Palo Alto Networks PAN-OS contains a critical out-of-bounds write vulnerability in the User-ID Authentication Portal (Captive Portal) service. The vulnerability affects PA-Series and VM-Series firewalls and allows unauthenticated attackers to execute arbitrary code with root privileges. Attackers can exploit this vulnerability by sending specially crafted packets to the affected systems. This represents a critical security risk as it provides complete system compromise without authentication requirements. The vulnerability is tracked as CVE-2026-0300 and has been assigned a high criticality rating.