Multi Step for Contact Form 7 <= 2.9.1 - Unauthenticated Stored Cross-Site Scripting via Multipart Field Name

The Multi Step for Contact Form 7 plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'multipart field name (array key)' parameter in all versions up to, and including, 2.9.1 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. Successful exploitation requires the target CF7 form to have the 'Save every step' option enabled and the Database for Contact Form 7 companion plugin to be active with its 'Expandable' table option enabled, both of which are disabled by default.