CVE-2026-86133 in Fireware OS
Summary
by MITRE • 09/30/2026
An integer underflow vulnerability in the WatchGuard Fireware OS IKE daemon (iked) allows a remote attacker who has completed the initial IKEv2 handshake to crash the iked process by sending a specially crafted encrypted IKEv2 message, resulting in a denial of service.
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Analysis
by VulDB Data Team • 09/30/2026
The identified security flaw resides within the Internet Key Exchange version 2 implementation, specifically affecting the iked daemon present in WatchGuard Fireware operating systems. This vulnerability is classified as an integer underflow condition that occurs during the processing of encrypted IKEv2 messages. The root cause lies in how the service handles arithmetic operations related to payload lengths or sequence numbers when parsing incoming packets from a peer who has already successfully completed the initial authentication and key exchange phases. By manipulating specific fields within these encrypted payloads, an attacker can trigger a mathematical error where a subtraction operation results in a negative value that is then interpreted as a large positive unsigned integer due to standard two's complement representation rules. This miscalculation leads to incorrect memory allocation or buffer boundary calculations during subsequent processing steps.
The operational impact of this vulnerability is severe and directly impacts the availability of network security infrastructure. Because the flaw causes the iked process to crash, it results in a complete denial of service for IKEv2 negotiations on the affected device. This disruption prevents new IPsec tunnels from being established and can cause existing secure connections to drop if they rely on periodic rekeying mechanisms managed by the daemon. For organizations relying on WatchGuard firewalls as their primary perimeter defense or site-to-site VPN gateway, this outage effectively blinds them to remote access requirements and disrupts inter-network communications that depend on encrypted tunnels. The attacker does not require administrative privileges but must possess network connectivity sufficient to reach the firewall's IKE service port, typically UDP 500 or UDP 4500.
From a threat modeling perspective, this vulnerability aligns with CWE-191, which defines integer underflow errors that can lead to buffer overflows or other memory corruption issues, although in this specific instance, it manifests primarily as a service crash rather than arbitrary code execution. The exploitation technique corresponds to the MITRE ATT&CK tactic of Impact, specifically subcategory Denial of Service via resource exhaustion or process termination. It also relates to Initial Access techniques involving valid accounts if the attacker compromises credentials prior to launching the attack, though the core exploit relies on protocol-level manipulation rather than credential theft. The requirement for a completed initial handshake means that while authentication is bypassed in terms of crashing the service, the attacker must still navigate past any pre-shared key or certificate validation checks required by the IKEv2 phase 1 negotiation.
Mitigation strategies should prioritize immediate patching to update the iked component to a version where integer arithmetic operations are properly validated against minimum bounds before processing. Network administrators can implement temporary workarounds such as restricting access to UDP ports 500 and 4500 from untrusted networks using ACLs if remote IKEv2 connectivity is not actively required by all users or sites. Additionally, deploying intrusion prevention systems with signatures capable of detecting malformed IKEv2 packets may provide a layer of defense against exploitation attempts while patches are being applied. Regular auditing of firewall logs for unexpected iked restarts can help identify potential probing activity targeting this specific vulnerability before it leads to service disruption.