---
id: CVE-2026-73548
title: >-
  Envoy is an open source edge and service proxy designed for cloud-native
  applications
summary: >-
  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.…
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N'
cwe:
  - CWE-444
vendor: envoyproxy
product: envoy
affected:
  - envoy < 1.36.10
  - 'envoy >= 1.37.0, < 1.37.6'
  - 'envoy >= 1.38.0, < 1.38.4'
  - 'envoy >= 1.39.0, < 1.39.1'
published: '2026-09-21'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T23:18:58.353'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-73548'
references:
  - url: >-
      https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361
    label: security-advisories@github.com
  - url: >-
      https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b
    label: security-advisories@github.com
  - url: >-
      https://github.com/envoyproxy/envoy/commit/4633b8cce3d15b8734eefe232e30c12b0140b91c
    label: security-advisories@github.com
  - url: >-
      https://github.com/envoyproxy/envoy/commit/bd6711f2617658e28dfa3df3dace6bfe3cfc0766
    label: security-advisories@github.com
  - url: 'https://github.com/envoyproxy/envoy/releases/tag/v1.36.10'
    label: security-advisories@github.com
  - url: 'https://github.com/envoyproxy/envoy/releases/tag/v1.37.6'
    label: security-advisories@github.com
  - url: 'https://github.com/envoyproxy/envoy/releases/tag/v1.38.4'
    label: security-advisories@github.com
  - url: 'https://github.com/envoyproxy/envoy/releases/tag/v1.39.1'
    label: security-advisories@github.com
  - url: >-
      https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2
    label: security-advisories@github.com
  - url: >-
      https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - cve.org
  - exploit-available
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-09-24T22:34:49.061262Z'
epss: 0.00665
epssPercentile: 0.49617
ingestedAt: '2026-09-21T19:51:58.874Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.