CVE-2026-100664 in Netty
Summary
by MITRE • 09/26/2026
Netty's HTTP/3 codec (io.netty:netty-codec-http3) versions 4.2.2.Final through 4.2.17.Final builds the HTTP/3 :authority pseudo-header from the HTTP/1 Host header before considering the authority of an absolute-form HTTP/1 request-target. In HttpConversionUtil.toHttp3Headers(HttpMessage, boolean) — reached via Http3FrameToHttpObjectCodec(false) — a non-empty Host header takes precedence over the request-target authority, contrary to the HTTP/1.1 rule that a server receiving an absolute-form request-target must ignore the Host header. In a Netty-based HTTP/1-to-HTTP/3 gateway, proxy, or protocol bridge, a remote client can send a request such as "GET https://trusted.example/admin HTTP/1.1" with "Host: attacker.example", causing components that validate, authorize, or route on the RFC-defined request-target authority to reach a different decision than the upstream HTTP/3 peer, which receives :authority derived from the conflicting Host header. This authority confusion can affect virtual-host routing, allow-list checks, backend selection, cache keys, and URL generation. The advisory reports integrity impact only (no code execution, memory corruption, or availability impact). Fixed in 4.2.18.Final.
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Analysis
by VulDB Data Team • 09/26/2026
The vulnerability identified in Netty's HTTP/3 codec within versions 4.2.2.Final through 4.2.17.Final represents a critical protocol compliance failure that undermines the integrity of request routing and access control mechanisms in systems acting as gateways, proxies, or bridges between HTTP/1.1 and HTTP/3 protocols. The core technical flaw resides within the HttpConversionUtil.toHttp3Headers method, which is invoked by the Http3FrameToHttpObjectCodec when processing incoming requests. In this conversion process, the implementation incorrectly prioritizes the value of the HTTP/1.1 Host header over the authority component extracted from an absolute-form request-target URI. This behavior directly contradicts RFC 7230 Section 5.3.2, which explicitly mandates that a server receiving an absolute-form request must ignore any Host header field present in the message and instead derive the target authority solely from the request-target itself. By failing to adhere to this standard, the codec allows a malicious client to manipulate the :authority pseudo-header sent upstream to HTTP/3 peers by supplying a deceptive Host header that differs from the actual destination specified in the URL path of the request.
This discrepancy creates a significant attack surface for authority confusion attacks, where downstream components make security decisions based on conflicting information depending on which protocol layer they inspect. For instance, an application firewall or access control list might validate permissions against the original HTTP/1.1 request-target authority, such as trusted.example, while the upstream HTTP/3 service processes the :authority pseudo-header derived from the attacker-controlled Host header, such as attacker.example. This divergence can lead to successful bypasses of virtual-host routing rules, incorrect backend server selection in load balancing scenarios, and misconfigured cache key generation that may serve stale or malicious content intended for a different host. Furthermore, URL generation logic within the application layer might produce links pointing to the wrong domain if it relies on the corrupted authority information propagated through the HTTP/3 headers. The impact is strictly limited to integrity violations related to routing and access control decisions rather than code execution or memory corruption, yet these logical flaws can facilitate unauthorized data access or service disruption by redirecting traffic to unintended destinations.
From a classification perspective, this vulnerability aligns with CWE-698, which describes the use of a request-target that does not match the intended server authority, and falls under MITRE ATT&CK technique T1071, specifically Application Layer Protocol: Web Protocols, where attackers exploit protocol ambiguities to evade detection or manipulate system behavior. The root cause is a failure in input validation and protocol adherence during format conversion, highlighting the risks associated with maintaining compatibility layers between different HTTP versions without strict enforcement of RFC standards. To mitigate this risk, organizations running Netty-based infrastructure must immediately upgrade to version 4.2.18.Final or later, where the logic has been corrected to prioritize the request-target authority over the Host header in accordance with HTTP/1.1 specifications. Until such an update is applied, administrators should consider implementing strict validation rules at the proxy layer that reject requests containing absolute-form URIs accompanied by mismatching Host headers, thereby enforcing consistency before the traffic reaches the vulnerable codec component.