For many Americans, a security code to an email account is the equivalent of the key to their physical home. Hackers are well aware of this fact, which is exactly why Yahoo was their newest victim of a security breech. On Thursday, the email provider announced that 450,000 Yahoo users' emails had been found and posted to a hacker’s website.

Only two days ago, the social networking site Formspring was the victim of a similar attack—having to disable almost 30 million registered users' passwords after someone breached its databases and posted 420,000 encrypted passwords on the Web. In June, LinkedIn was hacked with nearly 6.5 million user’s passwords obtained by a Russian hacker.

"If you're running a large network, there's a good chance one of your users' passwords has fallen into the hands of someone who intends to use it to access your network by impersonating your employee," Tom Cross, research director at network security firm Lancope, told USA Today. "We're beginning to see that passwords are an increasingly suspect security tool.”

Read it at USA Today.