Starwood hotels announced Friday that as many as 500 million guests may have had their information stolen in a data breach.

According to the Associated Press, Marriott said unauthorized access within its Starwood network had been happening for four years.

“This is one of the most significant data breaches in history given the size — about 500 million people are affected — and the sensitivity of the personal information that was stolen,” CreditCards.com industry analyst Ted Rossman said.

The company said that some customer’s credit cards and expiration dates may have been exposed. Although some guests had only their name and address stolen, others’ personal data include phone number, email address, passport number, Starwood Preferred Guest account information, date of birth, gender, arrival and departure information, reservation date and communication preferences, the AP reported.

Here are three things to do right away to protect your information:

1. Change your passwords

Don’t wait for an email saying your data might have been compromised. Change your passwords now. If you use the same password for several accounts, change those also — but not to the same thing. It’s best to have a different password for every account.

For even tighter security, enable two-factor authentication on those services that offer it. Two-factor authentication requires you to first log in with your username and password, and then input a code that is sent to you by email or SMS to finish logging in. The extra step may be a hassle, but it ensures that even if your password is stolen, hackers will not be able to access your accounts unless they have your phone.

2. Put a freeze on your credit

Putting a freeze on your credit can prevent anyone else from getting a card or loan in your name. According to the Federal Trade Commission, the alert stays on your report for a year, and you can get a new one after that year is up. It’s free to request an alert, and the credit bureau you contact must tell the other two about your alert.

To contact a credit bureau:

Equifax
Equifax.com/personal/credit-report-services
1-800-685-1111

Experian
Experian.com/help
1-888-397-3742

Transunion
TransUnion.com/credit-help
1-888-909-8872

3. Sign up with a credit monitoring service

Credit monitoring allows you to get early warnings if there is a change in your credit report or score. Both free and paid services are available. Consumersadvocate.org recently ranked the top nine credit monitoring services and what each provides.

It's top choice, Identity Guard offers monitoring, insurance,  credit reports and more for $17.99 a month after a 30-day free trial.

Marriott said Friday that it was providing guests the chance to enroll in WebWatcher free of charge for a year. WebWatcher monitors sites where personal information is shared and generates an alert to the consumer if evidence of the consumer's personal information is found. U.S. citizens who activate WebWatcher will also be provided fraud consultation services and reimbursement coverage for free, Marriott said. To activate WebWatcher, go to info.starwoodhotels.com.

Marriott said it has set up a dedicated website (info.starwoodhotels.com) and call center to answer questions guests might have about this incident.