CVE-1999-0861 is a vulnerability identified as a race condition in the SSL ISAPI filter used by Microsoft Internet Information Services (IIS) versions 2.0, 2.5, 3.0, and 4.0, as well as potentially other servers employing similar SSL ISAPI filters. The race condition occurs during the handling of SSL-encrypted communications, where concurrent processing threads may improperly synchronize access to sensitive data buffers. This flaw can cause portions of SSL-encrypted data to be leaked in plaintext, undermining the confidentiality guarantees of SSL/TLS. The vulnerability is classified under CWE-362 (Race Condition), indicating a timing issue that leads to improper access control or data exposure. Although the CVSS v2 base score is low (2.6), reflecting a low complexity of impact and no authentication required, the vulnerability specifically affects confidentiality (partial information disclosure) without impacting integrity or availability. The vulnerability was disclosed in 1999, and Microsoft has released patches (MS99-053) to address this issue. No known exploits have been reported in the wild, likely due to the age of the vulnerability and the obsolescence of affected IIS versions. However, systems still running these legacy IIS versions remain at risk of plaintext data leakage during SSL communications if unpatched. The vulnerability highlights the importance of proper synchronization in multi-threaded SSL processing components to prevent inadvertent data exposure.

For European organizations, the primary impact of CVE-1999-0861 is the potential leakage of sensitive information transmitted over SSL connections handled by affected IIS servers. Although the vulnerability does not allow modification or denial of service, the exposure of plaintext data could compromise confidentiality, potentially revealing credentials, session tokens, or other sensitive data. This risk is particularly relevant for organizations that still operate legacy IIS servers in their infrastructure, such as in industrial control systems, archival systems, or legacy web applications. Given the low CVSS score and absence of known exploits, the immediate risk is low; however, any exposure of plaintext data in transit can have compliance implications under GDPR and other European data protection regulations. Attackers with network access could exploit the race condition to intercept and reconstruct sensitive data, undermining trust in secure communications. The impact is mitigated if organizations have migrated to supported IIS versions or alternative web servers with updated SSL implementations.

European organizations should ensure that all IIS servers are upgraded to supported versions beyond IIS 4.0, as these legacy versions are no longer maintained or secure. For any remaining systems running affected IIS versions, immediate application of the Microsoft security patch MS99-053 is critical to eliminate the race condition vulnerability. Network segmentation and limiting exposure of legacy IIS servers to untrusted networks can reduce attack surface. Organizations should also consider deploying modern TLS termination proxies or load balancers that handle SSL/TLS offloading with secure, updated implementations. Regular vulnerability scanning and configuration audits should be conducted to identify legacy IIS instances. Additionally, monitoring network traffic for unusual plaintext data leakage patterns can help detect exploitation attempts. Finally, organizations should plan to retire or replace legacy systems that rely on outdated IIS versions to maintain compliance and security hygiene.