CVE-2026-73548

Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to 1.36.10, 1.37.6, 1.38.4, and 1.39.1, Envoy forwards data for a configured non-WebSocket HTTP upgrade before the upstream accepts the upgrade. An unauthenticated HTTP/2 client can place a complete HTTP/1.1 request in extended CONNECT data; Envoy downgrades the request, writes the data unframed to a keep-alive HTTP/1.1 upstream, and returns the socket to the shared pool while the smuggled response remains queued. A different downstream client can then receive the attacker's response. The relevant scope boundary is that webSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, and max_requests_per_connection set to 1 are not affected by the demonstrated path. This issue is fixed in versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1.

• CWE-444

• https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361
• https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b
• https://github.com/envoyproxy/envoy/commit/4633b8cce3d15b8734eefe232e30c12b0140b91c
• https://github.com/envoyproxy/envoy/commit/bd6711f2617658e28dfa3df3dace6bfe3cfc0766
• https://github.com/envoyproxy/envoy/releases/tag/v1.36.10
• https://github.com/envoyproxy/envoy/releases/tag/v1.37.6
• https://github.com/envoyproxy/envoy/releases/tag/v1.38.4
• https://github.com/envoyproxy/envoy/releases/tag/v1.39.1
• https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2
• https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2

Published

Sep 21, 2026

Last Modified

Sep 24, 2026

Status

Undergoing Analysis

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
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