Used in attacks· fix by 2026-05-09Not scored yet

Palo Alto Networks PAN-OS

CVE-2026-0300

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.

What this means for your business

  • It affects Palo Alto Networks PAN-OS. It matters if your company, or a supplier that handles your data, runs it.
  • Attackers are already using it. CISA added it to its list of flaws used in real attacks on 6 May 2026.
  • FIRST's prediction model gives it a 32% chance of attack attempts being seen in the next 30 days, ranking above 98% of all known flaws.

What to do

  1. 1Check whether your company or your suppliers run Palo Alto Networks PAN-OS.
  2. 2If you do, follow CISA's required action below. No patch reference has been published yet.
  3. 3Treat it as urgent. US federal agencies must fix it by 9 May 2026. That deadline does not apply to private companies, but it shows how seriously CISA takes it.
  4. 4Because attackers have used it, check the affected systems for signs of a break-in, not only for the update.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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’s wording, unedited.

Scoring

CVSS
Not yet scored

Dates

Published
2026-05-06
Added to KEV
2026-05-06
Remediation due
2026-05-09
Sources
CISA KEV