MUIS does not certify individual stocks and investment portfolios. It is up to the investor to assess the shares they are interested in, by getting more information from the companies themselves.
There are some companies which are already listed as Syariah-compliant. It is clear that these shares are permissible for Muslims to buy.
As for other companies that are not listed or certified per se as Syariah-Compliant, the principle is that it is permissible as long as the company is not involved in sectors such as gambling, weaponry, porcine products, and products that harm Muslims and the environment. In today's world, almost all big corporations are involved in some sort of Haram transactions. Thus, to further ascertain that a company is permissible to be invested on, or its shares be bought, the following should be taken into account:
a) No more that 15% of the company's nett income comes from interest and other prohibited transactions. If it is more than 15%, the company's stocks and shares cannot be bought.
b) Borrowing/equity ratio must not exceed 33%.The income you get from such investments must be purified first before consuming it. This can be done by giving an intelligent estimate of the percentage of non-halal transactions the company dealed with. Deduct that amount of percentage from your income and donate it to Islamic charitable organisations, such as mosques and institutions.