CVE-2025-64763 is a vulnerability identified in the Envoy proxy, a widely used high-performance edge, middle, and service proxy. The flaw exists in versions up to 1.36.2, specifically when Envoy is configured in TCP proxy mode to handle HTTP CONNECT requests. Normally, Envoy waits for a successful 2xx response before forwarding client data to the upstream TCP connection. However, in the affected versions, Envoy accepts and forwards client data before issuing this response. If an upstream forwarding proxy rejects the CONNECT request with a non-2xx status, this premature forwarding can cause a de-synchronization of the CONNECT tunnel state between Envoy and the upstream proxy. This state mismatch can lead to protocol confusion and potentially disrupt the expected tunnel behavior. The vulnerability stems from Envoy's default behavior to allow early CONNECT data to maintain compatibility with existing deployments. The issue is classified under CWE-693 (Protection Mechanism Failure), indicating a failure in properly enforcing protocol state protections. 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 score is 3.7 (low), reflecting that the vulnerability is remotely exploitable without authentication or user interaction but requires high attack complexity and results in limited confidentiality impact without affecting integrity or availability. No public exploits are known, and no patches are linked in the provided data, suggesting mitigation relies on configuration changes or upgrading to later versions beyond 1.36.2. This vulnerability is primarily relevant for deployments using Envoy as a TCP proxy handling CONNECT requests where upstream proxies may reject tunnel establishment.

The impact of CVE-2025-64763 on European organizations is generally low but context-dependent. Organizations using Envoy proxy in TCP proxy mode to handle CONNECT requests, especially in complex proxy chains with upstream forwarding proxies, may experience tunnel state desynchronization. This can lead to unexpected proxy behavior, potential disruptions in tunnel establishment, and degraded service reliability. While the vulnerability does not directly compromise confidentiality, integrity, or availability, it can cause operational issues in network traffic forwarding and proxy chaining scenarios. European cloud service providers, telecom operators, and enterprises relying on Envoy for edge or service proxying could face intermittent connectivity issues or degraded performance in proxy tunnels if the vulnerability is exploited or triggered unintentionally. The lack of known exploits and the requirement for specific proxy configurations limit widespread impact. However, organizations with strict network security policies or those operating critical infrastructure should consider this vulnerability to avoid subtle proxy failures that may complicate incident response or network troubleshooting. The vulnerability's low severity means it is unlikely to be exploited for direct data breaches but could be leveraged as part of a multi-stage attack chain or to cause denial-of-service conditions in proxy-dependent services.

To mitigate CVE-2025-64763, European organizations should take the following specific actions: 1) Identify all Envoy proxy deployments, particularly those configured in TCP proxy mode handling CONNECT requests. 2) Review proxy chains to determine if upstream forwarding proxies may reject CONNECT tunnels with non-2xx responses, which triggers the vulnerability. 3) 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 requiring a full Envoy upgrade. 4) Plan and execute upgrades to Envoy versions later than 1.36.2 where this issue is resolved or behavior improved. 5) Conduct thorough testing in staging environments to ensure enabling the flag does not disrupt legitimate traffic flows or existing deployments. 6) Monitor proxy logs for unusual CONNECT request failures or tunnel state errors that may indicate attempts to exploit or trigger the vulnerability. 7) Incorporate this vulnerability into incident response playbooks to quickly identify and remediate related proxy issues. 8) Coordinate with upstream proxy vendors or service providers to understand their CONNECT request handling and compatibility with Envoy configurations. These targeted steps go beyond generic patching advice by focusing on configuration tuning, environment-specific testing, and operational monitoring to minimize disruption while addressing the vulnerability.