CVE-1999-0861 in IIS
Summary
by MITRE
Race condition in the SSL ISAPI filter in IIS and other servers may leak information in plaintext.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability identified as CVE-1999-0861 represents a critical race condition flaw within the SSL ISAPI filter implementation in Microsoft Internet Information Services and similar web servers. This issue emerged during a period when secure web communications were rapidly evolving, and the complexity of implementing proper SSL/TLS encryption in web server environments was still being fully understood. The race condition occurs when multiple concurrent requests are processed simultaneously, creating a window where the system's state management becomes inconsistent, leading to potential information leakage.
The technical flaw manifests specifically within the ISAPI (Internet Server Application Programming Interface) filter mechanism that handles SSL encryption for web traffic. When multiple HTTP requests are processed concurrently through the same SSL ISAPI filter instance, the system fails to properly synchronize access to shared resources and memory buffers. This synchronization failure creates a temporal gap where plaintext data can be inadvertently exposed to unauthorized parties, particularly during the handshaking process or when processing encrypted connections. The vulnerability is particularly dangerous because it operates at the protocol level, affecting the fundamental security guarantees that SSL/TLS is designed to provide.
The operational impact of this vulnerability extends beyond simple information disclosure, as it fundamentally undermines the security model of encrypted web communications. Attackers can exploit this race condition to capture sensitive data that should remain encrypted, including session identifiers, authentication tokens, and potentially confidential business or personal information. The vulnerability affects not only Microsoft IIS but also other web servers that implement similar SSL ISAPI filter mechanisms, making it a widespread concern across the web server ecosystem. Organizations relying on these servers for secure communications face significant risk of data breaches, particularly in environments handling financial transactions, personal identification information, or corporate confidential data.
Mitigation strategies for CVE-1999-0861 require immediate attention through patch management and architectural modifications. Microsoft released security updates specifically addressing this vulnerability, and organizations should prioritize applying these patches to their affected systems. Additionally, administrators should consider implementing additional network-level protections such as firewall rules to limit access to SSL-enabled services and monitor for unusual traffic patterns that might indicate exploitation attempts. The vulnerability aligns with CWE-367, which describes time-of-check to time-of-use (TOCTOU) vulnerabilities, and maps to ATT&CK technique T1566, representing the exploitation of vulnerabilities for initial access. Organizations should also implement proper logging and monitoring to detect potential exploitation attempts and establish incident response procedures specifically addressing information leakage vulnerabilities. The remediation process must include thorough testing of patched systems to ensure that the race condition has been properly resolved without introducing new stability issues in the web server environment.