Input validation vulnerability in Really Simple Guest Post 1.0.6

The Really Simple Guest Post plugin is a WordPress plugin that helps people post content to a website. Unfortunately, versions of this plugin up to, and including, 1.0.6 have a security vulnerability. This means that an unauthorized user could gain access to the website and execute any code they wanted. This could be used to bypass security controls, steal sensitive information, or even upload and include other files that could also be used maliciously.