Palo Alto Networks has disclosed that threat actors may have attempted to unsuccessfully exploit a recently disclosed critical security flaw as early as April 9, 2026.
The vulnerability in question is CVE-2026-0300 (CVSS score: 9.3/8.7), a buffer overflow vulnerability in the User-ID Authentication Portal service of Palo Alto Networks PAN-OS software that could allow an unauthenticated attacker to execute arbitrary code with root privileges by sending specially crafted packets.
While fixes are expected to be released starting May 13, 2026, customers are advised to secure access to the PAN-OS User-ID Authentication Portal by restricting access to trusted zones, or by disabling it entirely if it's not used.
In an advisory issued Wednesday, the network security company said it's aware of limited exploitation of the flaw. It's tracking the activity under the CL-STA-1132, a suspected state-sponsored threat cluster of unknown provenance.
"The attacker behind this activity exploited CVE-2026-0300 to achieve unauthenticated remote code execution (RCE) in PAN-OS software. Upon successful exploitation, the attacker was able to inject shellcode into an nginx worker process," Palo Alto Networks Unit 42 said.
The cybersecurity company said it has observed unsuccessful exploitation attempts against a PAN-OS device starting April 9, 2026, a week after which the attackers managed to successfully obtain remote code execution against the appliance and inject shellcode.
As soon as initial access was achieved, the threat actors took steps to clear crash kernel messages, delete nginx crash entries and nginx crash records, and remove crash core dump files in an attempt to cover up the tracks.
Post-exploitation activities conducted by the adversary included conducting Active Directory (AD) enumeration and dropping additional payloads like EarthWorm and ReverseSocks5 against a second device on April 29, 2026. Both tools have been previously used by various China-nexus hacking groups.
"Over the last five years, nation-state threat actors engaged in cyber espionage have increasingly focused their efforts on edge-network technological assets, including firewalls, routers, IoT devices, hypervisors and various VPN solutions, which provide high-privilege access while often lacking the robust logging and security agents found on standard endpoints," Unit 42 said.
"The reliance of the attackers behind CL-STA-1132 on open-source tooling, rather than proprietary malware, minimized signature-based detection and facilitated seamless environment integration. This technical choice, combined with a disciplined operational cadence of intermittent interactive sessions over a multi-week period, intentionally remained below the behavioral thresholds of most automated alerting systems."
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