'We've Been Breached': Inside the Equifax Hack

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...