Palo Alto Networks has released an advisory warning that a critical buffer overflow vulnerability in its PAN-OS software has been exploited in the wild. The vulnerability, tracked as CVE-2026-0300, has been described as a case of unauthenticated remote code execution. It carries a CVSS score of 9.3 if the User-ID Authentication Portal is configured to enable access from the internet or any untrusted network. The severity comes down to 8.7 if access to the portal is restricted to only trusted internal IP addresses. "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 company said. According to Palo Alto Networks, the vulnerability has come under "limited exploitation," specifically targeting instances where the User-ID Authentication Portal has been left publicly accessible. The following versions are impacted by the flaw - - 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 The issue, as it stands, is unpatched, with Palo Alto Networks planning to release fixes starting May 13, 2026. The company also said the vulnerability is applicable only to PA-Series and VM-Series firewalls that are configured to use the User-ID Authentication Portal. "Customers following standard security best practices, such as restricting sensitive portals to trusted internal networks are at a greatly reduced risk," it added. In the absence of a patch, users are advised to either restrict User-ID Authentication Portal access to only trusted zones, or disable it entirely, if it's not required. Update The U.S. Cybersecurity and Infrastructure Security Agency (CISA), on May 6, 2026, added CVE-2026-0300 to its Known Exploited Vulnerabilities (KEV) catalog, requiring Federal Civilian Executive Branch (FCEB) agencies to apply the fixes or mitigations by May 9, 2026. "This vulnerability is specific to a limited number of customers with their User-ID Authentication Portal (Captive Portal) exposed to the public internet or untrusted IP addresses," a spokesperson for Palo Alto Networks told The Hacker News. "We have observed limited exploitation of this issue and are working to release software fixes, with the first updates expected to be available on May 13, 2026." "We have provided clear mitigation guidance to our customers to secure their environments immediately. This issue does not impact Cloud NGFW or Panorama appliances. We remain committed to a transparent, security-first approach to protect our global customer base." (The story was updated after publication to reflect the latest developments.)

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