CVE-2004-1109 in Personal Firewall
Summary
by MITRE
The FWDRV.SYS driver in Kerio Personal Firewall 4.1.1 and earlier allows remote attackers to cause a denial of service (CPU consumption and system freeze from infinite loop) via a (1) TCP, (2) UDP, or (3) ICMP packet with a zero length IP Option field.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 07/22/2025
The vulnerability described in CVE-2004-1109 represents a critical denial of service flaw within the FWDRV.SYS kernel driver component of Kerio Personal Firewall version 4.1.1 and earlier. This issue manifests when the firewall driver processes network packets containing malformed IP options with zero length fields, creating a condition that can be exploited remotely to consume excessive CPU resources and ultimately freeze the entire system. The vulnerability affects all three primary transport protocols TCP, UDP, and ICMP, making it particularly dangerous as attackers can leverage any of these protocols to trigger the malicious behavior. The flaw exists at the kernel level within the firewall driver, which processes incoming network traffic before it reaches the application layer, making it a fundamental security weakness in the system's network stack processing.
The technical root cause of this vulnerability lies in the improper validation and handling of IP option fields within the FWDRV.SYS driver. When a packet arrives with a zero length IP Option field, the driver fails to properly validate this condition and instead enters an infinite loop during processing. This infinite loop consumes 100% of available CPU cycles, effectively rendering the system unresponsive and causing a complete denial of service. The vulnerability maps to CWE-835, which specifically addresses the issue of infinite loops in software systems, and represents a classic example of inadequate input validation in kernel-mode drivers. The driver's failure to implement proper bounds checking and loop termination conditions creates a predictable execution path that attackers can exploit through carefully crafted network packets, making this vulnerability both reliable and easy to weaponize.
The operational impact of this vulnerability extends beyond simple system unavailability to encompass potential business disruption and security implications. Organizations relying on Kerio Personal Firewall for network protection face significant risks as attackers can remotely incapacitate systems without requiring authentication or specialized privileges. The infinite loop condition causes the system to become completely unresponsive, making it impossible for users to access services or perform administrative tasks. This type of denial of service attack can be particularly damaging in environments where system availability is critical, such as financial services, healthcare systems, or any infrastructure requiring continuous uptime. The vulnerability also represents a potential vector for distributed denial of service attacks when combined with other exploitation techniques, as multiple systems could be simultaneously targeted to create cascading failures across network segments.
Mitigation strategies for this vulnerability should focus on immediate remediation through software updates, as the vendor has released patches addressing the specific driver flaw. Organizations should prioritize updating to Kerio Personal Firewall version 4.1.2 or later, which contains the necessary fixes for the FWDRV.SYS driver. Network administrators should also implement additional protective measures such as packet filtering rules that can identify and drop malformed packets containing zero length IP options before they reach the vulnerable driver. The implementation of intrusion detection systems capable of recognizing patterns associated with this specific vulnerability can provide early warning capabilities. From an ATT&CK framework perspective, this vulnerability aligns with techniques categorized under privilege escalation and denial of service, specifically targeting the system's network stack and kernel components. Organizations should also consider implementing network segmentation and access controls to limit the potential impact of such attacks, while maintaining comprehensive logging and monitoring to detect exploitation attempts. Regular security assessments and vulnerability scanning should be conducted to identify similar flaws in other network security components and ensure overall system resilience against similar threats.