A vulnerability in rustls allows a network attacker to poison the client's per-server-name key-exchange record by sending an unauthenticated ServerHello selecting the X25519 group. This causes subsequent ClientHellos for that server name to carry only an X25519 share, potentially leading the handshake to complete with classical key exchange only, losing quantum-resistant confidentiality. No authentication bypass or key compromise occurs. The issue is fixed in rustls versions 0.23.46 and 0.24.0-dev.2.