High [CVE-2026-48743] Request desynchronization allows security policy bypass via HTTP/3 to HTTP/1 translation

Envoy is an open source edge and service proxy designed for cloud-native applications.

Prior to 1.35.11, 1.36.7, 1.37.3, and 1.38.1, Envoy can translate a downstream HTTP/3 request that is complete at the transport layer (HEADERS with FIN / headers-only close) but still carries a nonzero Content-Length into a complete upstream HTTP/1 request with unresolved body debt.

In an HTTP/1 upstream deployment where the origin replies before reading the declared body and keeps the connection reusable, the beginning of the next Envoy-generated upstream request can be consumed as the first request's body. The remaining bytes are then parsed by the origin as a new HTTP/1 request.

This was reproduced as a route-bypass/desync: direct /pwn was denied by Envoy, but the second downstream H3 stream received the response for backend-parsed GET /pwn HTTP/1.1. This vulnerability is fixed in 1.35.11, 1.36.7, 1.37.3, and 1.38.1.

This can lead to a request desynchronization, allowing an attacker to bypass security policies and access unauthorized resources. This is an Important flaw in Envoy, affecting Red Hat OpenShift Service Mesh and cloud.redhat.com deployments.

The vulnerability arises from improper handling of HTTP/3 to HTTP/1 request translation, where a complete HTTP/3 request with a non-zero Content-Length can lead to request desynchronization on the upstream HTTP/1 server.