Thomas Blattmann

**Name of the Vulnerable Software and Affected Versions** Mbed TLS versions 3.5.x through 3.5.x before 3.6.0 Mbed TLS versions prior to 3.6.0 **Description** An issue was discovered in Mbed TLS when negotiating the TLS version on the server side, it can fall back to the TLS 1.2 implementation of the protocol if it is disabled. If the TLS 1.2 implementation was disabled at build time, a TLS 1.2 client could put a TLS 1.3-only server into an infinite loop processing a TLS 1.2 ClientHello, resulting in a denial of service. If the TLS 1.2 implementation was disabled at runtime, a TLS 1.2 client can successfully establish a TLS 1.2 connection with the server. **Recommendations** For Mbed TLS versions 3.5.x before 3.6.0, update to version 3.6.0 or later to resolve the issue. For Mbed TLS versions prior to 3.6.0, update to version 3.6.0 or later to resolve the issue. As a temporary workaround, consider disabling the TLS 1.2 implementation to prevent a denial of service. Restrict access to the TLS 1.2 ClientHello to minimize the risk of exploitation.