A vulnerability was found in HSC Cybersecurity Mailinspector up to 5.3.2-3. Affected by this issue is some unknown functionality of the file /mailinspector/mliUserValidation.php of the component URL Handler. The manipulation of the argument error_description results in cross site scripting. The attack may be performed from remote. The exploit has been made public and could be used. Upgrading to version 5.4.0 can resolve this issue. You should upgrade the affected component. The vendor was contacted early and responded very professional: "We have already implemented the fix and made a hotfix available to affected customers, ensuring mitigation while the official release 5.4.0 has not yet been published. This allows customers to address the issue immediately, outside the regular release cycle."