CVE-2026-0300: Palo Alto Networks PAN-OS Out-of-bounds Write Vulnerability

Should you act?

Exploited: patch now if you run it

CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Yes, if you run Palo Alto Networks PAN-OS. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2026-05-06, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2026-05-09; treat that as the outer limit, not the target.
• EPSS 31.7%, percentile 98: FIRST’s model estimates a 31.7% chance of exploitation activity in the next 30 days, higher than 98% of all scored CVEs (very high). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.

What it is

A buffer overflow vulnerability in the User-ID™ Authentication Portal (aka Captive Portal) service of Palo Alto Networks PAN-OS software allows an unauthenticated attacker to execute arbitrary code with root privileges on the PA-Series and VM-Series firewalls by sending specially crafted packets. The risk of this issue is greatly reduced if you secure access to the User-ID™ Authentication Portal per the best practice guidelines https://knowledgebase.paloaltonetworks.com/KCSArticleDetail by restricting access to only trusted internal IP addresses. Prisma Access, Cloud NGFW and Panorama appliances are not impacted by this vulnerability.

CISA’s summary

Palo Alto Networks PAN-OS contains an out-of-bounds write vulnerability in the User-ID Authentication Portal (aka Captive Portal) service that can allow an unauthenticated attacker to execute arbitrary code with root privileges on the PA-Series and VM-Series firewalls by sending specially crafted packets.

Weakness type: CWE-787.

Product: Palo Alto Networks PAN-OS · Assigned by: palo_alto · Published: 2026-05-06

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: added 2026-05-06; federal due date 2026-05-09. Known ransomware use: Unknown.
• CISA required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. Until the vendor releases an official fix, the following workaround should be implemented: - Restrict User-ID Authentication Portal access to only trusted zones. - Disable User-ID Authentication Portal if not required. 5/13/2026: Palo Alto has released a variety of patches. If these are relevant to your environment, please apply the designated patch.
• CISA SSVC assessment: exploitation active, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 31.72% probability of exploitation activity in the next 30 days; percentile 98.

Timeline

• 2026-05-06 — Added to CISA’s Known Exploited Vulnerabilities catalog
• 2026-05-06 — CVE record published by palo_alto
• 2026-05-09 — CISA remediation deadline for US federal agencies
• 2026-10-15 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. security.paloaltonetworks.com security.paloaltonetworks.com

Vulnerability records

1. CVE.org record CVE-2026-0300 cve.org
2. NVD entry CVE-2026-0300 nvd.nist.gov

Government

1. CISA Known Exploited Vulnerabilities catalog entry cisa.gov

Other references from the CVE record and advisory

1. cert-portal.siemens.com cert-portal.siemens.com

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