- Heartland Payment System
- TJX
- Epsilon
- RSA
- Stuxnet
- Department of Veterans Affairs
- Sony PlayStation Network
- ESTsoft
- Gawker Media
- Google, etc.
- VeriSign
- CardSystems
- AOL
- Monster.com
- Fidelity National Information Services
http://www.csoonline.com/article/700263/the-15-worst-data-security-breaches-of-the-21st-century
Out of all these I think the most brutal security breach was AOL because they got domain hijacked. But I think the VeriSign and Playstation hack effected more people and on a much larger scale. The Playstation network I remember took a VERY long time to recover from the hack and then was hacked again. Security is so important but no matter how much you secure something someone can almost always fine a back door especially if they have the time, resources and money.