PayPal is upgrading the certificate for www.paypal.com to SHA-256. This endpoint is also used by merchants using the Instant Payment Notification (IPN) product.
A detailed FAQ has been provided here:
Merchant Security System Upgrade Guide
Also the microsite:
2015-2016 SSL Certificate Change
For most online store owners the primary concern is that your server has the correct new G5 certificate installed. Instructions for testing can be found here:
If you’re running CentOS or RedHat then the command is:
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awk -v cmd='openssl x509 -noout -subject' '/BEGIN/{close(cmd)};{print | cmd}' \ < /etc/pki/tls/certs/ca-bundle.crt | grep G5 |