{"id":"CVE-2025-64763","aliases":["BIT-envoy-2025-64763","GHSA-rj35-4m94-77jh"],"url":"https://o3.security/vulnerability/CVE-2025-64763","summary":"Envoy forwards early CONNECT data in TCP proxy mode","details":"## Summary\n\nForwarding of early CONNECT data in TCP proxy mode.\n\n## Details\n\nPer [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.\n\nThe 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. \n\n## Impact\n\nDe-synchronization of CONNECT tunnel state if a forwarding proxy upstream from Envoy responds with a non 2xx status.\n\n## Attack vector(s)\nSending data for a CONNECT request before receiving 2xx response.\n\n## Patches\nUsers should upgrade to v1.36.3, v1.35.7, v1.34.11 or v1.33.13\n\n## Credits\n\n[chasingimpact](https://github.com/chasingimpact) (Patrick)","published":"2025-12-03T18:13:58.496Z","modified":"2026-08-12T03:51:12.516212238Z","cvss":{"score":3.7,"severity":"LOW","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Go","name":"github.com/envoyproxy/envoy","fixedVersion":"1.36.3"},{"ecosystem":"Go","name":"github.com/envoyproxy/envoy","fixedVersion":"1.35.7"},{"ecosystem":"Go","name":"github.com/envoyproxy/envoy","fixedVersion":"1.34.11"},{"ecosystem":"Go","name":"github.com/envoyproxy/envoy","fixedVersion":"1.33.13"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/64xxx/CVE-2025-64763.json"},{"type":"ADVISORY","url":"https://github.com/envoyproxy/envoy/security/advisories/GHSA-rj35-4m94-77jh"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-64763"},{"type":"PACKAGE","url":"https://github.com/envoyproxy/envoy"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:12.516212238Z"}}