From blog.sucuri.net
If you haven’t updated your WordPress website since October 2013, this wouldn’t affect you, but we strongly hope that is not the case! There’s a new object injection vulnerability which affects WordPress versions 3.7 to 5.7.1. Be sure to get updated to 5.7.2 as soon as possible!
According to WPScan, the new object injection vulnerability is due to versions of PHPMailer library between 6.1.8 and 6.4.0. The original CVE can be found here.