Could be the largest heist of its kind

1. Russian hackers have stolen 1.2 billion username and password combinations.
2. The digital credentials came from ~420,000 sites.
3. The sites included small and well-known ones.
4. The hackers make money by logging into social and email accounts and posting about fake products.
5. This could be the largest theft of username and passwords.
6. Security experts say users should monitor their accounts for unauthorized messages being sent out.