Rustls incorrectly accepts TLS 1.3 handshake messages at an encryption level that violates RFC 8446 record‑boundary alignment when a key‑changing message is present in the same record. The library treats a plaintext EncryptedExtensions as encrypted, allowing an attacker to inject plaintext handshake data. This input‑validation flaw (CWE‑20) can expose handshake traffic and grant a malicious peer the ability to manipulate subsequent protocol steps.

The vulnerability exists in Rustls library versions from 0.23.13 up to and including 0.23.44. It affects all applications that incorporate these Rustls crates, including web servers, clients, and any service that uses the Rustls TLS implementation.

The CVSS score of 6.9 categorizes this as medium severity. EPSS data is not available, and the issue is not listed in the CISA KEV catalog. Exploitation requires the attacker to interact with the TLS handshake, typically as a peer or on‑path entity. The attack vector is network‑based, and while it does not provide remote code execution, it enables the attacker to send crafted plaintext messages and potentially disclose sensitive handshake information.