The rustls library versions 0.23.44 and 0.23.45 contain an information disclosure flaw. When an Encrypted Client Hello (ECH) request is rejected, the start_traffic() routine continues to send plaintext data queued earlier from ClientConnection::writer(). Thus data intended for an inner server is forwarded to the public‑facing server, exposing application data. This flaw is an instance of improper handling of failure conditions (CWE-696) and can lead to unintended data leakage during the TLS handshake.

The affected product is the RustLS TLS library from the RustLS Project, specifically versions 0.23.44 and 0.23.45. Any systems that incorporate these releases for HTTPS or other TLS‑enabled services are potentially impacted.

The CVSS score of 4.6 classifies the vulnerability as moderate; however, because the leakage occurs during the TLS handshake, an attacker who can initiate connections to the server could obtain arbitrary application data that had been buffered before the handshake. The EPSS score is not available, and the vulnerability is not listed in the CISA KEV catalog, indicating that no documented exploit exists. Nonetheless, the risk remains due to data exposure that can be triggered remotely by sending crafted ECH requests. The attack vector requires an attacker's ability to craft an ECH handshake that is rejected, but no privileged access or additional conditions are necessary beyond normal client connectivity to the target server.