Why Should This Issue Be Considered Important?
Sucuri discovered a critical cross-site scripting (XSS) issue (defined) within the Jetpack plugin caused by how it validates the email address submitted via the contact form module within the plugin.
How Can I Protect Myself From This Issue?
Please update to JetPack version 3.7.1 or later (at the time of writing, version 3.7.2 is available). Instructions for updating WordPress plugins are provided here. Installation instructions for JetPack are provided here.
I hope that the above information is useful to you in securing your WordPress site from this flaw if you make use of the JetPack plugin.