Impact

The impact of this is medium, because it gives the privileged users (with the ability to edit pages) a mechanism to perform remote code execution via XSS. At a minimum, the vulnerability represents a bypass of security controls put in place to mitigate this form of attack.

The remote code execution can be performed because XSS would allow an attacker to execute functionality on behalf of a stolen administrative account - the facility to install custom plugins would then allow said attacker to install a plugin containing a web shell and thus garner access to the underlying system.

References

https://owasp.org/www-project-top-ten/2017/A7_2017-Cross-Site_Scripting_(XSS)
https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html
https://cwe.mitre.org/data/definitions/79.html

For more information

Please contact contact@pentest.co.uk