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CVE-2026-73548

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

HIGH
7.5
CVSS Score
Published: September 21, 2026 at 07:35 PM UTC
Updated: October 5, 2026 at 02:19 PM UTC
1 product affected across 1 vendorEnvoy

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Severity
HIGH7.5
Exploit prediction
EPSS 0.5%
SSVC
PoC

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

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Envoy· Envoyproxy
95%
v < 1.36.10v < 1.37.6v < 1.38.4v < 1.39.11.37.0 ≤ v < 1.37.61.38.0 ≤ v < 1.38.41.39.0 ≤ v < 1.39.1
Affected range stops below 1.36.10, so it looks safe — our inference, not a vendor statement
Affected range stops below 1.37.6, so it looks safe — our inference, not a vendor statement
Affected range stops below 1.38.4, so it looks safe — our inference, not a vendor statement
Affected range stops below 1.39.1, so it looks safe — our inference, not a vendor statement

References (9)

x_refsource_CONFIRMExploitVendor Advisory
Source: cverecord
https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2
x_refsource_MISCPatch
Source: cverecord
https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361
x_refsource_MISCPatch
Source: cverecord
https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b
x_refsource_MISCPatch
Source: cverecord
https://github.com/envoyproxy/envoy/commit/4633b8cce3d15b8734eefe232e30c12b0140b91c
x_refsource_MISCPatch
Source: cverecord
https://github.com/envoyproxy/envoy/commit/bd6711f2617658e28dfa3df3dace6bfe3cfc0766
x_refsource_MISCRelease Notes
Source: cverecord
https://github.com/envoyproxy/envoy/releases/tag/v1.36.10
x_refsource_MISCRelease Notes
Source: cverecord
https://github.com/envoyproxy/envoy/releases/tag/v1.37.6
x_refsource_MISCRelease Notes
Source: cverecord
https://github.com/envoyproxy/envoy/releases/tag/v1.38.4

SSVC — Action Prioritization

SSVCPoCAutomatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

7.5HIGHGitHub_M
Attack Vector
NETWORK
Complexity
LOW
Privileges
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
HIGH
Integrity
NONE
Availability
NONE
Vector?

Quick Info

StatusANALYZED
CWE
CWE-444

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