Magazine Companion <= 1.2.3 - Authenticated (Contributor+) Stored Cross-Site Scripting

The Magazine Companion plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'headerHtmlTag' attribute in the bnm-blocks/featured-posts-1 block in all versions up to, and including, 1.2.3. This is due to insufficient input sanitization and output escaping when using user-supplied values as HTML tag names. This makes it possible for authenticated attackers, with contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.