CVE-1999-0667 identifies a fundamental vulnerability in the Address Resolution Protocol (ARP), which is used to map IP addresses to MAC addresses within local area networks. The vulnerability arises because ARP lacks any authentication mechanism, allowing any host on the same local network segment to send spoofed ARP replies. This spoofing can poison the ARP cache of other devices, causing them to associate an IP address with a malicious MAC address controlled by the attacker. As a result, attackers can perform man-in-the-middle (MITM) attacks, intercepting, modifying, or blocking network traffic. Additionally, attackers can conduct denial of service (DoS) attacks by poisoning ARP caches with invalid mappings, disrupting normal network communications. The vulnerability has a CVSS score of 10, indicating critical severity with network vector, low attack complexity, no authentication required, and full impact on confidentiality, integrity, and availability. Despite its age and fundamental nature, this vulnerability remains relevant because ARP is still widely used in IPv4 networks, and the protocol itself has not been redesigned to include security features. No official patches exist since ARP is a protocol standard rather than a software product, so mitigation relies on network design and security controls rather than software updates.

For European organizations, the impact of ARP spoofing can be significant, especially in environments relying heavily on IPv4 LANs such as corporate offices, data centers, and industrial control systems. Successful exploitation can lead to interception of sensitive data, including credentials and confidential communications, undermining data confidentiality and integrity. It can also disrupt network availability by causing denial of service conditions. This can affect critical infrastructure sectors such as finance, healthcare, manufacturing, and government agencies, potentially leading to operational disruptions and regulatory compliance issues under GDPR and other data protection laws. The ease of exploitation means that attackers with local network access, including malicious insiders or compromised devices, can leverage this vulnerability to escalate attacks or move laterally within networks.

Mitigation requires a multi-layered approach beyond generic advice: 1) Implement Dynamic ARP Inspection (DAI) on managed switches to validate ARP packets against trusted DHCP snooping databases, preventing unauthorized ARP replies. 2) Use static ARP entries for critical servers and network devices where feasible to eliminate reliance on dynamic ARP resolution. 3) Segment networks using VLANs to limit the broadcast domain and reduce the attack surface for ARP spoofing. 4) Deploy network intrusion detection systems (NIDS) with ARP spoofing detection capabilities to alert on suspicious ARP activity. 5) Employ encrypted communication protocols (e.g., TLS, IPsec) to protect data even if ARP spoofing occurs. 6) Regularly audit and monitor network traffic for anomalies indicative of ARP poisoning. 7) Educate network administrators and users about the risks and signs of ARP spoofing attacks.