CVE-2026-73548 — Envoy: Cross-user response poisoning via a generic (non-WebSocket) HTTP upgrade on Envoy's shared backend pool

Description (CNA)

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 (CNA)

Vendor / product	Versions
envoyproxy / envoy	affected < 1.36.10; affected >= 1.37.0, < 1.37.6; affected >= 1.38.0, < 1.38.4; affected >= 1.39.0, < 1.39.1

CWE

CWE-444

References

ADP enrichment

CISA-ADP

No English description supplied.

https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2