Grav before 3.9.2 fails to validate untrusted Host headers in the sendInvitationEmail() function when constructing token-bearing invitation links. Attackers can manipulate the Host header to poison invitation links and redirect users to attacker-controlled domains, bypassing the require_trusted_host protection which only covers password reset flows.

Grav CMS versions before 3.9.2 are vulnerable to Host header injection in the sendInvitationEmail() function. Attackers can manipulate the HTTP Host header to poison invitation links sent to users, redirecting them to attacker-controlled domains. The vulnerability exists because the require_trusted_host protection only covers password reset flows and not invitation email flows. Token-bearing invitation links can be crafted to exfiltrate tokens or perform phishing attacks. This represents a bypass of existing security controls within the Grav CMS framework. The fix is available in Grav version 3.9.2 and above.