The plugin settings are visible to all registered users in the dashboard and are lacking any sanitisation. As a result, any registered user, such as subscriber, can leave an XSS payload in the plugin settings, which will be triggered by any user visiting them, and could allow for privilege escalation. The vendor decided to close the plugin.
WordPress Contact Form Check Tester plugin <= 1.0.2 - Cross-Site Scripting (XSS) vulnerability
Contact Form Check Tester <= 1.0.2 - Broken Access Control to Cross-Site Scripting (XSS)
WordPress Contact Form Check Tester 1.0.2 Plugin - Broken Access Control Vulnerability
WordPress Contact Form Check Tester 1.0.2 XSS / Access Control