Post SMTP < 3.9.0 - Unauthenticated Stored Cross-Site Scripting via 'event_type'

Description

The plugin is vulnerable to Stored Cross-Site Scripting via the ‘event_type’ parameter 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. The vulnerability is only exploitable when the Post SMTP Pro plugin is also installed and its Reporting and Tracking extension is enabled.