NIST deprecates RSA-2048 from 2030. Find every one of them before attackers do.
🐹
🐹 Go
Not in CISA KEV
LOW severity

CVE-2025-64763 — envoy

LOW

CVE-2025-64763 is a low-severity (CVSS 3.7) CWE-693 vulnerability in github.com/envoyproxy/envoy. A fix is available for github.com/envoyproxy/envoy — see the affected versions and patch details below.

Envoy forwards early CONNECT data in TCP proxy mode

Also known asBIT-envoy-2025-64763GHSA-rj35-4m94-77jh
Published
Updated
Affected
4 pkgs
Patched
4 / 4
Exploits
None indexed
Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2025-64763.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs23th percentile — riskier than 23% of all scored CVEsHighest risk
0.00%0.27%0.54%0.81%0.0%0.3%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

4 pkgs affected
🐹github.com/envoyproxy/envoy🐹github.com/envoyproxy/envoy🐹github.com/envoyproxy/envoy🐹github.com/envoyproxy/envoy

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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.reloadable_features.reject_early_connect_data 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.

Impact

De-synchronization of CONNECT tunnel state if a forwarding proxy upstream from Envoy responds with a non 2xx status.

Attack vector(s)

Sending data for a CONNECT request before receiving 2xx response.

Patches

Users should upgrade to v1.36.3, v1.35.7, v1.34.11 or v1.33.13

Credits

chasingimpact (Patrick)

Affected Packages

4 total 4 fixed
EcosystemPackageVulnerable rangeFix
🐹Gogithub.com/envoyproxy/envoy≥ 1.36.0&&< 1.36.31.36.3go get github.com/envoyproxy/envoy@v1.36.3
🐹Gogithub.com/envoyproxy/envoy≥ 1.35.0&&< 1.35.71.35.7go get github.com/envoyproxy/envoy@v1.35.7
🐹Gogithub.com/envoyproxy/envoy≥ 1.34.0&&< 1.34.111.34.11go get github.com/envoyproxy/envoy@v1.34.11
🐹Gogithub.com/envoyproxy/envoyall versions1.33.13go get github.com/envoyproxy/envoy@v1.33.13

Affected Products

1 product · 4 configurations
Application
envoyenvoyproxy
≥ 1.36.0 && < 1.36.3
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/envoyproxy/envoy, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update github.com/envoyproxy/envoy to 1.36.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-64763 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This vulnerability is rated Moderate for Red Hat. Envoy, when operating in TCP proxy mode for CONNECT requests, forwards client data before a 2xx response. This can lead to a de-synchronized CONNECT tunnel state if an upstream proxy rejects the tunnel establishment. This behavior is enabled by default in affected…

Workaround published by Red Hat
To mitigate this issue, set the `envoy.reloadable_features.reject_early_connect_data` runtime flag to reject CONNECT requests that send data before a 2xx response. This configuration change may require a service reload or restart to take effect.
Source: Red Hat security advisory for CVE-2025-64763 (CC BY 4.0)

Frequently Asked Questions

## Summary Forwarding of early CONNECT data in TCP proxy mode. ## Details Per [RFC 7231-4.3.6](https://www.rfc-editor.org/rfc/rfc7231#section-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
O3 Security · Impact-Aware SCA

Is CVE-2025-64763 in your dependencies?

Find it across Go, including transitive dependencies.

Scan my dependencies How O3 SCA works
CVE-2025-64763: envoy — Fixed in 1.36.3