Grav CMS versions before 3.9.2 do not validate the Host header used when the sendInvitationEmail() routine creates a token‑bearing invitation link. The result is that an attacker can supply a forged Host header, causing the system to construct a link that points to an arbitrary domain of the attacker’s choice. When a user receives the poisoned invitation and clicks the link, the victim is redirected to a malicious site where credentials can be harvested or malware can be delivered. This flaw is a form of forgery in trusted input (CWE‑350).

All installations of getgrav’s Grav CMS that expose the sendInvitationEmail feature and are running any release before 3.9.2 are vulnerable. The flaw does not depend on any particular sub‑version or host configuration, so any instance that accepts external invitation email requests can be compromised.

Based on the description, it is inferred that the attack vector involves sending a crafted HTTP request containing a forged Host header to the publicly accessible sendInvitationEmail endpoint. The CVSS score of 8.7 reflects a high severity related to phishing, and the EPSS score is currently unavailable, which suggests that exploitation is possible but may not be widespread. The vulnerability is not listed in the CISA KEV catalog, so there is no known active exploitation in the wild yet. Nevertheless, a low‑to‑moderate risk of exploitation exists, particularly in environments that expose the invitation endpoint to unauthenticated users.