CVE-2000-0457 is a high-severity vulnerability affecting Microsoft Internet Information Server (IIS) versions 4.0 and 5.0. The vulnerability resides in the ISM.DLL component, which handles certain file requests. Specifically, remote attackers can exploit this flaw by requesting a file and appending a large number of encoded spaces (%20) followed by a .htr extension. This manipulation causes the server to improperly process the request, allowing attackers to read arbitrary file contents on the server. The vulnerability is commonly referred to as the ".HTR File Fragment Reading" or "File Fragment Reading via .HTR" vulnerability. It enables attackers to bypass normal access controls and retrieve sensitive files, potentially including configuration files, source code, or other critical data. The CVSS score of 7.5 (AV:N/AC:L/Au:N/C:P/I:P/A:P) indicates that the vulnerability can be exploited remotely over the network without authentication, with low attack complexity, and results in partial confidentiality, integrity, and availability impacts. Although no known exploits in the wild have been reported, the vulnerability's nature and ease of exploitation make it a significant risk. Microsoft has released patches to address this issue, detailed in security bulletin MS00-031. The vulnerability primarily affects legacy IIS installations that remain unpatched, which may still be present in some environments despite the age of the software.

For European organizations, this vulnerability poses a substantial risk, especially for those still operating legacy IIS 4.0 or 5.0 servers, which might be found in industrial, governmental, or legacy application environments. Successful exploitation can lead to unauthorized disclosure of sensitive information, including internal configuration files, credentials, or proprietary data, compromising confidentiality. The partial integrity impact suggests that attackers might also manipulate some data or server responses, while availability could be affected if attackers leverage the vulnerability to disrupt services. Given the remote and unauthenticated nature of the exploit, attackers can target vulnerable servers from anywhere, increasing the attack surface. European organizations in sectors such as government, finance, healthcare, and critical infrastructure, where legacy systems may still be in use, are particularly at risk. The exposure of sensitive data could lead to regulatory non-compliance under GDPR, reputational damage, and potential financial losses. Although the vulnerability is over two decades old, unpatched legacy systems remain a common vector for attackers, making this a relevant threat for organizations with outdated infrastructure.

1. Immediate application of the official Microsoft patch MS00-031 to all IIS 4.0 and 5.0 servers is critical to remediate the vulnerability. 2. Conduct a thorough inventory of all IIS servers in the environment to identify any legacy versions still in operation. 3. Where possible, upgrade legacy IIS servers to supported versions or migrate to modern web server platforms to eliminate exposure to this and other legacy vulnerabilities. 4. Implement strict network segmentation and firewall rules to restrict external access to legacy IIS servers, limiting exposure to trusted internal networks only. 5. Deploy web application firewalls (WAFs) with custom rules to detect and block suspicious requests containing excessive encoded spaces or .htr extensions. 6. Monitor web server logs for unusual request patterns indicative of exploitation attempts, such as repeated requests with appended %20 sequences and .htr suffixes. 7. Educate IT and security teams about the risks of legacy systems and enforce policies to phase out unsupported software. 8. Regularly review and update incident response plans to include scenarios involving legacy system exploitation.