CVE-1999-0253 is a high-severity vulnerability affecting Microsoft Internet Information Server (IIS) versions 1.0, 2.0, and 3.0, specifically when the 'iis-fix' hotfix is installed. This vulnerability allows remote attackers to read the source code of ASP (Active Server Pages) programs by exploiting a URL encoding flaw. By substituting a dot ('.') with its URL-encoded equivalent '%2e' in the URL, an attacker can bypass normal request handling and access the raw ASP source files instead of the processed output. This exposure reveals sensitive server-side script logic, including business logic, database queries, and potentially embedded credentials or configuration details. The vulnerability is remotely exploitable without authentication and requires only a crafted HTTP request, making it relatively easy to exploit. The CVSS v2 score is 7.5, reflecting a high impact on confidentiality, integrity, and availability, as attackers can gain insight into server-side code, potentially facilitating further attacks such as code injection or privilege escalation. No patch is available for this vulnerability, and there are no known exploits in the wild currently documented. However, given the age of the affected IIS versions (released in the mid-1990s), modern systems are unlikely to be affected unless legacy infrastructure remains in use. This vulnerability highlights the risks of legacy software and incomplete hotfixes that inadvertently introduce new attack vectors.

For European organizations, the impact of CVE-1999-0253 depends largely on whether legacy IIS servers (versions 1.0, 2.0, or 3.0) are still operational within their infrastructure. If such legacy systems are in use, attackers could remotely access sensitive ASP source code, leading to exposure of intellectual property, business logic, and sensitive configuration data. This could facilitate further exploitation such as injection attacks, unauthorized data access, or service disruption. Confidentiality is primarily at risk, but integrity and availability could also be impacted if attackers leverage the disclosed source code to craft more damaging attacks. Although modern IIS versions are not affected, some European organizations in sectors with long IT lifecycles (e.g., manufacturing, government, or critical infrastructure) might still run legacy systems, increasing their risk. Additionally, the lack of a patch means organizations must rely on compensating controls or system upgrades. The vulnerability's ease of exploitation and remote nature make it a significant risk if legacy IIS servers are exposed to the internet or untrusted networks.

Given the absence of an official patch, European organizations should prioritize the following mitigations: 1) Immediate identification and inventory of any IIS 1.0, 2.0, or 3.0 servers, especially those with the 'iis-fix' hotfix installed. 2) Decommission or upgrade legacy IIS servers to supported, modern versions that have robust security controls and are actively maintained. 3) If legacy systems must remain operational, isolate them from public networks using network segmentation and strict firewall rules to prevent unauthorized remote access. 4) Implement web application firewalls (WAFs) capable of detecting and blocking malicious URL encoding attempts, such as requests containing '%2e' in suspicious contexts. 5) Conduct regular security assessments and penetration testing focused on legacy infrastructure to detect potential exploitation attempts. 6) Monitor web server logs for unusual URL patterns indicative of exploitation attempts. 7) Educate IT staff about the risks of legacy software and the importance of timely upgrades and patch management. These steps go beyond generic advice by focusing on legacy system management, network isolation, and proactive detection tailored to this specific vulnerability.