CVE-2026-78011: WatchGuard Fireware OS iked DoS Vulnerability

CVE-2026-78011 Overview

CVE-2026-78011 is an integer underflow vulnerability [CWE-191] in the WatchGuard Fireware OS iked process. The iked daemon handles Internet Key Exchange (IKE) negotiation for Virtual Private Network (VPN) sessions. A remote, unauthenticated attacker can send specially crafted network traffic to trigger the underflow. Successful exploitation creates a Denial of Service (DoS) condition in VPN processing on affected firewall appliances.

Critical Impact
Unauthenticated network-based attackers can disrupt VPN availability on WatchGuard Fireware OS devices without any user interaction or prior access.

Affected Products

WatchGuard Fireware OS (see the WatchGuard CVE-2026-78011 Advisory for affected versions)
WatchGuard Firebox appliances running the vulnerable iked process
Deployments exposing IKE/VPN services to untrusted networks

Discovery Timeline

2026-08-28 - CVE-2026-78011 published to the National Vulnerability Database (NVD)
2026-09-03 - Last updated in NVD database

Technical Details for CVE-2026-78011

Vulnerability Analysis

The flaw resides in the iked process, which brokers IKE and IPsec negotiations for VPN clients and site-to-site tunnels. An integer underflow occurs during parsing of attacker-controlled fields in specially crafted network traffic. When a size or length value is decremented below zero, it wraps into a very large unsigned integer. Downstream logic then operates on invalid buffer bounds, corrupting internal state and causing the iked process to terminate or hang.

Because iked is central to VPN session establishment, its failure interrupts existing tunnels and blocks new negotiations. Attackers do not need credentials, user interaction, or any pre-existing tunnel to reach the vulnerable code path. The vulnerability affects availability only; there is no reported impact to confidentiality or integrity.

Root Cause

The root cause is classified as [CWE-191] Integer Underflow (Wrap or Wraparound). The iked parser subtracts a header or field length from a remaining-buffer counter without validating that the counter is greater than or equal to the value being subtracted. The resulting wrap produces an oversized length used in later memory operations or loops, destabilizing the process.

Attack Vector

Exploitation is network-based and unauthenticated. An attacker reaches the vulnerable code path by sending crafted IKE packets to a Fireware appliance exposing VPN services on the internet or an accessible network segment. No authentication, cryptographic handshake completion, or user interaction is required. Repeated packets can be used to sustain the DoS condition against VPN processing.

No public proof-of-concept exploit is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. The EPSS (Exploit Prediction Scoring System) indicates a low near-term exploitation probability at the time of publication.

Detection Methods for CVE-2026-78011

Indicators of Compromise

Unexpected restarts or crashes of the iked process on Fireware appliances
VPN tunnel establishment failures and dropped IKE Phase 1 or Phase 2 negotiations
Sudden loss of remote-access or branch-office VPN connectivity affecting multiple users simultaneously
Bursts of malformed IKE (UDP/500 or UDP/4500) traffic from a single or small set of source addresses

Detection Strategies

Monitor Fireware system logs for iked service termination, respawn, or watchdog events
Inspect firewall telemetry for anomalous IKE packet rates and malformed IKE payloads on UDP/500 and UDP/4500
Correlate VPN client disconnection spikes with inbound traffic patterns to identify targeted DoS activity
Baseline normal VPN session churn and alert on statistically significant deviations

Monitoring Recommendations

Forward Fireware syslog and diagnostic logs to a centralized SIEM or data lake for correlation and retention
Alert on process-crash events tied to iked and on repeated Phase 1 negotiation failures
Track upstream network sensors for crafted IKE traffic targeting firewall management or VPN interfaces
Review high-availability failover events, which may indicate iked instability under attack

How to Mitigate CVE-2026-78011

Immediate Actions Required

Review the WatchGuard CVE-2026-78011 Advisory and identify affected Fireware OS versions in your environment
Apply the vendor-supplied Fireware OS update to remediated versions as soon as maintenance windows allow
Restrict IKE/IPsec exposure to known peer addresses using geo-blocking or explicit allow-lists where feasible
Enable high-availability pairing so a secondary appliance can absorb VPN load if iked fails on the primary

Patch Information

WatchGuard has published guidance and fixed versions in the vendor advisory. Consult the WatchGuard CVE-2026-78011 Advisory for the specific fixed Fireware OS releases and upgrade procedures for your appliance model. Apply patches through the standard Fireware update workflow and verify version strings after upgrade.

Workarounds

Limit inbound IKE traffic (UDP/500 and UDP/4500) to trusted peer IP addresses using policy rules
Disable Mobile VPN and branch-office VPN interfaces on appliances that do not require them until patching completes
Deploy upstream rate limiting or scrubbing for IKE traffic to reduce the impact of malformed packet floods
Ensure change-management and rollback plans are in place before applying firmware updates to production firewalls

bash

# Example: restrict IKE traffic to known VPN peers on an upstream device
# (adapt syntax to your environment; validate before deployment)
iptables -A INPUT -p udp --dport 500  -s <trusted_peer_ip> -j ACCEPT
iptables -A INPUT -p udp --dport 4500 -s <trusted_peer_ip> -j ACCEPT
iptables -A INPUT -p udp --dport 500  -j DROP
iptables -A INPUT -p udp --dport 4500 -j DROP