#bufferoverflow

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 affecting PA-Series and VM-Series firewalls.

The Vulnerability: CVE-2026-0300

This is a buffer overflow vulnerability in the User-ID Authentication Portal (also known as Captive Portal) service of Palo Alto Networks PAN-OS software. The flaw allows an unauthenticated attacker to execute arbitrary code with root privileges on affected firewalls by sending specially crafted packets.

Severity Ratings:

- CVSS 9.3 (Critical): When User-ID Authentication Portal is configured to enable access from the internet or any untrusted network - CVSS 8.7 (High): When access to the portal is restricted to only trusted internal IP addresses

The difference in severity scores highlights a critical security principle: network segmentation and access control can significantly reduce risk exposure, even for severe vulnerabilities.

Active Exploitation Confirmed

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. This is not a theoretical threat—attackers are actively scanning for and exploiting vulnerable systems in the wild.

The "limited" nature of exploitation suggests that:

- Attackers are being selective, targeting high-value or easily accessible victims - Exploitation requires specific configurations (publicly exposed portals) - The threat landscape may escalate as exploit code matures or spreads

Affected Versions

PAN-OS 12.1 is the primary affected version. Critically, the issue is currently unpatched, with Palo Alto Networks planning to release fixes starting May 13, 2026.

This creates a dangerous window where:

- Attackers know about the vulnerability - Active exploitation is occurring - No official patch is available yet - Organizations must rely on mitigations rather than fixes

The vulnerability is applicable only to PA-Series and VM-Series firewalls that are configured to use the User-ID Authentication Portal. Organizations not using this feature are not affected.

Why This Matters

Palo Alto Networks firewalls are deployed at the security perimeter of countless enterprises, government agencies, and critical infrastructure organizations. They are trusted to inspect, filter, and control network traffic. A remote code execution vulnerability with root privileges means:

- Complete Firewall Compromise: Attackers can bypass all security policies, inspect or modify traffic, and pivot into internal networks - Persistence: Root access allows attackers to install backdoors that survive reboots and configuration changes - Lateral Movement: Compromised firewalls provide a strategic position to attack other systems on the network - Data Exfiltration: Attackers can intercept sensitive traffic passing through the firewall - Supply Chain Risk: Managed service providers (MSPs) managing multiple client firewalls could see widespread compromise

Immediate Mitigation Steps

In the absence of a patch, Palo Alto Networks recommends the following actions:

1. Restrict Portal Access (Preferred)

Limit User-ID Authentication Portal access to only trusted zones and internal IP addresses. This should be done through:

- Firewall management interface access controls - Network segmentation (place management interfaces on isolated VLANs) - VPN requirements for remote management - IP allowlisting for administrative access 2. Disable the Portal (If Not Required)

If your organization does not actively use the User-ID Authentication Portal feature, disable it entirely. This eliminates the attack surface completely.

3. Monitor for Suspicious Activity

Implement enhanced monitoring for:

- Unexpected firewall configuration changes - Unusual outbound connections from firewall management interfaces - Authentication failures or anomalies on the Captive Portal - Unexpected system processes or resource usage on firewall appliances 4. Network-Level Protections

Deploy additional layers of defense:

- Ensure firewalls are not directly exposed to the internet - Use upstream firewalls or WAFs to filter traffic to management interfaces - Implement intrusion detection/prevention systems (IDS/IPS) to detect exploitation attempts

Reflection: The Pattern of Perimeter Vulnerabilities

CVE-2026-0300 follows a disturbing pattern we've seen repeatedly in network security infrastructure:

1. Management Interface Exposure

Time and again, critical vulnerabilities exploit the gap between "trusted" and "untrusted" network boundaries. The User-ID Authentication Portal is designed for internal user tracking, yet when exposed to the internet, it becomes a direct path to root compromise. This raises fundamental questions:

- Why are management portals ever internet-accessible by default? - Should features like Captive Portal require explicit opt-in for external access? - Are vendors doing enough to enforce secure-by-default configurations? 2. The Patch Gap Problem

The period between vulnerability disclosure and patch availability is when organizations are most vulnerable. In this case, Palo Alto Networks has confirmed active exploitation but won't release a fix until May 13, 2026. This creates a 7+ day window (from early May disclosure) where:

- Defenders are powerless to fully remediate - Attackers have free rein against unpatched systems - Mitigations become the only line of defense

This highlights the importance of:

- Coordinated Disclosure: Vendors should ideally have patches ready before public disclosure - Emergency Mitigations: Clear, actionable guidance for the unpatched window - Rapid Response: Organizations must have processes to implement mitigations within hours, not days 3. The Human Factor

The vulnerability specifically targets configurations where the User-ID Portal is publicly accessible. This is almost always a configuration choice, not a default state. Yet, such configurations persist because:

- Convenience overrides security (easier to access from anywhere) - Lack of awareness about exposure (teams don't audit their own configs) - Legacy decisions (set up years ago, forgotten) - Third-party management (MSPs may have different security standards) 4. Critical Infrastructure at Risk

Palo Alto firewalls protect some of the most sensitive networks on the planet—financial institutions, healthcare systems, government agencies, and critical infrastructure. A single exploited firewall can provide attackers with:

- Visibility into all network traffic - Ability to modify or inject malicious traffic - Platform for attacking downstream systems - Potential for widespread disruption if used destructively

The "limited exploitation" characterization should not create complacency. Nation-state actors, ransomware groups, and cybercriminals all have strong incentives to target firewall infrastructure.

Lessons for Security Teams

1. Audit Your Exposure

Immediately verify whether your Palo Alto firewalls have the User-ID Authentication Portal enabled and whether it's accessible from untrusted networks. Use tools like:

- Palo Alto Networks Expedition (configuration analysis) - External vulnerability scans from internet-facing IPs - Firewall log analysis for unexpected access attempts 2. Assume Breach

Given active exploitation, operate under the assumption that attackers are scanning for and targeting vulnerable systems. Implement detection capabilities that assume the perimeter may be compromised:

- Monitor firewall integrity and configuration changes - Deploy network detection and response (NDR) tools - Implement zero-trust principles for internal traffic 3. Prepare for Patch Day

When patches are released on May 13, have a rapid deployment plan ready:

- Test patches in non-production environments immediately - Schedule maintenance windows for emergency updates - Coordinate with change management to expedite approval - Have rollback plans in case of patch issues 4. Review Vendor Security Posture

This incident should prompt broader questions about your network security vendor relationships:

- How quickly do they respond to critical vulnerabilities? - Do they provide clear mitigation guidance? - Are their default configurations secure? - What's their track record for patch timelines?

Broader Industry Implications

CVE-2026-0300 is part of a larger pattern of critical infrastructure vulnerabilities we've seen in 2026:

- Network Infrastructure Targeting: Attackers increasingly focus on routers, firewalls, and switches as high-value targets - Authentication Bypass: Unauthenticated RCE vulnerabilities eliminate the need for credential theft - Root Privilege Escalation: Direct root access maximizes attacker control - Zero-Day to N-Day Acceleration: Time between disclosure and exploitation continues to shrink

Organizations must adapt by treating network perimeter security as a living, breathing defense layer that requires constant vigilance—not a "set it and forget it" solution.

Timeline

- Early May 2026: Palo Alto Networks discovers active exploitation - May 2026: Advisory released, vulnerability disclosed as CVE-2026-0300 - May 13, 2026 (Planned): Patch release date for PAN-OS 12.1 - Ongoing: Limited exploitation continues against unpatched, exposed systems

Conclusion

CVE-2026-0300 is a stark reminder that even the most trusted security infrastructure can become an attacker's greatest asset when vulnerabilities are exploited. The combination of active exploitation, root-level access, and a patch gap creates a perfect storm that demands immediate attention.

Organizations running Palo Alto PA-Series or VM-Series firewalls with PAN-OS 12.1 must act now—not when the patch is released. Restrict or disable the User-ID Authentication Portal, enhance monitoring, and prepare for rapid patch deployment. The cost of inaction could be complete network compromise.

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So, 0x04 was pretty easy- it was a simple integer overflow attack; in all honesty I solved this without analyzing the source code well and took the time to analyze it after.

I understand the flow of the C now since the analysis above. I may actually make it standard practice to do this before I look into exploitation as it might help my speed! But yeah... Bit of a silly one in my opinion: I don’t think you’d come across anything like this realistically.

The first two bytes of the given file are stored into bar.len.

If len+1 bitwise AND 0xff (255) is greater than 1024, “Bad dog.“ is returned- else the contents of the file is returned starting at offset 2...

What? Like was this challenge written at 2AM? :’)

Anyway, my approach was pretty clever in it’s simplicity- this is why I don’t look at hints, you find different ways to solve things!

My payload is pretty simple: <1034 padding/shellcode><4 bytes junk (originally I thought it was EBP since input here overflowed the EBP register; as the program exits normally after our shellcode without segfault I was wrong, probably because the file closes first and thus EBP is changed by this function)><4 byte EIP/NOP address><4 byte expected file pointer>

I found the file pointer address when providing 1043 bytes of input to the file and performing a backtrace via GDB: I could see that the offsets I’d provided were correct as EIP and EBP were populated, but without the correct file pointer the program was constantly segfaulting before the point of EIP execution.

The second address is the desired file pointer. I think this was needed as the execve call works by essentially replacing program memory with the memory of the new process, inheriting the relevant file descriptors to do this.

Our payload led to the file pointer’s (or ESI’s) LSB being overwritten by the terminating ‘0a‘ newline character, meaning it was invalid and attempts to close the file by execve before executing /bin/sh were unsuccessful.

Reflection

I don’t really think much of this challenge if I’m honest- I don’t think there’s any sort of scenario in which anyone would encounter code with logic as random as the above.

However, I did notice how much my understanding of stack attacks is developing at a low level: as I said, I basically constructed this payload myself from simple trial and error (and a bit of logic!)

All these challenges sort of become trivial after you’ve spent most your something awesome doing CTFs (including reverse engineering) and doing the COMP6841 CTF, although I thought I would dump up my solutions here anyway. I use a nifty exploitation library known as pwntools which simplifies the process of the exploit. A number of key things (commands) I’ll explain with regards to some of the things I do in this:

process() - Pretty self explanatory, just launch the specified process.

recvline() - Read exactly 1 line of input, i.e. till ‘\n’.

sendline() - Sends the given input, terminated by a ‘\n’.

cyclic() - Produces a de Bruijn sequence of order 4 of the specified length using a given alphabet.

wait() - Wait for the program to finish executing.

Coredump() - Read in the dump after terminating execution; think of this as the ‘current state of the program’ before termination.

ELF() - Reads in the ELF file for the program; contains ‘symbol look-ups and and a relocatable table’.

interactive() - Reconnects program up to STDIN and STDOUT.

The key part of all these exploits is using cyclic to determine the offset of the return address; since every set of 4 bytes is unique, we can determine at what offset the return address is based on the seg fault address. We read this from ‘core.eip’ - it contains the instruction pointer when the program dies. All we need to do now is overwrite this with the symbol for win - we can just use ‘objdump’ to find the name of the symbol and then specify it in the program. I should probably mention you can ‘just strings’ all the overflows except for 1.1.2 to get the flag.

1.0-basic

1.1-whereami

1.1.2-returnToSqrOne

1.2-blind

Bufferover-flow Challenges by LachJones (my fork)

Basic

So we’ve been given two files, basic and basic.c.

Basically, we need to change a char on the stack from ‘A’ to ‘B’.

So because we have access to the .c file, I know the buffer is a size of 32 bytes. So using pwntools, I can develop a payload set for the 33th byte to be an 8-bit integer of 66, the ascii value for ‘B’.

Boom - works

whereami

Same kind of deal, we’ve been given the binary and a .c file.

Here, the binary is telling you where the program is about to jump to (address value), so using cyclic, I can determine the size of the payload easily.

So the buffer size is 64 bytes. Similarly, prepare a pwn python script and inject the payload of the given hex value for the win address.

Blind

So here, we aren’t provided the source code, so we’re going to have crack open the binary.

Inputting a cyclic string in gdb (to read seg fault address) yields:

Which means the buffer is 72 bytes long. Similarly to above, a python script is generated.

Looking in the binary, the function win is found at the address of: 0x80484d6, and hence, we know what our payload is.

Boom