CVE-1999-1148 in IIS
Summary
by MITRE
FTP service in IIS 4.0 and earlier allows remote attackers to cause a denial of service (resource exhaustion) via many passive (PASV) connections at the same time.
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Analysis
by VulDB Data Team • 04/20/2026
The vulnerability described in CVE-1999-1148 represents a critical resource exhaustion flaw affecting Internet Information Services version 4.0 and earlier implementations. This issue specifically targets the File Transfer Protocol service component within Microsoft IIS, which serves as a fundamental web server platform for hosting websites and applications during the late 1990s and early 2000s. The vulnerability stems from the FTP service's inadequate handling of concurrent passive connection requests, creating a pathway for malicious actors to exploit system resources through deliberate overuse of the PASV command functionality.
The technical mechanism behind this vulnerability operates through the passive FTP mode implementation where the FTP server listens for incoming connections from clients. When multiple simultaneous passive connections are established, the IIS FTP service fails to properly manage these connections, leading to resource exhaustion. Each passive connection consumes system resources including memory, file handles, and network sockets. Attackers can leverage this flaw by initiating numerous concurrent PASV commands, effectively exhausting the system's available resources and causing the FTP service to become unresponsive or crash entirely. This behavior aligns with CWE-400, which categorizes resource exhaustion vulnerabilities as those that allow attackers to consume system resources beyond normal operational limits.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire server's stability and availability. When the FTP service becomes unresponsive due to resource exhaustion, legitimate users cannot establish FTP connections, leading to denial of service for authorized personnel and potentially disrupting business operations. The vulnerability affects the broader system because the FTP service operates at the core level of IIS, meaning that resource exhaustion in this component can cascade into affecting other services running on the same server instance. This type of attack represents a classic denial of service vector that can be executed with minimal technical expertise, making it particularly dangerous for organizations relying on IIS 4.0 or earlier versions.
Security practitioners should recognize this vulnerability as part of the broader ATT&CK framework's denial of service tactics, specifically targeting the service availability aspect of system security. Organizations must implement immediate mitigations including connection rate limiting, implementing firewall rules to restrict FTP connection attempts, and applying the appropriate security patches released by Microsoft. The vulnerability highlights the importance of proper resource management in server applications and demonstrates how seemingly simple protocol implementations can create significant security risks. Additionally, this issue underscores the necessity of regular security assessments and patch management procedures to prevent exploitation of known vulnerabilities. The affected systems should be upgraded to newer versions of IIS that have addressed these resource management deficiencies, as the original IIS 4.0 implementation lacked adequate protection against such resource exhaustion attacks.