CVE-1999-1591 is a high-severity vulnerability affecting Microsoft Internet Information Services (IIS) versions 4.0 Service Pack 4 (SP4) and 6.0. The vulnerability arises because IIS, in these versions without certain hotfixes, fails to enforce authentication under specific conditions. This allows remote attackers to bypass authentication mechanisms entirely. The issue was demonstrated using Microsoft Visual InterDev 6.0, which could connect to the IIS server and gain unauthorized access without providing valid credentials. The vulnerability impacts confidentiality, integrity, and availability, as unauthorized users can potentially access sensitive data, modify web content, or disrupt web services. The CVSS score of 7.5 (AV:N/AC:L/Au:N/C:P/I:P/A:P) reflects that the vulnerability is remotely exploitable over the network without authentication, requires low attack complexity, and can lead to partial compromise of confidentiality, integrity, and availability. Although no official patches or hotfixes are currently available, the vulnerability is well-documented and known since 1999. Exploitation does not require user interaction, making it a significant risk for unpatched IIS servers still in operation. Given the age of the affected IIS versions, modern environments are less likely to be impacted, but legacy systems or specialized industrial or governmental applications might still be vulnerable.

For European organizations, the impact of this vulnerability can be substantial if legacy IIS 4.0 SP4 or 6.0 servers remain in use. Unauthorized access to web servers can lead to data breaches involving sensitive personal or corporate data, violating GDPR and other data protection regulations. Integrity compromise could allow attackers to alter website content, inject malicious code, or deface public-facing portals, damaging organizational reputation and trust. Availability impacts could disrupt critical web services, affecting business continuity. Sectors such as government, healthcare, finance, and critical infrastructure operators that may still rely on legacy systems are particularly at risk. Furthermore, the lack of authentication requirements under certain conditions could facilitate lateral movement within internal networks if exploited, increasing the scope of damage. Although no known exploits are currently active in the wild, the vulnerability’s characteristics make it a potential target for opportunistic attackers scanning for legacy IIS servers in Europe.

Given the absence of official patches, European organizations should prioritize the following mitigations: 1) Immediate identification and inventory of IIS servers running versions 4.0 SP4 or 6.0, especially those exposed to external networks. 2) Decommission or upgrade legacy IIS servers to supported versions with active security updates. 3) If upgrading is not immediately feasible, restrict network access to vulnerable IIS servers using firewalls or network segmentation to limit exposure to trusted internal users only. 4) Employ Web Application Firewalls (WAFs) with rules designed to detect and block unauthorized access attempts and anomalous Visual InterDev connections. 5) Implement strict monitoring and logging of IIS server access to detect suspicious authentication bypass attempts. 6) Harden IIS configurations by disabling unnecessary services and features, and enforce strong authentication mechanisms where possible. 7) Conduct regular vulnerability scans and penetration tests focused on legacy systems to identify and remediate weaknesses proactively. 8) Educate IT staff about the risks associated with legacy IIS versions and the importance of timely system upgrades.