CVE-1999-0561 in IIS
Summary
by MITRE
IIS has the #exec function enabled for Server Side Include (SSI) files.
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Analysis
by VulDB Data Team • 04/18/2026
The vulnerability described in CVE-1999-0561 represents a critical security flaw in Microsoft Internet Information Services version 4.0 and earlier versions that allows unauthorized execution of server-side commands through Server Side Include functionality. This issue specifically relates to the #exec directive being enabled within SSI files, which creates a dangerous attack surface where malicious users can execute arbitrary commands on the web server. The vulnerability stems from the default configuration of IIS where the #exec function remains enabled, allowing attackers to inject and execute system commands directly through specially crafted SSI files.
The technical flaw manifests when the web server processes SSI directives that contain the #exec command, which can execute operating system commands and return the output to the client. This functionality is inherently dangerous because it allows attackers to leverage the web server's privileges to perform actions such as reading system files, executing programs, accessing network resources, or even gaining full system control. The vulnerability exists because the #exec directive is enabled by default in IIS 4.0 and earlier versions, providing attackers with a straightforward path to command execution without requiring additional authentication or privilege escalation. This represents a classic example of insecure input handling and privilege escalation through server-side processing.
The operational impact of this vulnerability is severe and far-reaching, as it allows attackers to completely compromise web servers running vulnerable IIS versions. An attacker can use the #exec function to execute commands such as dir, type, or system-specific commands that reveal system information, access sensitive files, or even establish backdoors. The vulnerability enables attackers to perform reconnaissance, escalate privileges, and potentially gain complete control over the affected system. This can result in data breaches, system compromise, denial of service attacks, and unauthorized access to network resources that are typically protected by the server's security controls. The impact extends beyond individual server compromise to potentially affect entire network infrastructures that rely on compromised IIS servers as entry points.
Organizations should immediately disable the #exec directive in SSI configurations and ensure that all IIS servers are updated to versions that either disable this functionality by default or provide proper access controls. The recommended mitigation includes modifying the SSI configuration to remove or restrict the #exec directive, implementing proper input validation for all SSI files, and applying the latest security patches from Microsoft. Additionally, organizations should consider implementing network segmentation, access control lists, and monitoring solutions to detect and prevent exploitation attempts. This vulnerability aligns with CWE-78 and CWE-88 categories related to command injection and improper input sanitization, and represents a technique that attackers can use to achieve persistent access through the ATT&CK framework's Execution and Privilege Escalation phases. Regular security assessments and vulnerability scanning should be implemented to ensure that SSI configurations remain secure and that no unauthorized execution capabilities exist within web server environments.