CVE-2001-0493 in Small HTTP Server
Summary
by MITRE
Small HTTP server 2.03 allows remote attackers to cause a denial of service via a URL that contains an MS-DOS device name such as aux.
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Analysis
by VulDB Data Team • 05/11/2019
The vulnerability identified as CVE-2001-0493 affects Small HTTP Server version 2.03, a lightweight web server implementation that was commonly used in embedded systems and legacy environments. This issue represents a classic denial of service vulnerability that exploits the server's handling of specially crafted URLs containing Microsoft DOS device names. The flaw demonstrates how operating system specific conventions can create unexpected security implications when implemented in network services. The vulnerability specifically targets the server's URL parsing mechanism, where it fails to properly sanitize input containing device names such as aux, con, prn, nul, and other reserved DOS filenames that have special significance in the Windows operating system.
The technical root cause of this vulnerability lies in the server's inadequate input validation and sanitization processes. When the Small HTTP Server receives a request containing a URL with a DOS device name, it attempts to process this path in a manner that conflicts with the underlying operating system's file handling mechanisms. The server's file system operations become compromised when encountering these reserved names, as the operating system treats them as special device files rather than regular file paths. This misinterpretation causes the server to either crash during processing or become unresponsive while attempting to handle the malformed request. The vulnerability operates at the application layer and can be exploited through simple HTTP requests without requiring authentication or specialized tools, making it particularly dangerous for publicly accessible web servers.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be leveraged to create persistent denial of service conditions that may require manual intervention to resolve. Attackers can repeatedly send malicious requests containing DOS device names to keep the server in a non-responsive state, effectively rendering the web service unavailable to legitimate users. This vulnerability is particularly concerning in embedded systems or IoT devices where the server may be running with elevated privileges and where restart procedures might be difficult or impossible to execute remotely. The attack vector demonstrates how legacy system conventions can create security weaknesses in modern network services, especially when developers fail to account for operating system-specific behaviors in their input handling code. This vulnerability also highlights the importance of proper input validation and the potential for operating system level artifacts to create unexpected security implications in network applications.
Mitigation strategies for this vulnerability should focus on implementing robust input validation mechanisms that filter out reserved device names before they can be processed by the server. The most effective approach involves modifying the server's URL parsing logic to recognize and reject patterns that match DOS device names, preventing these special characters from being passed to the underlying file system operations. Network administrators should also consider implementing web application firewalls or intrusion prevention systems that can detect and block requests containing known malicious patterns. Additionally, upgrading to newer versions of the Small HTTP Server or migrating to more modern web server implementations that properly handle input validation would provide long-term protection. This vulnerability aligns with CWE-20, which describes improper input validation, and represents a classic example of how operating system specific conventions can create security weaknesses in network applications. The ATT&CK framework categorizes this as a denial of service attack technique that exploits application layer vulnerabilities, and the remediation strategies should include both immediate patching and long-term architectural improvements to prevent similar issues in other components of the system.