CVE-2025-64763 is a vulnerability identified in the Envoy proxy software, specifically affecting versions up to 1.36.2. Envoy is widely used as a high-performance edge, middle, and service proxy in modern cloud-native environments. The vulnerability occurs when Envoy is configured in TCP proxy mode to handle HTTP CONNECT requests. In this mode, Envoy accepts and forwards client data to the upstream TCP connection before issuing a 2xx success response. If an upstream forwarding proxy rejects the CONNECT request with a non-2xx status, Envoy's early forwarding of client data causes a de-synchronization of the CONNECT tunnel state. This de-synchronization can lead to inconsistent protocol states between Envoy and upstream proxies, potentially causing connection disruptions or unexpected behavior in the proxy chain. The root cause is a protection mechanism failure (CWE-693) where Envoy does not properly validate the connection establishment sequence before forwarding data. By default, Envoy allows early CONNECT data to maintain backward compatibility with existing deployments. However, a runtime flag, envoy.reloadable_features.reject_early_connect_data, can be enabled to reject CONNECT requests that send data before a 2xx response, preventing the state desynchronization. The CVSS v3.1 base score is 3.7, reflecting a low severity due to the vulnerability's limited impact on confidentiality, integrity, and availability, and the requirement for specific proxy configurations. No public exploits or active attacks have been reported. The vulnerability primarily affects environments where Envoy is used as a TCP proxy handling CONNECT requests with upstream proxies that may reject connections. This scenario is common in complex proxy chains or service mesh architectures.

For European organizations, the impact of CVE-2025-64763 is generally low but context-dependent. Organizations using Envoy as a TCP proxy in service meshes, cloud-native applications, or edge proxy deployments may experience connection disruptions or degraded service reliability if upstream proxies reject CONNECT requests and early client data is forwarded. This could lead to intermittent failures in establishing TCP tunnels, affecting applications relying on CONNECT-based proxying such as HTTPS proxying or tunneling protocols. While the vulnerability does not directly compromise data confidentiality or integrity, the desynchronized tunnel state could cause operational issues, increased troubleshooting complexity, and potential denial of service in proxy chains. Organizations with strict compliance or high availability requirements should consider this risk more seriously. The lack of known exploits reduces immediate threat, but the vulnerability could be leveraged in targeted attacks to disrupt proxy services. European cloud service providers, telecom operators, and enterprises with complex proxy infrastructures are most at risk. The impact on availability and operational continuity is the primary concern rather than data breach or privilege escalation.

To mitigate CVE-2025-64763, European organizations should: 1) Identify all Envoy proxy instances running affected versions (<=1.36.2) configured in TCP proxy mode handling CONNECT requests. 2) Enable the runtime flag envoy.reloadable_features.reject_early_connect_data to reject early client data before a 2xx response, preventing tunnel state desynchronization. This flag can be set dynamically without restarting Envoy, allowing gradual rollout. 3) Where possible, upgrade Envoy to a version beyond 1.36.2 where this behavior is corrected or better controlled. 4) Review upstream proxy configurations to ensure they do not reject CONNECT requests unexpectedly, reducing the chance of state desynchronization. 5) Implement monitoring and alerting on proxy connection errors and tunnel failures to detect potential issues early. 6) Test proxy chains in staging environments to validate CONNECT request handling and early data acceptance behavior. 7) Document and train network and security teams on this vulnerability to ensure rapid response if issues arise. These steps go beyond generic patching advice by focusing on configuration flags, upstream proxy coordination, and operational monitoring specific to this vulnerability.