Technical Deep Dive: CVE-2026-0300 Pre-Authentication RCE in PAN-OS

Attack Overview

CVE-2026-0300 is a critical out-of-bounds write vulnerability (CWE-787) affecting the Palo Alto Networks PAN-OS User-ID Authentication Portal (Captive Portal). This flaw allows an unauthenticated, network-based attacker to achieve arbitrary code execution with root privileges. Given its position on the network perimeter and the high privilege level of the vulnerable component, this vulnerability represents a severe risk to organizational infrastructure. It is currently being exploited in the wild and is documented in the CISA Known Exploited Vulnerabilities (KEV) catalog.

Step-by-Step Execution

The exploitation of CVE-2026-0300 typically follows a structured progression:

1. Reconnaissance: Threat actors identify target firewalls where the User-ID Authentication Portal is exposed on L3 interfaces, specifically those with "Response Pages" enabled.
2. Delivery: The attacker sends a specially crafted HTTP POST request to the Authentication Portal endpoint.
3. Trigger: The vulnerable service (typically authd) parses the malformed request. The lack of strict bounds checking on specific input fields leads to an out-of-bounds write.
4. Exploitation: The memory corruption allows the attacker to overwrite the stack, hijacking the execution flow to run arbitrary code as the root user.
5. Persistence and Action: Once root access is established, attackers generally deploy tunneling tools and commence Active Directory (AD) enumeration for further lateral movement.

Code-Level Mechanics

The vulnerability is a classic stack-based buffer overflow. It resides within the packet-processing logic of the User-ID Authentication Portal. When the service parses incoming HTTP requests, it fails to perform adequate length validation on specific input fields before copying the data into internal memory buffers.

An attacker can provide an oversized payload that exceeds the allocated buffer size, overwriting adjacent memory. By precisely crafting this payload, the attacker can overwrite the return address on the stack. When the vulnerable function attempts to return, execution is redirected to an attacker-controlled location, such as memory-resident shellcode or a Return-Oriented Programming (ROP) chain, leading to full system compromise.

// Conceptual representation of the vulnerability
void handle_auth_request(char *user_input) {
    char buffer[1024];
    // UNSAFE: Copying user input without length check
    strcpy(buffer, user_input);
}

Payload Behavior

Observed exploitation attempts have utilized the following techniques:

• Initial Foothold: Execution of memory-resident shellcode to establish a pivot point without leaving immediate forensic traces on the disk.
• Tunneling: Frequent deployment of open-source tunneling tools, including Chisel and Ligolo-ng, to facilitate stable communication and bypass internal security controls.
• Post-Exploitation: Following the initial breach, attackers utilize custom scripts for AD enumeration and move laterally into the internal network via the compromised firewall.

Detection Opportunities

Organizations should focus on the following telemetry to identify potential exploitation:

• Process Monitoring: Monitor for frequent crashes or core dumps associated with the authd or captive portal processes, which may indicate failed exploitation attempts or memory corruption.
• Network Analysis: Identify anomalous outbound connections originating from the firewall itself, particularly those using non-standard ports or known tunneling protocols.
• Security Signatures: Ensure that Palo Alto Networks Threat ID 510019 is active and configured with a 'Reset' or 'Drop' action to intercept known exploit patterns.

Evidence Gaps

While the vulnerability mechanics are known to be a stack-based buffer overflow in authd, the specific malformed packet structure and the exact length-check failure point in the source code have not been publicly disclosed by the vendor. Additionally, while Chisel and Ligolo-ng have been observed, the full breadth of the custom post-exploitation scripts used for AD enumeration remains uncharacterized.