Bug 2418664 (CVE-2025-64763)

Summary: CVE-2025-64763 Envoy: Envoy: De-synchronized CONNECT tunnel state due to early data handling in TCP proxy mode
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in Envoy. This vulnerability allows a de-synchronized CONNECT tunnel state via accepting client data before issuing a 2xx response and forwarding it to the upstream TCP (Transmission Control Protocol) connection when configured in TCP (Transmission Control Protocol) proxy mode to handle CONNECT requests.
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Description OSIDB Bzimport 2025-12-03 19:02:09 UTC 
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.reloadable_features.reject_early_connect_data 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.