CVE-1999-1533 is a high-severity vulnerability affecting the Eicon Technology Diva LAN ISDN modem. The vulnerability arises from the modem's HTTP service, specifically the login.htm file, which accepts a password argument. A remote attacker can exploit this vulnerability by sending an excessively long password argument to the login.htm endpoint. This input causes the modem to hang, effectively resulting in a denial of service (DoS). The vulnerability does not require authentication or user interaction, making it remotely exploitable over the network. The CVSS score of 7.5 reflects the critical nature of this vulnerability, with a vector indicating network attack vector (AV:N), low attack complexity (AC:L), no authentication required (Au:N), and impacts on confidentiality, integrity, and availability (C:P/I:P/A:P). Although the vulnerability was published in 1999 and no patches are available, the affected versions are specifically 3.2.3 and 3.3 of the product, which is identified as interscan_viruswall by Trend Micro, though the root cause is in the Eicon Technology Diva LAN ISDN modem's HTTP service. No known exploits in the wild have been reported, but the potential for disruption remains significant due to the nature of the DoS condition. The vulnerability impacts the availability of the modem, which could disrupt ISDN network connectivity and related communications relying on this hardware.

For European organizations, the impact of this vulnerability can be substantial, particularly for those still using legacy ISDN infrastructure with Eicon Technology Diva LAN ISDN modems. A successful DoS attack could disrupt critical network communications, affecting business operations, especially in sectors relying on ISDN for voice, data, or integrated services. This could lead to operational downtime, loss of productivity, and potential financial losses. Additionally, the compromise of confidentiality and integrity indicated by the CVSS vector suggests that the vulnerability might allow some level of unauthorized data exposure or manipulation, although the primary impact is denial of service. Given that ISDN technology is less common today but may still be in use in certain industrial, governmental, or telecommunications environments in Europe, the threat remains relevant for organizations with legacy systems. The lack of available patches means organizations must rely on compensating controls to mitigate risk.

Since no patches are available for this vulnerability, European organizations should implement specific mitigations to reduce exposure. First, isolate the affected ISDN modems from direct internet access by placing them behind firewalls or network segmentation to restrict access to the HTTP service on the modem. Implement strict access control lists (ACLs) to allow only trusted IP addresses to communicate with the modem's management interface. Monitor network traffic for abnormal HTTP requests, particularly those with unusually long parameters targeting login.htm, to detect and block potential exploitation attempts. Where possible, replace legacy Eicon Technology Diva LAN ISDN modems with modern, supported hardware that does not have this vulnerability. Additionally, organizations should conduct regular security assessments of their ISDN infrastructure and maintain up-to-date network documentation to quickly identify and isolate vulnerable devices. Employing intrusion detection/prevention systems (IDS/IPS) with custom signatures to detect exploitation attempts can further enhance defense.