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Palo Alto Networks has published fixes for CVE-2026-0300, a critical PAN-OS flaw that attackers have exploited in the wild. The buffer overflow affects the User-ID Authentication Portal, also called Captive Portal, on PA-Series and VM-Series firewalls when specific portal and network-interface settings expose it to untrusted traffic. Unit 42 describes the activity as likely state-sponsored, but its report does not identify a country or government.
What is the Palo Alto firewall zero-day?
CVE-2026-0300 is a buffer overflow in the PAN-OS User-ID Authentication Portal. Palo Alto Networks rates it critical, with a CVSS score of 9.3, and says an unauthenticated attacker could send specially crafted packets to execute code with root privileges on an affected firewall. The vendor says the flaw was discovered in production use. Palo Alto Networks’ CVE-2026-0300 advisory identifies the affected products as PA-Series and VM-Series firewalls; Cloud NGFW, Prisma Access, and Panorama are not impacted.
Is my Palo Alto firewall exposed?
The vulnerability applies only when the User-ID Authentication Portal is configured, and exposure requires both of these conditions:
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →- The Authentication Portal is enabled under Device > User Identification > Authentication Portal Settings.
- An interface management profile with Response Page enabled is attached to an L3 interface in a zone that can receive untrusted or internet traffic.
A portal reachable from the public internet or another untrusted network presents the greatest risk. Review both settings, not just whether the firewall is running PAN-OS.
Was CVE-2026-0300 exploited in the wild?
Yes. Palo Alto Networks reports limited exploitation against User-ID Authentication Portals exposed to untrusted IP addresses or the public internet. In its advisory, the company says: “Limited exploitation has been observed targeting Palo Alto Networks User-ID™ Authentication Portals that are exposed to untrusted IP addresses and/or the public internet.” That observation does not mean every vulnerable firewall has been compromised. The vendor has not established a broader number of affected customers or confirmed compromises in the cited advisory.
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Unit 42 tracks the activity as CL-STA-1132. Its threat brief describes attackers achieving unauthenticated remote code execution, injecting shellcode into an nginx worker process, and later using the publicly available tunneling tools EarthWorm and ReverseSocks5. Unit 42 also reports log cleanup and Active Directory enumeration using service-account credentials it says were likely obtained from the firewall.
Who is behind the attacks?
Unit 42 characterizes CL-STA-1132 as a cluster of likely state-sponsored threat activity. Its reviewed report does not name a sponsoring government or attribute the activity to a particular country. “Likely state-sponsored” is therefore the limit of the attribution claim; it should not be read as a confirmed identification of a state actor.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsWhich PAN-OS versions fix CVE-2026-0300?
The fix depends on the installed PAN-OS release train and branch. The vendor’s advisory lists multiple branch-specific fixed releases, including the following examples:
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| PAN-OS branch | Fixed versions listed in the advisory |
|---|---|
| 12.1 | 12.1.4-h5 or 12.1.7 and later |
| 11.2 | 11.2.4-h17, 11.2.7-h13, 11.2.10-h6, or 11.2.12 and later |
| 11.1 | Branch-specific fixes through 11.1.13-h5 or 11.1.15 and later |
| 10.2 | Branch-specific fixes through 10.2.18-h6 |
These are examples, not a universal upgrade target: check the advisory’s full version table against the firewall’s exact installed branch and release before upgrading. Administrators on unsupported older PAN-OS versions should move to a supported version that includes the fix. Palo Alto Networks published the advisory on May 5, 2026 and updated it May 28, 2026; its current release-specific guidance supersedes earlier reports that patches were still pending. CERT-EU’s May 6 advisory recorded CVSS 9.3 but reflected the patch status at that time.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should administrators do?
Install the branch-specific fix
Use Palo Alto Networks’ current CVE-2026-0300 version table to select a fixed release for the firewall’s installed PAN-OS branch. Do not assume that a fixed version from another branch applies to your installation.
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Reduce exposure while arranging an upgrade
- Restrict Authentication Portal access to trusted zones.
- Disable Response Page in interface management profiles attached to L3 interfaces in zones that can receive untrusted or internet traffic.
- If the Authentication Portal is not required, disable it.
Enable the available Threat Prevention protection
Eligible Threat Prevention subscribers can enable Threat ID 510019 with Applications and Threats content version 9097-10022. Decoder support requires PAN-OS 11.1 or later. Palo Alto presents this as an additional protection; check the advisory and your subscription and platform details to determine applicability. It is not a substitute for installing a fixed PAN-OS version.
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