CVE-1999-0874 is a critical buffer overflow vulnerability affecting Microsoft Internet Information Server (IIS) version 4.0. The flaw arises when the server processes malformed HTTP requests targeting files with specific extensions: .HTR, .IDC, or .STM. These extensions are associated with IIS's server-side scripting and data access components. An attacker can craft a malicious request that overflows a buffer in the IIS process, leading to memory corruption. This can cause the server to crash, resulting in a denial of service (DoS). The vulnerability does not require any authentication and can be exploited remotely over the network, making it highly accessible to attackers. The Common Vulnerabilities and Exposures (CVE) entry assigns a CVSS v2 base score of 10.0, indicating maximum severity. The vector metrics (AV:N/AC:L/Au:N/C:C/I:C/A:C) reflect that the attack can be launched remotely without authentication, and it impacts confidentiality, integrity, and availability. Although no known exploits in the wild have been documented, the vulnerability's nature and severity make it a significant risk. Microsoft has released a security bulletin (MS99-019) providing patches and mitigation guidance. The underlying weakness is a classic buffer overflow (CWE-119), a well-understood category of memory corruption bugs that can lead to arbitrary code execution or service disruption. Given the age of IIS 4.0, this vulnerability is largely historical but remains relevant in legacy environments that have not been updated or patched.

For European organizations, the impact of this vulnerability primarily concerns availability and potential service disruption. IIS 4.0 was widely used in the late 1990s and early 2000s, so legacy systems or industrial control environments still running this version are at risk. A successful exploit can cause denial of service, interrupting web services and potentially affecting business operations, customer access, and internal communications. Although the vulnerability also affects confidentiality and integrity per the CVSS vector, practical exploitation beyond DoS is less documented. However, the risk of arbitrary code execution cannot be ruled out, which could lead to further compromise. European organizations with legacy infrastructure, especially in sectors such as manufacturing, utilities, or government agencies that may still rely on outdated IIS versions, are vulnerable. The disruption could have cascading effects on critical services. Additionally, compliance with European data protection regulations (e.g., GDPR) mandates timely patching of known vulnerabilities to protect personal data, so unpatched systems could expose organizations to regulatory penalties.

1. Immediate application of the official Microsoft patch MS99-019 is the primary mitigation step. This patch addresses the buffer overflow and should be deployed on all IIS 4.0 servers. 2. If patching is not immediately feasible, organizations should consider isolating IIS 4.0 servers from untrusted networks or restricting access via firewalls to trusted IP ranges only. 3. Disable or restrict handling of .HTR, .IDC, and .STM file extensions if these are not required by the application, reducing the attack surface. 4. Monitor network traffic for malformed HTTP requests targeting these extensions, using intrusion detection systems (IDS) or web application firewalls (WAF) with custom rules. 5. Plan and execute migration from IIS 4.0 to supported, modern versions of IIS or alternative web servers to eliminate legacy vulnerabilities and improve security posture. 6. Conduct regular vulnerability assessments and penetration testing to identify residual risks in legacy systems. 7. Maintain comprehensive incident response plans to quickly address any exploitation attempts.