CVE-1999-0191 is a vulnerability found in Microsoft Internet Information Server (IIS) version 3.0, specifically involving the newdsn.exe CGI script. This script is designed to create new Data Source Names (DSNs) for ODBC connections. The vulnerability allows remote, unauthenticated attackers to overwrite arbitrary files on the server by exploiting improper input validation in the newdsn.exe script. Because the CGI script runs with the privileges of the IIS process, successful exploitation can lead to modification of critical files, potentially enabling attackers to alter server behavior, inject malicious code, or disrupt services. The vulnerability does not require any authentication and can be triggered remotely over the network, making it relatively easy to exploit. The CVSS score of 6.4 (medium severity) reflects the potential confidentiality and integrity impacts, with no direct impact on availability. Although no patches are available due to the age of the software, the vulnerability remains a concern for legacy systems still running IIS 3.0. No known exploits are currently reported in the wild, but the risk remains given the ease of exploitation and the critical nature of file overwrite capabilities.

For European organizations, the impact of this vulnerability could be significant if legacy IIS 3.0 servers are still in use, particularly in environments where these servers host sensitive or critical applications. Successful exploitation could lead to unauthorized modification of configuration files, web content, or executable scripts, resulting in data integrity breaches and potential unauthorized access. This could compromise confidential information, damage organizational reputation, and disrupt business operations. Although IIS 3.0 is largely obsolete, some industrial control systems, legacy applications, or archival systems might still rely on it, especially in sectors with long equipment lifecycles such as manufacturing or utilities. The vulnerability's ability to be exploited remotely without authentication increases the risk of attacks originating from outside the organization, including from hostile actors targeting European infrastructure or intellectual property.

Given that no official patches are available for IIS 3.0, European organizations should prioritize decommissioning or upgrading affected servers to supported versions of IIS or alternative web servers. If immediate upgrade is not feasible, organizations should implement network-level protections such as firewall rules to restrict access to the newdsn.exe CGI script or the entire IIS server from untrusted networks. Disabling or removing the newdsn.exe script entirely is a critical mitigation step to eliminate the attack vector. Additionally, organizations should conduct thorough audits to identify any legacy IIS 3.0 deployments and isolate them within segmented network zones with strict access controls. Regular monitoring and logging of web server activity can help detect any attempts to exploit this vulnerability. Employing web application firewalls (WAFs) with custom rules to block suspicious requests targeting newdsn.exe can provide an additional layer of defense. Finally, organizations should ensure that backups of critical files are maintained and tested for restoration to recover quickly from any potential file overwrites.