CVE-1999-1537 is a vulnerability affecting Microsoft Internet Information Server (IIS) versions 3.0 and 4.0. The core issue lies in IIS's inability to differentiate between web pages that require encryption and those that do not. Specifically, IIS treats all HTTPS requests uniformly, even for files that are normally served without encryption. When a remote attacker sends SSL requests to the HTTPS port for these unencrypted files, IIS is forced to perform additional processing to encrypt and send the files over SSL. This extra workload can be exploited to cause resource exhaustion on the server, leading to a denial of service (DoS) condition. The vulnerability does not impact confidentiality or integrity directly, as it does not allow unauthorized data access or modification, but it affects availability by overwhelming server resources. The CVSS score of 5.0 (medium severity) reflects this impact, with an attack vector of network (no physical or local access needed), low attack complexity, no authentication required, and no impact on confidentiality or integrity, only availability. No patches are available for this vulnerability, and there are no known exploits in the wild. Given the age of the affected IIS versions (released in the late 1990s), this vulnerability is primarily relevant in legacy environments still running IIS 3.x or 4.x. Modern IIS versions have addressed this issue. The vulnerability highlights the importance of proper handling of encrypted versus unencrypted content to avoid unnecessary resource consumption and potential DoS scenarios.

For European organizations, the impact of CVE-1999-1537 is generally low in modern contexts because IIS 3.x and 4.x are obsolete and rarely used in production environments. However, organizations with legacy systems or critical infrastructure still relying on these IIS versions could face denial of service attacks that disrupt web services. Such disruption could affect availability of internal or external web applications, potentially impacting business operations, customer access, or critical services. In sectors like government, healthcare, or finance where legacy systems may persist, the risk is more pronounced. Additionally, denial of service attacks could be leveraged as part of a broader attack campaign to distract or degrade defenses. The lack of patches means mitigation must rely on compensating controls. Overall, the threat is limited in scope but could cause operational interruptions in legacy-dependent environments.

Given that no patches are available for IIS 3.x and 4.x, European organizations should prioritize the following mitigations: 1) Upgrade to supported, modern versions of IIS or alternative web servers that properly distinguish between encrypted and unencrypted content and have current security support. 2) If upgrade is not immediately feasible, isolate legacy IIS servers from direct internet exposure by placing them behind firewalls or reverse proxies that can filter and limit HTTPS requests, particularly those targeting unencrypted files. 3) Implement network-level rate limiting or intrusion prevention systems (IPS) to detect and block abnormal SSL request patterns that could indicate resource exhaustion attempts. 4) Monitor server performance and logs for unusual spikes in SSL traffic or resource usage to enable early detection of potential DoS attacks. 5) Consider disabling HTTPS on legacy IIS servers if encryption is not required, or configure SSL termination at a front-end device to offload SSL processing and reduce server load. 6) Conduct regular security audits to identify legacy systems and plan their decommissioning or replacement to reduce exposure to known vulnerabilities.