The Post Snippets plugin for WordPress is vulnerable to Stored Cross-Site Scripting in all versions up to, and including, 4.0.19. This is due to insufficient output escaping of imported snippet content when rendering JavaScript variables in the post editor. Specifically, the jqueryUiDialog() method in WPEditor.php embeds snippet content directly into JavaScript string literals without escaping double quotes (the quote-escaping code on line 214 is commented out).

When snippets are imported via the Import/Export feature, the content bypasses WordPress's wp_magic_quotes() (which would otherwise add protective backslashes), allowing double quotes in snippet content to break out of the JavaScript string context. This makes it possible for authenticated attackers, with Administrator-level access and above, to inject arbitrary web scripts via a malicious import file that execute whenever any administrator accesses a post editor page. Please note that this does not affect single-site installations as administrators already have the unfiltered_html capability.