CVE-2000-0339 is a high-severity vulnerability affecting ZoneAlarm firewall versions 2.1.10 and earlier. The core issue lies in the firewall's failure to properly filter UDP packets originating from source port 67, which is typically used by DHCP servers. This flaw allows remote attackers to bypass firewall rules by sending specially crafted UDP packets with a source port of 67. Because the firewall does not inspect or block these packets as expected, attackers can potentially gain unauthorized access to internal network resources or manipulate network traffic. The vulnerability impacts confidentiality, integrity, and availability, as attackers can circumvent security controls, inject malicious traffic, or disrupt legitimate communications. The CVSS score of 7.5 (AV:N/AC:L/Au:N/C:P/I:P/A:P) reflects that the vulnerability is remotely exploitable over the network without authentication, with low attack complexity, and can lead to partial compromise of confidentiality, integrity, and availability. No patch is available for this vulnerability, and there are no known exploits in the wild, likely due to the age of the product version and the evolution of firewall technologies since 2000. However, systems still running these legacy versions remain at risk if exposed to untrusted networks. The vulnerability is particularly relevant for organizations relying on ZoneAlarm 2.1.10 or earlier as a perimeter defense, especially in environments where DHCP traffic is common and trusted, as attackers can masquerade malicious packets as legitimate DHCP responses or requests to bypass firewall protections.

For European organizations, the impact of this vulnerability can be significant if legacy ZoneAlarm firewall versions are still in use, particularly in small to medium enterprises or legacy industrial control environments where older software persists. Successful exploitation could lead to unauthorized access to internal systems, data leakage, or disruption of network services. This could compromise sensitive business information, intellectual property, or personal data protected under GDPR, leading to regulatory penalties and reputational damage. Additionally, attackers could leverage this bypass to pivot within the network, escalating privileges or deploying malware. The lack of a patch means organizations must rely on compensating controls, increasing operational complexity. Given the vulnerability's age, it is less likely to affect modern enterprise environments but remains a risk in legacy or poorly maintained systems.

Since no patch is available, European organizations should implement the following specific mitigations: 1) Immediately identify and inventory all systems running ZoneAlarm 2.1.10 or earlier and plan for their upgrade or replacement with supported firewall solutions. 2) Implement network segmentation to isolate legacy systems from critical assets and limit exposure to untrusted networks. 3) Deploy additional network-level filtering on perimeter routers or next-generation firewalls to block or scrutinize UDP packets with source port 67 originating from untrusted sources. 4) Monitor network traffic for anomalous UDP packets with source port 67 that do not correspond to legitimate DHCP servers. 5) Enforce strict DHCP server authentication and validation mechanisms to prevent spoofing. 6) Educate network administrators about this vulnerability to ensure awareness and prompt incident response. 7) Where possible, disable or restrict DHCP traffic on network segments that do not require it to reduce attack surface.