A huge data breach has rocked the world of personal information security. Four months ago, a notorious hacking group claimed they stole a massive amount of sensitive data from a major data broker. Now, a member of this group has leaked most of this stolen data for free on an online marketplace that deals in stolen information.

This breach involves Social Security numbers and other crucial personal details, potentially putting millions at risk for identity theft and fraud. Teresa Murray, a consumer protection expert, warns that this could be the worst data breach we've seen. “If this is as big as it seems, it’s a major wake-up call,” she says. “People who haven’t been careful should be especially concerned.”

The hacking group, known as USDoD, had claimed back in April that they stole records of 2.9 billion people from National Public Data (NPD), a company that provides personal details for background checks. They initially tried to sell this information for $3.5 million. ...