In Mbed TLS libraries version 3.5.0 through 4.0.0, the logic that manages TLS 1.3 session resumption fails to correctly verify the client’s identity when the session is resumed. This flaw allows an attacker to forge a client identity and obtain the privileges of a legitimate client, effectively bypassing authentication controls and potentially gaining unauthorized access to services that rely on TLS 1.3 and client certificates.

The vulnerability affects the Mbed TLS cryptographic library, which is widely deployed in embedded and IoT devices. All builds that include the affected versions (3.5.0 up to 4.0.0) are impacted; no specific vendor or product variants are named beyond the library itself.

The flaw carries a CVSS score of 9.1 and an EPSS probability of less than 1 percent, indicating high severity but low current exploitation likelihood. It is not recorded in the CISA KEV catalog. An attacker would need the ability to initiate a TLS 1.3 handshake from a client that has previously established a session and then reuse the session resumption context, potentially on a network they control. The exploitation requires control over the client and the ability to influence the TLS handshake, but remains a significant risk for any server that uses Mbed TLS for client authentication.