CVE-1999-0240 describes a vulnerability in certain network filters or firewalls that improperly handle fragmented SYN packets containing IP reserved bits. Specifically, these security devices allow fragmented TCP SYN packets where the IP header includes reserved bits set, which violates the intended filtering policies. The reserved bits in the IP header are meant to be unused or set to zero, and their presence can be leveraged to bypass firewall rules or filtering logic. Fragmented packets complicate inspection because the firewall must reassemble fragments or inspect them individually, and improper handling can allow malicious SYN packets to pass through undetected. Since SYN packets initiate TCP connections, allowing malformed or policy-violating SYN packets can enable attackers to circumvent firewall rules, potentially leading to unauthorized access or denial of service. The vulnerability has a CVSS score of 7.5 (high severity), indicating a network attack vector with low complexity, no authentication required, and impacts on confidentiality, integrity, and availability. Although this vulnerability was published in 1999 and no patches are available, it remains relevant for legacy systems or outdated firewall implementations that have not been updated to properly handle fragmented packets with reserved IP bits.

For European organizations, this vulnerability poses a risk primarily to network perimeter defenses relying on outdated or improperly configured firewalls and filters. If exploited, attackers could bypass firewall rules, potentially gaining unauthorized access to internal networks or disrupting services via crafted SYN packets. This could lead to data breaches, service interruptions, or lateral movement within corporate networks. Organizations in sectors with critical infrastructure, finance, healthcare, or government services are particularly at risk due to the sensitivity of their data and the potential impact of service disruption. Although modern firewalls and intrusion prevention systems have largely mitigated this issue, legacy devices still in use in some European enterprises or public sector networks could be vulnerable. The impact is compounded by the fact that no patches are available, so mitigation relies on configuration changes or device replacement.

European organizations should audit their network security devices to identify any legacy firewalls or filters that might be susceptible to this vulnerability. Specific mitigation steps include: 1) Upgrading or replacing outdated firewall hardware and software with modern solutions that correctly handle fragmented packets and enforce IP header policies. 2) Configuring firewalls to drop fragmented packets with reserved IP bits or to perform deep packet inspection and reassembly before applying filtering rules. 3) Implementing network segmentation to limit exposure if a firewall is bypassed. 4) Employing intrusion detection/prevention systems (IDS/IPS) that can detect anomalous fragmented SYN packets and alert or block them. 5) Regularly reviewing firewall rules and logs for signs of suspicious fragmented packet traffic. 6) Conducting penetration testing to verify that firewall policies cannot be bypassed using fragmented packets. These measures go beyond generic advice by focusing on legacy device identification, configuration tuning, and layered defense strategies.