The best way to keep track of the recent security issues and get the latest information is to register your Moodle site with moodle.org.
By registering your Moodle site, your email address is added to the low-volume mailing list for important and most up-to-date information, including new and point releases and notifications such as security alerts.
We highly recommend you register your site.
Otherwise, after each release, all important security issues are published in this forum, which you can subscribe to (moodle.org account required). You can also follow moodlesecurity on Twitter, which will be updated as much as possible.
Please note that if you subscribe to the security forum and Twitter options, there will be a delay of up to one week until the information becomes available.