CVE-2026-73548: Envoyproxy Envoy

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

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.

Affected products

• Envoyproxy Envoy: before 1.36.10 (fixed in 1.36.10); from 1.37.0, before 1.37.6 (fixed in 1.37.6); from 1.38.0, before 1.38.4 (fixed in 1.38.4); from 1.39.0, before 1.39.1 (fixed in 1.39.1)

Published 2026-09-21. Last modified 2026-10-05.