static int ssl_write_certificate_verify( mbedtls_ssl_context *ssl )
{
    const mbedtls_ssl_ciphersuite_t *ciphersuite_info =
        ssl->transform_negotiate->ciphersuite_info;
    int ret;

    MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> write certificate verify" ) );

    if( ( ret = mbedtls_ssl_derive_keys( ssl ) ) != 0 )
    {
        MBEDTLS_SSL_DEBUG_RET( 1, "mbedtls_ssl_derive_keys", ret );
        return( ret );
    }

    if( ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_PSK ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_RSA_PSK ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECDHE_PSK ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_DHE_PSK ||
        ciphersuite_info->key_exchange == MBEDTLS_KEY_EXCHANGE_ECJPAKE )
    {
        MBEDTLS_SSL_DEBUG_MSG( 2, ( "<= skip write certificate verify" ) );
        ssl->state++;
        return( 0 );
    }

    MBEDTLS_SSL_DEBUG_MSG( 1, ( "should never happen" ) );
    return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
}

The product reads data past the end, or before the beginning, of the intended buffer.