Envoy forwards early CONNECT data in TCP proxy mode

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2025-64763: 0.32% estimated exploitation activity in the next 30 days; percentile 25.41 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Forwarding of early CONNECT data in TCP proxy mode. Details Per RFC 7231-4.3.6 the sender of CONNECT (and all inbound proxies) switch to tunnel mode only after receiving 2xx response. However in TCP proxy mode, Envoy accepts client data before it has issued a 2xx response and eagerly proxies it to an established TCP connection. This creates possibility of a de-synchronized tunnel state if a proxy upstream from Envoy responds with a status other an 2xx. The RFC does not specify the behavior in case an early CONNECT data is received and early CONNECT data is common as a latency reduction mechanism. To prevent disruption to existing deployments Envoy will by default allow early CONNECT data. Setting the envoy.reloadablefeatures.rejectearlyconnectdata runtime flag to true will cause CONNECT requests that send data before 2xx response to be rejected. This options should be enabled if there are intermediaries upstream from Envoy that may reject establishment of a CONNECT tunnel.…

Published by the source

5 Dec 2025, 18:12 UTC

Last modified by the source

10 Dec 2025, 20:48 UTC

Source last checked

26 Oct 2026, 15:13 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-rj35-4m94-77jh

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# Patch Next advisory brief: CVE-2025-64763

Advisory: Envoy forwards early CONNECT data in TCP proxy mode
IDs: CVE-2025-64763, GHSA-rj35-4m94-77jh
Record: https://patchnext.com/advisories/cve-2025-64763 (revision 1, source checked 26 Oct 2026, 15:13 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/envoyproxy/envoy 1.36.3, 1.35.7, 1.34.11, 1.33.13. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/envoyproxy/envoy: affected >= 1.36.0, <= 1.36.2; >= 1.35.0, <= 1.35.6; >= 1.34.0, <= 1.34.10; <= 1.33.12; fixed in 1.33.13, 1.36.3, 1.35.7, 1.34.11

## Suggested inspection (read-only)
- Go github.com/envoyproxy/envoy: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/envoyproxy/envoy.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rj35-4m94-77jh (GitHub-reviewed advisory GHSA-rj35-4m94-77jh lists affected ranges: Go github.com/envoyproxy/envoy >=1.36.0 =1.35.0 =1.34.0 =0 <=1.33.12)

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