Leave it Home. Never carry your Social Security card — what if your wallet or purse is stolen? The same goes for your Medicare card (which includes your Social), unless you’re seeing a health care provider for the first time. If you like the security of having an ID on you, carry a photocopy of your Medicare card with several digits blanked out.
When Asked, Don’t Tell. Only a few organizations have a legal right to your Social — your employer, banks and lenders, investment funds, the IRS and government-funded programs such as workers’ compensation. When asked by others, just say no. The more your number is out there, the greater the risk of identity theft.
Guard the Final Four. Although most widely used and shared, the last four digits are in fact the most important to protect. These are truly random and unique; the first five numbers represent when and where your Social Security card was issued. Scammers can get those numbers by knowing your birth date and hometown. So don’t use the last four as a PIN. Don’t share them in emails. Ask companies to use an alternative identifier.
Source: AARP Money
Protect Social Security Number, Avoid Identity Theft