Get an IRS Identity Protection (IP) PIN

From the IRS:

An identity protection PIN (IP PIN) is a six-digit number that prevents someone else from filing a tax return using your Social Security number (SSN) or individual taxpayer identification number (ITIN). The IP PIN is known only to you and the IRS. It helps verify your identity when you file your electronic or paper tax return. Even though you may not have a filing requirement, an IP PIN still protects your account.