CVE-2025-64763

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.

Attack vector(s)

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

Credits

chasingimpact (Patrick)

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

CVE-2025-64763 has a CVSS score of 3.7 (Low). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.36.3, 1.35.7, 1.34.11, 1.33.13); upgrading removes the vulnerable code path.

Kodem intelligence

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

1. What is CVE-2025-64763? CVE-2025-64763 is a low-severity security vulnerability in github.com/envoyproxy/envoy (go), affecting versions >= 1.36.0, <= 1.36.2. It is fixed in 1.36.3, 1.35.7, 1.34.11, 1.33.13.
2. How severe is CVE-2025-64763? CVE-2025-64763 has a CVSS score of 3.7 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/envoyproxy/envoy are affected by CVE-2025-64763? github.com/envoyproxy/envoy (go) versions >= 1.36.0, <= 1.36.2 is affected.
4. Is there a fix for CVE-2025-64763? Yes. CVE-2025-64763 is fixed in 1.36.3, 1.35.7, 1.34.11, 1.33.13. Upgrade to this version or later.
5. Is CVE-2025-64763 exploitable, and should I be worried? Whether CVE-2025-64763 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-64763 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-64763?
   • Upgrade github.com/envoyproxy/envoy to 1.36.3 or later
   • Upgrade github.com/envoyproxy/envoy to 1.35.7 or later
   • Upgrade github.com/envoyproxy/envoy to 1.34.11 or later
   • Upgrade github.com/envoyproxy/envoy to 1.33.13 or later