The vulnerability exists in Grav CMS versions prior to 2.0.15 within the detectXss() function. It fails to properly detect event-handler attributes when there is an unpaired quote in an unquoted attribute value, allowing authenticated editors to inject event handlers like onerror= that execute stored XSS payloads in visitor browsers. This is a stored XSS vulnerability requiring authenticated editor privileges and involves bypassing input validation due to improper parsing of attribute values.

Successful exploitation allows authenticated editors to inject malicious event-handler attributes that execute arbitrary JavaScript in the browsers of visitors viewing the affected page content. This can lead to compromise of user sessions, theft of sensitive data, or other client-side impacts. The vulnerability has a high impact on confidentiality and integrity, with no direct impact on availability.

A fix is available in Grav version 2.0.15. Users should upgrade to version 2.0.15 or later to remediate this vulnerability. No additional mitigation steps are indicated in the advisory.