The extent to which stock ownership is considered active partnership in a corporation is a critical question in numerous areas of Halakha. Conceivably, by buying a single share of stock a person could find himself committing transgressions from all four sections of the Shulchan Arukh:
- from Orach Chaim – Shabbat prohibitions such as profiting from one’s business, employing Jews and working animals; possessing leaven during Passover;
- from Yoreh De’ah – benefiting from a mixture of milk and meat, from avoda zara, and from orla; lending and borrowing at interest, doing business in forbidden foodstuffs;
- from Even Ha-Ezer – being a partner in licentious activities;
- from Choshen Mishpat – being a partner in robbery, withholding of wages, or commission of damages.
Source: http://www.jlaw.com/Articles/stockmarket.html