protocol standards. The root cause is typically categorized under CWE-693, which refers to Protection Mechanism Failure, specifically where the system fails to enforce intended security controls due to incorrect interpretation of input data types.
The operational impact of this vulnerability can vary depending on the deployment context but generally results in degraded performance and potential functional failures for applications relying on advanced HTTP/2 or HTTP/3 features. Clients expecting full protocol support may experience timeouts, connection resets, or incomplete data transmission because the server does not acknowledge the extended capabilities required by modern web frameworks. Furthermore, this behavior can be exploited to bypass security policies that are tied specifically to extended request types. For instance, if an organization has implemented access controls or filtering rules based on specific HTTP/3 extensions for traffic inspection or load balancing, these mechanisms may fail silently as the requests appear as standard CONNECTs, thereby evading detection and control measures designed to monitor advanced protocol usage.
This issue also intersects with ATT&CK technique T1071, Application Layer Protocol: Web Protocols, where adversaries might exploit misconfigured servers to tunnel malicious traffic through allowed ports without triggering alerts associated with non-standard protocols. By forcing the connection into a legacy mode, attackers could potentially hide command-and-control communications or data exfiltration attempts within what appears to be normal web proxy activity. The lack of proper protocol enforcement creates an ambiguity that security tools often struggle to distinguish from legitimate configuration errors, making detection more difficult for network monitoring systems that rely on clear protocol signatures.
Mitigation strategies should focus on ensuring strict compliance with RFC 9113 for HTTP/2 and RFC 9114 for HTTP/3 regarding the handling of extended methods. Server administrators must update their proxy software or web servers to versions that correctly parse and process extended CONNECT requests according to specification rather than defaulting to legacy behavior. It is critical to verify that configuration files do not explicitly disable support for newer protocol features under the guise of compatibility mode unless absolutely necessary. Additionally, implementing rigorous input validation at the application layer can help detect anomalous request patterns before they reach the core protocol handler. Regular security audits and penetration testing should include scenarios designed to test protocol version negotiation and method handling to ensure that extended requests are not inadvertently downgraded or ignored by intermediate infrastructure components.