CVE-2009-4444 in IIS
Summary
by MITRE
Microsoft Internet Information Services (IIS) 5.x and 6.x uses only the portion of a filename before a ; (semicolon) character to determine the file extension, which allows remote attackers to bypass intended extension restrictions of third-party upload applications via a filename with a (1) .asp, (2) .cer, or (3) .asa first extension, followed by a semicolon and a safe extension, as demonstrated by the use of asp.dll to handle a .asp;.jpg file.
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Analysis
by VulDB Data Team • 02/19/2017
Microsoft Internet Information Services versions 5.x and 6.x contain a critical file extension handling vulnerability that stems from improper filename parsing logic. The flaw occurs when IIS processes filenames containing semicolon characters, where the server only considers the portion of the filename preceding the semicolon to determine the file extension for security policy enforcement. This behavior creates a significant bypass mechanism for attackers seeking to circumvent file upload restrictions implemented by third-party applications. The vulnerability is classified under CWE-15 as improper neutralization of special elements, specifically related to the handling of semicolon characters in file paths. The technical implementation of this flaw allows malicious users to upload files with dangerous extensions such as .asp, .cer, or .asa followed by a semicolon and a seemingly safe extension like .jpg or .gif, effectively tricking the system into treating the malicious file as a legitimate image or document.
The operational impact of this vulnerability extends beyond simple file upload restrictions, as it enables remote code execution through the exploitation of IIS's handling of dynamic server pages. When a file such as test.asp;.jpg is uploaded, the system recognizes the extension as .jpg for initial upload validation but then processes the actual file through the .asp handler due to the semicolon delimiter, allowing attackers to execute malicious code on the server. This vulnerability specifically affects the IIS 5.x and 6.x versions where the web server's extension mapping logic does not properly sanitize semicolon characters from filenames during security checks. The attack vector is particularly dangerous because it leverages the legitimate functionality of IIS's extension handling while bypassing the security controls implemented by third-party upload applications. According to ATT&CK framework, this vulnerability maps to T1190 - Exploit Public-Facing Application and T1059 - Command and Scripting Interpreter, as it enables attackers to execute commands through the IIS server's scripting capabilities. The flaw essentially creates a path traversal and execution bypass mechanism that can be exploited across multiple attack stages, from initial access through to privilege escalation.
The mitigation strategies for this vulnerability require immediate implementation of IIS updates and configuration changes to address the core parsing issue. Organizations should apply the relevant Microsoft security patches that fix the filename extension handling logic in IIS versions 5.x and 6.x. Additionally, administrators must implement strict filename validation on the application layer to prevent semicolon characters from being processed in uploaded filenames. Network-level restrictions should be configured to block files with dangerous extensions regardless of their presence in semicolon-delimited names. The vulnerability also necessitates a comprehensive review of existing upload restrictions and implementation of proper file type validation that considers the entire filename rather than just the portion before delimiters. Security monitoring should be enhanced to detect suspicious file upload patterns and unusual execution of server-side scripts. Organizations should also consider implementing web application firewalls and additional security controls to prevent exploitation attempts. The remediation process must include thorough testing of file upload functionalities to ensure that the fix does not break legitimate application functionality while effectively blocking the semicolon-based bypass mechanism. This vulnerability demonstrates the critical importance of proper input validation and the potential consequences of incomplete security implementations in web server software.