External risk intelligence

Fireware OS WireGuard Stack Overflow Denial of Service

CVE advisorySeverity: CRITICAL (CVSS 9.5)

CVE-2026-103594

The vulnerability requires a man-in-the-middle attacker to intercept and tamper with TLS-decrypted traffic specifically between a Firebox appliance and WatchGuard Cloud. Because this attack surface is restricted to the internal communication channel between the device and the vendor's cloud management service, it is not a direct public-facing internet endpoint.

Out-of-bounds Write

Halo Surface Signal: 2 out of 5 — less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in Fireware OS affecting its WireGuard configuration processing. This issue could allow an attacker with specific network access to disrupt services or potentially execute unauthorized code through tampering with configuration data. The primary concern is confirming if our environment is exposed to this specific attack path.

A configuration flaw could disrupt services.
Understand exposure to specific network access.
Confirm relevance and ensure no direct impact.

Attack Path

How an attacker could exploit the issue

An attacker with the ability to intercept and modify TLS-decrypted network traffic between a Firebox appliance and WatchGuard Cloud could potentially exploit this vulnerability. By tampering with a FireCloud Gateway configuration payload, the attacker could trigger a buffer overflow, leading to a denial of service and possibly the execution of arbitrary code.

Requires man-in-the-middle access.
Tampering with gateway configuration payload.
Denial of service and code execution.

Live Threat

Current exploitation, exposure, and threat context

A remote attacker with man-in-the-middle access to TLS-decrypted traffic between a Firebox and WatchGuard Cloud could trigger a stack-based buffer overflow. This may allow for denial of service and potentially arbitrary code execution when tampering with a FireCloud Gateway configuration payload.

System data and service behavior at risk.
Tampering with TLS-decrypted traffic.
Denial of service or code execution.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in Fireware OS impacts the WireGuard configuration processing, potentially allowing for Denial of Service or arbitrary code execution. Initial triage should focus on identifying all instances of the affected technology, assessing their exposure and criticality, and confirming the accountable owner for remediation planning.

Ownership: Infrastructure and Security teams.
Verify: Affected technology reachability and criticality.
Action: Plan remediation based on identified risk.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is Fireware OS and its role in WatchGuard devices?

Fireware OS is the primary operating system running on WatchGuard Firebox appliances. It manages network security functions, including firewall rules and VPN connectivity. In this context, it handles the configuration processing for WireGuard tunnels, which allows for secure, high-performance site-to-site or remote-access networking.

What does CWE-121 mean for CVE-2026-103594?

CWE-121 refers to a stack-based buffer overflow. This vulnerability occurs when the software writes more data to a memory buffer located on the stack than it is designed to hold. For this CVE, it means that by sending a specially crafted configuration payload, an attacker can overwrite adjacent memory, leading to system crashes or potentially unauthorized command execution.

How can an attacker trigger this vulnerability?

An attacker must achieve a man-in-the-middle position to intercept and modify TLS-decrypted traffic moving between a Firebox appliance and WatchGuard Cloud. Simply sending malicious traffic to the device's public interface does not trigger this; the attacker must specifically tamper with the FireCloud Gateway configuration payload during transmission.

Do I need to worry about this vulnerability?

Halo Surface Signal labels the attack surface as Unlikely because the exploit requires specialized access to the internal communication channel between the device and its cloud management service. It is not an open, internet-facing endpoint. You should prioritize this if your network architecture or potential attacker access patterns could allow traffic interception between your appliance and the cloud service.

When should I begin remediation for this issue?

Start by identifying all deployed Firebox appliances running Fireware OS. Verify their current configuration and ensure that ownership is assigned to the infrastructure and security teams. Use this triage phase to confirm the criticality of each device and plan your update schedule according to your organization's standard lifecycle and maintenance processes.

References

psirt.watchguard.com/CVE-2026-103594

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.