CVE-2026-73548

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

Published: Sep 21, 2026Updated: Sep 23, 2026 Sources: CVE List NVDCWE-444

7.5HIGH

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.

No summary for this CVE yet.

CVSS Vector Breakdown

Exploitability

AV:NAttack Vector

Network

AC:LAttack Complexity

Low

PR:NPrivileges Required

None

UI:NUser Interaction

None

Scope

S:UScope

Unchanged

Impact

C:HConfidentiality

High

I:NIntegrity

None

A:NAvailability

None

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Weaknesses

CWE-444

Affected Products

envoyproxy

oss-project·USaka envoy proxy, envoy

envoyOn-prem

Cloud & SaaS / api-gateway

Exploitability

No known exploits, KEV entries, or remediation guidance available for this vulnerability yet.

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Attack Graph

Products CVE Techniques Tactics

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MITRE ATT&CK

1 technique

Collection

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References

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

github.com

https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361

github.com

https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b

github.com

and 6 more references View all →

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