Gutentools <= 1.1.3 - Authenticated (Contributor+) Stored Cross-Site Scripting via Post Slider Block Attributes

The Gutentools plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the Post Slider block's block_id attribute in all versions up to, and including, 1.1.3. This is due to insufficient input sanitization and output escaping combined with a custom unescaping routine that reintroduces dangerous characters. 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.