Description
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
References10
- github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-… ExploitVendor Advisory
- github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-… ExploitVendor Advisory
- github.com/envoyproxy/envoy/commit/309855626966cff176155c821043… Patch
- github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d… Patch
- github.com/envoyproxy/envoy/commit/4633b8cce3d15b8734eefe232e30… Patch
- github.com/envoyproxy/envoy/commit/bd6711f2617658e28dfa3df3dace… Patch
- github.com/envoyproxy/envoy/releases/tag/v1.36.10 Release Notes
- github.com/envoyproxy/envoy/releases/tag/v1.37.6 Release Notes
- github.com/envoyproxy/envoy/releases/tag/v1.38.4 Release Notes
- github.com/envoyproxy/envoy/releases/tag/v1.39.1 Release Notes
Metrics
- CVSS 3.1
-
7.5
HIGH
Score source: CNA
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N - EPSS — exploit probability
- 0% percentile: 40 · as of 8 November 2026
- CWE
- CWE-444
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