The theft of the email addresses and other account details of 500 million Yahoo users is a golden opportunity for cyber-criminals working with bad-bots.
Those who use Yahoo to provide their principal email account should have been concerned by news of the leak. Many others, with an old Yahoo account that they do not use much, may not have been that bothered. Is such complacency justified? That depends, to gauge the risk you need to understand the cyber-crime opportunity represented by 500 million accounts.