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Vulnerability record · CVE-2026-73548 · published 21 September 2026

CVE-2026-73548: HTTP request smuggling vulnerability

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.

7.5 CVSS 3.1 High EPSS 0.48% · top 61.1% CWE-444 · HTTP request smuggling Undergoing Analysis
7.5CVSS 3.1 base score
0.48%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
10References
24 Sep 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

References

LinkTags
https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361
https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b
https://github.com/envoyproxy/envoy/commit/4633b8cce3d15b8734eefe232e30c12b0140b91c
https://github.com/envoyproxy/envoy/commit/bd6711f2617658e28dfa3df3dace6bfe3cfc0766
https://github.com/envoyproxy/envoy/releases/tag/v1.36.10
https://github.com/envoyproxy/envoy/releases/tag/v1.37.6
https://github.com/envoyproxy/envoy/releases/tag/v1.38.4
https://github.com/envoyproxy/envoy/releases/tag/v1.39.1
https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2
https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2

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Source: NIST National Vulnerability Database (record CVE-2026-73548), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.