CVE-1999-0278 is a vulnerability affecting Microsoft Internet Information Server (IIS) versions 3.0 and 4.0. The flaw allows remote attackers to obtain the source code of ASP (Active Server Pages) files by appending the string "::$DATA" to the URL of the ASP file. This technique exploits the way IIS handles alternate data streams (ADS) on the underlying NTFS file system. By requesting the ASP file with the "::$DATA" suffix, the server returns the raw source code of the ASP script instead of executing it. This exposure can reveal sensitive information such as database connection strings, authentication credentials, or business logic embedded in the ASP code. The vulnerability requires no authentication and can be exploited remotely over the network with low complexity. The CVSS v2 score is 5.0 (medium severity), reflecting the confidentiality impact (partial disclosure of source code) without affecting integrity or availability. Microsoft issued patches to address this issue, documented in security bulletin MS98-003. No known exploits in the wild have been reported, but the vulnerability remains a significant risk if unpatched due to the sensitive nature of source code disclosure.

For European organizations, this vulnerability poses a risk of confidential information leakage, which can lead to further attacks such as credential theft, privilege escalation, or business logic manipulation. Organizations relying on IIS 3.0 or 4.0 to serve ASP applications may inadvertently expose their source code, undermining intellectual property protection and compliance with data protection regulations like GDPR. The exposure of sensitive configuration details could facilitate lateral movement within networks or targeted attacks against critical infrastructure. Although these IIS versions are legacy and largely replaced, some legacy systems in European industries, government agencies, or critical infrastructure sectors might still be in use, increasing the risk. The vulnerability does not directly impact system availability or integrity but compromises confidentiality, which can have cascading effects on organizational security posture.

1. Immediate patching: Apply the security updates provided by Microsoft in bulletin MS98-003 to eliminate the vulnerability. 2. Upgrade IIS: Migrate from IIS 3.0 or 4.0 to supported, modern versions of IIS that do not have this vulnerability and receive ongoing security updates. 3. Restrict access: Implement network-level controls such as firewalls or reverse proxies to restrict access to legacy IIS servers, limiting exposure to untrusted networks. 4. Monitor logs: Enable detailed logging and monitor for suspicious URL requests containing the "::$DATA" suffix or other anomalous patterns indicating attempts to access source code. 5. Code review and secrets management: Review ASP code for embedded sensitive information and remove or secure credentials using environment variables or secure vaults. 6. Segmentation: Isolate legacy IIS servers from critical network segments to reduce potential impact if compromised. 7. Incident response readiness: Prepare to respond to potential data leakage incidents by having forensic and remediation plans in place.