Palo Alto Networks has disclosed that threat actors may have attempted to unsuccessfully exploit a recently disclosed critical security flaw as early as April 9, 2026. The vulnerability in question is CVE-2026-0300 (CVSS score: 9.3/8.7), a buffer overflow vulnerability in the User-ID Authentication Portal service of Palo Alto Networks PAN-OS software that could allow an unauthenticated attacker to execute arbitrary code with root privileges by sending specially crafted packets. While fixes are expected to be released starting May 13, 2026, customers are advised to secure access to the PAN-OS User-ID Authentication Portal by restricting access to trusted zones, or by disabling it entirely if it's not used. As additional mitigation, the company is recommending that organizations disable Response Pages in the Interface Management Profile for any L3 interface where untrusted or internet traffic can ingress. Customers with Advanced Threat Prevention can also block exploitation attempts by enabling Threat ID 510019 from Applications and Threats content version 9097-10022. In an advisory issued Wednesday, the network security company said it's aware of limited exploitation of the flaw. It's tracking the activity under the CL-STA-1132, a suspected state-sponsored threat cluster of unknown provenance. "The attacker behind this activity exploited CVE-2026-0300 to achieve unauthenticated remote code execution (RCE) in PAN-OS software. Upon successful exploitation, the attacker was able to inject shellcode into an nginx worker process," Palo Alto Networks Unit 42 said. The cybersecurity company said it has observed unsuccessful exploitation attempts against a PAN-OS device starting April 9, 2026, a week after which the attackers managed to successfully obtain remote code execution against the appliance and inject shellcode. As soon as initial access was achieved, the threat actors took steps to clear crash kernel messages, delete nginx crash entries and nginx crash recor