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Introduction
A newly disclosed buffer overflow vulnerability (CWE-787) has been discovered in the Palo Alto Networks PAN-OS software’s User-ID Authentication Portal (Captive Portal), receiving a CVSS 4.0 score of 9.3 (Critical). This unpatched zero-day is already under “limited exploitation” targeting systems where the portal is exposed to the public internet or untrusted networks.
Learning Objectives
- Root-Level RCE Understanding: Learn how an unauthenticated attacker can exploit a buffer overflow with a specially crafted packet to execute arbitrary code with root privileges without credentials, gaining full control over the firewall.
- Assess Your Exposure: Identify if your PAN-OS devices are in the affected versions range and if the vulnerable portal is enabled, especially on internet-facing interfaces.
- Implement Immediate Mitigation: Master step-by-step configurations to restrict access to trusted zones or disable the Captive Portal entirely, effectively neutralizing the attack vector until an official patch is released.
You Should Know
1. Technical Analysis of the Buffer Overflow Vulnerability
This vulnerability exists due to a boundary error within the User-ID Authentication Portal service. A remote attacker can send specially crafted packets to the device, triggering an out-of-bounds write. This allows the attacker to overwrite adjacent memory, redirecting the program’s execution flow to inject and run arbitrary code with the highest root privileges on the system.
Who is at Risk? The flaw affects PA-Series and VM-Series firewalls running specific branches of PAN-OS version 10.2.x, 11.1.x, 11.2.x, and 12.1.x that have the User-ID Authentication Portal configured. Prisma Access, Cloud NGFW, and Panorama appliances are not impacted by this vulnerability.
Step-by-step guide for manual verification includes command-line testing spanning Linux, Windows, and firewall management interfaces to confirm your vulnerable status:
Option 1: Check PAN-OS Version and Portal Status (Firewall CLI)
Connect to your Palo Alto Networks firewall via SSH and use the following command to check your software version:
show system info | match "sw-version"
This command returns your current PAN-OS version. Compare the output against the vulnerable version matrix below:
Affected Versions (Use this as a reference)
- 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
Next, verify if the Captive Portal is configured on an interface facing an untrusted zone:
show running config | match "set deviceconfig setting captive-portal"
A non-empty or configured result indicates an enabled Captive Portal, potentially exposing the attack surface.
Option 2: Remote Scanning (Linux)
From an external Linux machine with `nmap` installed, scan for the default User-ID portal port. Use this command:
nmap -p 443,6082 <target-firewall-ip> -sV --script=http-title
– -p 443,6082: Specifies common HTTPS and default User-ID portal ports.
– -sV: Enables version detection.
– --script=http-title: Fetches the web page title to help identify the portal service.
A response indicating a “Captive Portal” or “User-ID” login page over an untrusted network confirms public exposure.
Option 3: Exposed Portal Check (Windows with PowerShell)
Administrators on a Windows jump box can use PowerShell’s `Test-NetConnection` to check if the firewall service is externally reachable:
Test-NetConnection -ComputerName <target-firewall-ip> -Port 443
If the command returns TcpTestSucceeded: True, the web interface is accessible. Following this, you can use Invoke-WebRequest:
Invoke-WebRequest -Uri "https://<target-firewall-ip>/php/login.php" -SkipCertificateCheck
Be careful, this request could trigger logging on the target firewall.
2. Threat Landscape: Active Exploitation in the Wild
As of May 6, 2026, this vulnerability is being actively exploited in limited, targeted campaigns. Palo Alto Networks has officially noted that “limited exploitation has been observed targeting User-ID Authentication Portals that are exposed to untrusted IP addresses and/or the public internet”.
Attack Vector: Network-based, low attack complexity, requiring no privileges or user interaction.
Impact: Total compromise of firewall confidentiality, integrity, and availability. Attackers can decrypt VPN traffic, modify security policies, pivot into internal networks, and install persistent backdoors.
Step‑by‑step guide for detection using Linux tools
Set up a simple packet capture to monitor for potential network scanning. Run this `tcpdump` command to capture all traffic to your firewall’s external interface:
sudo tcpdump -i eth0 host <firewall-external-ip> and (tcp port 443 or tcp port 6082) -c 1000 -w pan-capture.pcap
– -i eth0: Replace `eth0` with your monitoring interface.
– host <firewall-external-ip>: Filters traffic to and from your firewall’s IP.
– \(tcp port 443 or tcp port 6082\): Listens for suspicious internet scans on web ports.
– -c 1000: Captures 1000 packets.
– -w pan-capture.pcap: Saves the capture to a file.
After capturing, analyze the file with `strings` or `Wireshark` to look for malformed HTTP requests:
strings pan-capture.pcap | grep -i "user-id|captive|overflow"
Look for irregular patterns, extremely long user agent strings, or malformed POST requests.
Proactive Hunt using Linux `grep` on Firewall Logs
Export your system logs and use `grep` to hunt for exploitation indicators:
grep -i "user-id.buffer|portal.overflow|captive.exception" /var/log/pan/firewall.log
3. Effective Mitigation: Restrict Access to Trusted Zones
The most effective mitigation is to restrict access to the User-ID Authentication Portal to only trusted internal IP addresses. This reduces the attack surface from a CVSS 9.3 (Critical) to an 8.7 (High) and dramatically decreases the likelihood of exploitation.
Step‑by‑step console guide for access restriction via PAN-OS CLI:
1. Connect to your firewall via SSH.
2. Enter configuration mode:
configure
3. Access the Captive Portal settings:
set deviceconfig setting captive-portal
4. If no whitelist exists, create a new access list that only includes your internal IP ranges:
set deviceconfig setting captive-portal permitted-ip <internal_subnet_in_CIDR>
For example: `set deviceconfig setting captive-portal permitted-ip 192.168.1.0/24`
5. Verify the new setting:
show deviceconfig setting captive-portal
6. If you have an existing access list, add the internal IP to the permitted list:
set deviceconfig setting captive-portal permitted-ip <existing_ip> <new_internal_subnet>
For example: `set deviceconfig setting captive-portal permitted-ip 10.0.0.0/8`
7. Commit the changes:
commit
4. Hardening: Disabling the Captive Portal
If your organization does not rely on the User-ID Authentication Portal, disabling it entirely is the safest short-term mitigatio. The service is non-essential for basic firewall operations and is often only used for web authentication or guest access.
Step‑by‑step guide to disable via command line:
1. Access the firewall CLI via SSH.
2. Enter configuration mode:
configure
3. Disable the Captive Portal service with the following command：
set deviceconfig setting captive-portal disable yes
4. Confirm the configuration with:
show deviceconfig setting captive-portal
The output should show `disabled: yes`.
- Commit the change to apply the new settings:
commit
After the commit completes, the vulnerable listener will be terminated.
- Final validation: from an external machine, try to access
https://<firewall-ip>/. A connection timeout or connection refused error indicates the portal is no longer serving on the public interface, verifying successful mitigation.
5. Monitoring, Patch Strategy & Threat Prevention
Given that a patch is scheduled for release on May 13, 2026, it’s critical to plan for seamless integration of the security update. The official Palo Alto Networks advisory states the fix will be applied to the following hotfix releases: >=12.1.4-h5 (ETA: 05/13), >=11.2.7-h13 (ETA: 05/13), and >=11.1.4-h33 (ETA: 05/13).
Step‑by‑step guide for patch preparation using Linux and Windows:
1. Inventory your devices from Linux: Use `nmap` to perform a network sweep for all potential PAN-OS hosts:
nmap -sn <your-internal-subnet> | grep "Palo Alto" -B 2
This identifies active Palo Alto devices in your internal network.
2. Automate Patch Installation (Conceptual via API): For larger deployments, PAN-OS supports configuration via API. This `curl` command from a Linux management server initiates a dynamic update check:
curl -k -u '<api_key>:' 'https://<firewall-ip>/api/?type=op&cmd=<show><system><dynamic-updates><installed-version>panos-patch</installed-version></dynamic-updates></show></request>'
This checks for the latest patch status via API, verifying availability. The best practice is to stage downloads during maintenance windows.
3. Enable Threat Prevention Signatures (Windows-based via WebUI or CLI) : Palo Alto Networks released a Threat Prevention signature for PAN-OS 11.1 and higher on May 5, 2026. To activate it from CLI:
request dynamic-updates install type threat version latest
This command forces the firewall to immediately fetch and install the latest threat prevention definitions, providing a protective layer against crafted exploit packets.
4. Monitor Traffic for Exploitation: Use the PAN-OS CLI to monitor Threat logs for any blocked activity related to CVE-2026-0300:
show log threat-direction both | match "CVE-2026-0300"
This search will show you if the threat signature has blocked an attempt against your firewall.
What Undercode Say
- The Real Danger Lies in Misconfiguration: The most alarming aspect of this vulnerability is not just the flaw itself, but how common the risky configuration might be. Many organizations expose authentication portals to the internet for convenience, especially in hybrid or remote work setups. This creates an attack surface that adversaries actively scan for, making this vulnerability particularly exploitable in real-world conditions.
- Root-Level Access Changes the Game: Remote code execution vulnerabilities are always serious, but achieving root privileges elevates this issue into a worst-case scenario. With full system control, attackers can disable security features, manipulate traffic flows, and embed persistent backdoors that survive reboots and even some updates.
- “Limited Exploitation” Doesn’t Mean Low Risk: The phrase “limited exploitation” often leads organizations to underestimate urgency. In reality, this typically means attackers are selectively targeting high-value systems or testing the exploit before scaling operations. Historically, many vulnerabilities labeled this way later evolved into widespread attacks once exploitation techniques became more refined.
Expected Output:
- Secure the User-ID Authentication Portal immediately by restricting access to trusted internal IP addresses via the `permitted-ip` command.
- If not required, fully disable the Captive Portal service with
set deviceconfig setting captive-portal disable yes. - Prepare for the May 13, 2026 patch by inventorying PAN-OS devices with `nmap` and enabling the latest Threat Prevention signatures.
Prediction
The existence of a public proof-of-concept (PoC) on GitHub will inevitably lead to widespread weaponization of this exploit within days post-patch, if not sooner. Once a patch is officially released, attackers will race to reverse-engineer the fix to craft operational exploits for unpatched devices. Organizations that fail to immediately apply the May 13 patch can expect to see opportunistic scans and massive-scale exploitation campaigns within a two-week window after the patch release, targeting the long tail of exposed, unpatched firewalls in enterprise environments. This vulnerability strongly mirrors the dynamics of the 2024 PAN-OS zero-day exploits, where a 9.8 CVSS flaw was eventually leveraged by state-sponsored hackers to backdoor thousands of appliances.
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Reported By: Hackermohitkumar Pan – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
