CVE‑2026‑0300 is a buffer overflow vulnerability in the PAN‑OS User‑ID Authentication Portal service. Successful exploitation enables unauthenticated remote code execution with root-level privileges on affected devices. According to Palo Alto Networks Unit 42, exploitation involves injecting shellcode into an nginx worker process running on the PAN‑OS appliance[2].

You are only affected if you are running the following configuration[3]:

• User-ID Authentication Portal configured in the User-ID Authentication Portal Settings page. You can verify the configuration by going to Device > User Identification > Authentication Portal Settings -> Enable Authentication Portal (applies to both transparent and redirect modes) and
• An interface management profile with response pages enabled and associated with an external/internet-accessible interface. You can verify the configuration by going to Network > Interface > Select the interface > Advanced Tab > Create Management Interface Profile

CVE

CVE-2026-0300

Affected Products

PAN-OS 12.1 – < 12.1.4-h5, < 12.1.7
PAN-OS 11.2 – < 11.2.4-h17, < 11.2.7-h13, < 11.2.10-h6, < 11.2.12
PAN-OS 11.1 – < 11.1.4-h33, < 11.1.6-h32, < 11.1.7-h6, < 11.1.10-h25, < 11.1.13-h5, < 11.1.15
PAN-OS 10.2 – < 10.2.7-h34, < 10.2.10-h36, < 10.2.13-h21, < 10.2.16-h7, < 10.2.18-h6

Exploitation

Palo Alto Networks observed unsuccessful exploitation attempts starting April 9, 2026. Approximately one week later, threat actors were able to successfully achieve remote code execution and inject shellcode into a vulnerable device. The activity has been attributed to a suspected state‑sponsored threat cluster designated CL‑STA‑1132[2].

Recommended Actions

Palo Alto are currently working on publishing updates that resolves this vulnerability, estimated times of arrival can be found here[3].
In the meantime, Truesec recommends that you follow the vendors advice for mitigation[3]:

• Restrict User-ID Authentication Portal access to only trusted zones and in addition, disable Response Pages in the Interface Management Profile attached to every L3 interface in any zone where untrusted/internet traffic can ingress. Keep Response Pages enabled only on interfaces in trust/internal zones where legitimate users’ browsers ingress.
• Disable User-ID Authentication Portal if not required.

References

[1] https://unit42.paloaltonetworks.com/captive-portal-zero-day/
[2] https://thehackernews.com/2026/05/pan-os-rce-exploit-under-active-use.html
[3] https://security.paloaltonetworks.com/CVE-2026-0300