CVE-2025-64763: Envoy forwards early CONNECT data in TCP proxy mode
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.
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)
Other sources
Envoy is a high-performance edge/middle/service proxy. In 1.33.12, 1.34.10, 1.35.6, 1.36.2, and earlier, when Envoy is configured in TCP proxy mode to handle CONNECT requests, it accepts client data before issuing a 2xx response and forwards that data to the upstream TCP connection. If a forwarding proxy upstream from Envoy then responds with a non-2xx status, this can cause a de-synchronized CONNECT tunnel state. By default Envoy continues to allow early CONNECT data to avoid disrupting existing deployments. The envoy.reloadablefeatures.rejectearlyconnectdata runtime flag can be set to reject CONNECT requests that send data before a 2xx response when intermediaries upstream from Envoy may reject establishment of a CONNECT tunnel.
— MITRE
Affected Software
Remediation
Event History
Frequently Asked Questions
What is the severity of CVE-2025-64763?
CVE-2025-64763 is considered a critical vulnerability affecting Envoy versions up to 1.36.2.
How do I fix CVE-2025-64763?
To fix CVE-2025-64763, upgrade to Envoy version 1.36.3 or later.
What versions of Envoy are affected by CVE-2025-64763?
CVE-2025-64763 affects Envoy versions 1.33.12, 1.34.10, 1.35.6, and 1.36.2 and earlier.
What type of vulnerability is CVE-2025-64763?
CVE-2025-64763 is a security vulnerability in the TCP proxy mode of Envoy that allows data leakage.
What can attackers do with CVE-2025-64763?
Attackers can exploit CVE-2025-64763 to intercept and manipulate client data before a successful connection is established.