The Wall Street Journal

The Equifax hack was one of the most significant in recent years. Here’s how it went down.

The crisis has sent shock waves through the industry, spooked consumers and sparked investigations. A focus for inquiry is a software glitch that appears to be how the intruders got into the company’s systems.

On March 8, researchers at Cisco Systems Inc. reported an online security flaw that allowed hackers to break into servers around the internet. Cisco urged users to upgrade their systems immediately with a newly issued fix.

Equifax Inc. was among the companies using the flawed software. On Friday, it said its technology experts at the time worked to identify and patch vulnerable systems.

In late July, though, Atlanta-based Equifax discovered suspicious traffic on its system—and found the same security flaw still existed in some areas. The company’s security staff again addressed the problem, according to Equifax, but by then it was too late.

From about mid-May to July 30, hackers ransacked vast troves of information at the credit-reporting company. The breach potentially exposed about 143 million Americans’ personal information, including names, addresses, dates of birth and Social Security numbers. The revelations have shaken the company, as well as confidence in a linchpin of the financial system, and triggered a federal criminal investigation.

Much remains unknown about the hack attack and how it burrowed so deeply inside the company. Investigators, security experts and Equifax itself are focusing on what the company did or didn’t do right in the run-up to

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