Equifax Data Breach Due to Failure to Install Patches

"the Equifax data compromise was due to their failure to install the security updates provided in a timely manner."

Source: MEDIA ALERT: The Apache Software Foundation Confirms Equifax Data Breach Due to Failure to Install Patches Provided for Apache® Struts™ Exploit : The Apache Software Foundation Blog

As simple as that apparently. Keep up to date with patching.