CVE-2026-73548

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in. High risk of data exposure.

Exploit odds

0.5% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-08-12 20:53 UTC ReservedThe CVE id was reserved by its CNA
2. 2026-09-21 20:17 UTC PublishedDisclosed publicly on CVE.org and NVD
3. 2026-10-05 14:19 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.5% in the next 30 days (EPSS).
• Severity: 7.5 HIGH (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2026-09-21 20:17 UTC

Last modified

2026-10-05 14:19 UTC

Exploit probability

0.5% · EPSS, next 30 days

Weakness

CWE-444

Record status

Analyzed

Affected

• Envoyproxy Envoy < 1.36.10
• Envoyproxy Envoy < 1.37.6
• Envoyproxy Envoy < 1.38.4
• Envoyproxy Envoy < 1.39.1

Attack profile CVSS 3.1 · 7.5

Attack vector

Network

Complexity

Low

Privileges needed

None

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

None

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2026-73548.md and follow it. Check whether this project is affected by CVE-2026-73548. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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