https://www.irs.gov/identity-theft-central

Tax-related identity theft happens when someone steals your personal information to commit tax fraud. Your taxes can be affected if your Social Security number is used to file a fraudulent return or to claim a refund or credit.
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Protect Yourself — Learn about the warning signs and what you can do to safeguard your Social Security number.

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What to Do if You are a Victim — If your Social Security number is compromised and you know or suspect you are a victim of tax-related identity theft, the IRS recommends these additional steps.