FreeScout's input sanitization function Helper::stripDangerousTags() removes certain dangerous HTML tags but does not remove <style> tags. An attacker with mailbox settings access can inject CSS in the mailbox signature, which is rendered unescaped in conversation views. Due to a permissive Content Security Policy allowing 'unsafe-inline' styles, this CSS can exfiltrate CSRF tokens from users viewing the conversation. With the stolen CSRF token, the attacker can perform privileged actions such as creating admin accounts or changing credentials, effectively escalating privileges from agent to admin. This vulnerability is a result of an incomplete fix for a prior XSS issue and is resolved in FreeScout version 1.8.213.

An attacker with mailbox settings permissions (admin or agent) can inject malicious CSS to steal CSRF tokens from other users viewing mailbox conversations. This enables the attacker to perform any state-changing actions on behalf of the victim, including privilege escalation to admin level. The vulnerability compromises confidentiality and integrity but does not affect availability.

Upgrade FreeScout to version 1.8.213 or later, where the input sanitization function has been updated to properly remove <style> tags and prevent CSS injection. Until upgraded, users with mailbox settings access should be considered a high risk, and access should be restricted if possible. Patch status is not explicitly stated in the vendor advisory content provided, but version 1.8.213 contains the fix.