Is it permissible to work as a programmer for stock trading software for companies on the stock exchange, knowing that some of these companies may be involved in prohibited activities or deal with interest (riba)? Is this considered aiding in sin, or does the sin fall upon the buyer? What is the ruling on accepting work in a company that claims not to deal with prohibited stocks until this is verified?

If the company deals exclusively in permissible (halal (definition)) stocks, then there is no harm in developing a program for buying and selling these stocks through the stock exchange. However, if it deals in both impermissible (haram (definition)) and permissible stocks, then it is not permissible to assist it in that, whether through programming or otherwise, due to the Almighty's saying: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression). For sin befalls whoever aids in the forbidden. Therefore, if you are certain that the company deals only in permissible stocks, there is no harm in accepting work with it.