10. ESTsoft Breach
South Korean software maker ESTsoft announced a data security breach in 2011 that exposed the account credentials of millions of users. Hackers reportedly planted malware on one of its update servers, exposing the personal information of 35 million users. The breach impacted every site that used the firms' Web applications. The data included names, birth dates, user IDs and hashed passwords. The company develops a wide range of applications, including antivirus software.