Simply put, you can't invest money with any reputable company without handing over a SSN or other Tax ID#. Uncle Sam requires it so that they can get your tax info. So if you are asking, should you trust your money to a legitimate company, the answer is it's preferable to the other options. Any company must store the SSN on their servers. If your concern is putting it into a web form vs a paper and mailing it or calling it in on the phone, really, security on a web browser is better than the security on a POTS phone or the postal service. Sure, your computer could be hacked right now. If you are really concerned, buy a new computer and use it for nothing but online banking. It's typically best practice to assume that any computer that has touched the raw internet is unclean. If you don't want to buy a new computer, than you obviously don't care much about your security, so than the answer to your poll is stop worrying about it and just give them the info.