The Post Grid Gutenberg Blocks – PostX plugin for WordPress is vulnerable to Stored Cross-Site Scripting via displayname User Field in all versions up to, and including, 5.1.0 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. WordPress core's preuserdisplayname filter encodes &, <, and > but leaves double-quotes intact (ENTNOQUOTES), allowing a Subscriber-level user to store a double-quote in their displayname via /wp-admin/profile.php that subsequently breaks out of the alt="" attribute at ArchiveTitle.php:144.