AIKIDO-2026-376044
h2 is vulnerable to HTTP Request Smuggling
53
Medium Risk
This Affects:
h2TL;DR
The h2 HTTP/2 stack accepts request header blocks that contain more than one Host header and forwards every value to the consuming application. When the consumer downgrades the HTTP/2 request to HTTP/1.1, the emitted request carries two Host header lines. That divergence between an intermediary and a backend is a request smuggling primitive that can be abused to bypass front-end security controls. The fix validates the authority in _validate_host_authority_header() and rejects header blocks that carry duplicate Host or :authority values by raising a protocol error.
Who does this affect?
You are affected if you are using a version that falls within the vulnerable range and your application forwards or downgrades the received HTTP/2 request to an HTTP/1.1 backend.
Background info
h2 is vulnerable to HTTP Request Smuggling in versions 0.0.1 - 4.4.0.
How to fix this
Upgrade the h2 library to the patch version.
Links
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