CVE-1999-0725 is a high-severity vulnerability affecting Microsoft Internet Information Server (IIS) versions 3.0 and 4.0 when configured with a default language setting of Chinese, Korean, or Japanese. This vulnerability, known as the "Double Byte Code Page" issue, allows a remote attacker to view the source code of certain files on the web server. The root cause lies in IIS's handling of double-byte character sets (DBCS) used in East Asian languages, which can cause the server to incorrectly process file extensions and bypass normal content handling rules. As a result, attackers can request files that should be processed by the server (such as ASP scripts) but instead receive the raw source code, exposing sensitive information like database connection strings, application logic, and credentials. The vulnerability requires no authentication and can be exploited remotely over the network, with a moderate attack complexity. The CVSS v2 base score is 7.1, reflecting a high impact on confidentiality but no impact on integrity or availability. Microsoft released patches in 1999 (MS99-022) to address this issue by correcting the handling of double-byte character sets in IIS. Although no known exploits have been reported in the wild, the vulnerability remains critical for legacy systems still running these IIS versions with affected language settings.

For European organizations, this vulnerability poses a significant risk primarily to those still operating legacy IIS 3.0 or 4.0 servers configured for East Asian languages. Exposure of source code can lead to leakage of sensitive business logic, credentials, and other confidential data, potentially enabling further attacks such as privilege escalation, data breaches, or lateral movement within networks. While the direct impact is confidentiality loss, the indirect consequences could include regulatory non-compliance (e.g., GDPR) due to data exposure, reputational damage, and financial losses. Organizations in Europe with subsidiaries, partners, or customer bases in East Asia might be more likely to have such configurations. Additionally, multinational companies with legacy infrastructure or those supporting multilingual websites could be at risk. Although modern IIS versions and configurations are not affected, the presence of unpatched legacy systems in critical infrastructure or industrial sectors could present a notable attack surface.

1. Immediate patching: Apply the official Microsoft security update MS99-022 to all affected IIS 3.0 and 4.0 servers to fix the double-byte character handling flaw. 2. Upgrade IIS: Migrate legacy IIS servers to supported, modern versions that do not exhibit this vulnerability and benefit from ongoing security updates. 3. Configuration review: Verify and restrict default language settings to only those necessary for business operations, avoiding East Asian default languages unless required. 4. Access controls: Implement strict network segmentation and firewall rules to limit external access to legacy IIS servers, reducing exposure to remote attacks. 5. Source code protection: Use web server configurations or application-level controls to prevent direct access to source code files regardless of language settings. 6. Monitoring and detection: Deploy intrusion detection systems and log analysis to identify suspicious requests that attempt to exploit double-byte character encoding bypasses. 7. Legacy system audit: Conduct comprehensive audits to identify any remaining IIS 3.0 or 4.0 instances and prioritize their remediation or decommissioning.