Microsoft stopped the support for XP, but for WannaCry attack they made exclusion:

• Microsoft Customer Guidance for WannaCrypt attacks
• Microsoft Security Bulletin MS17-010

If you have too many XP as a pos stations you can use registry hack to enable again Windows Update for extra 5 years.

• Zdnet: Registry hack enables continued updates for Windows XP
• US DOD may still be receiving Windows XP patches from Microsoft 
• Registry hack enables continued updates for Windows XP
• Продлеваем получение обновлений безопасности для Windows XP еще на 5 лет
• How to get security updates for Windows XP until April 2019