CVE-2003-1491 in Personal Firewall
Summary
by MITRE
Kerio Personal Firewall (KPF) 2.1.4 has a default rule to accept incoming packets from DNS (UDP port 53), which allows remote attackers to bypass the firewall filters via packets with a source port of 53.
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Analysis
by VulDB Data Team • 03/08/2021
Kerio Personal Firewall version 2.1.4 contains a critical configuration flaw that undermines its core security posture through an overly permissive default rule set. The vulnerability manifests as a default rule that accepts incoming packets specifically from DNS traffic using UDP port 53, creating an unintended backdoor that remote attackers can exploit to circumvent all other firewall filtering mechanisms. This design decision fundamentally compromises the firewall's ability to enforce network security policies, as it allows unrestricted ingress from any source that can spoof or manipulate DNS traffic patterns.
The technical exploitation of this vulnerability occurs through a sophisticated packet manipulation technique where attackers craft malicious UDP packets with a source port of 53, mimicking legitimate DNS responses. This approach leverages the inherent trust placed in DNS traffic by the firewall's default configuration, effectively bypassing all other security controls that would normally block unauthorized incoming connections. The flaw represents a classic example of insufficient access control and improper privilege management, aligning with CWE-284 which addresses inadequate access control mechanisms in network security implementations.
From an operational impact perspective, this vulnerability exposes systems protected by KPF 2.1.4 to a wide range of potential attacks including but not limited to port scanning, service exploitation, and lateral movement within network perimeters. The default rule essentially creates a persistent entry point that remains active regardless of custom firewall policies configured by administrators, making it particularly dangerous in enterprise environments where multiple security layers are expected to work in conjunction. Attackers can utilize this bypass to establish covert communication channels, exfiltrate data, or deploy malicious payloads without triggering any firewall alerts or blocking mechanisms.
The security implications extend beyond immediate exploitation as this vulnerability demonstrates a fundamental flaw in the firewall's rule evaluation process and default security configuration. Organizations implementing KPF 2.1.4 may experience false security assumptions, leading to inadequate network monitoring and incident response procedures. This vulnerability aligns with ATT&CK technique T1071.004 which covers application layer protocol usage for command and control communications, as attackers can leverage the legitimate DNS traffic pattern to maintain persistence. The flaw also represents a failure in principle of least privilege enforcement, where the firewall grants unnecessary access based on service port rather than source address validation, a pattern commonly associated with weak network segmentation practices.
Mitigation strategies should include immediate rule modification to restrict DNS traffic sources to trusted internal servers only, implementation of source address validation, and deployment of additional network monitoring to detect anomalous DNS traffic patterns. Administrators must also consider implementing network access control lists that specifically block incoming DNS responses from external sources and establish comprehensive firewall rule auditing procedures to identify similar configuration flaws. The vulnerability underscores the critical importance of regular security assessments and proper default configuration management, as these issues can persist undetected for extended periods, providing attackers with persistent access to protected network resources.