CVE-2003-0526 in ISA Server
Summary
by MITRE
Cross-site scripting (XSS) vulnerability in Microsoft Internet Security and Acceleration (ISA) Server 2000 allows remote attackers to inject arbitrary web script via a URL containing the script in the domain name portion, which is not properly cleansed in the default error pages (1) 500.htm for "500 Internal Server error" or (2) 404.htm for "404 Not Found."
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Analysis
by VulDB Data Team • 08/12/2025
The vulnerability described in CVE-2003-0526 represents a critical cross-site scripting flaw within Microsoft Internet Security and Acceleration ISA Server 2000, specifically targeting the server's default error handling mechanisms. This security weakness resides in the way the ISA Server processes and displays error messages when encountering malformed URLs or inaccessible resources. The flaw manifests when the server encounters a request with a malicious script embedded within the domain name portion of a URL, which then gets reflected back to users through the default error pages without proper input sanitization. This particular vulnerability falls under the CWE-79 category of Cross-Site Scripting, which is classified as a common web application security weakness that enables attackers to inject client-side scripts into web pages viewed by other users.
The technical exploitation of this vulnerability occurs through the manipulation of URL structures where attackers can embed malicious JavaScript code within the domain name component of a web address. When the ISA Server encounters such a malformed request and generates an error response, it fails to properly sanitize the domain name portion before displaying it in the default error pages. The specific error pages affected are 500.htm for internal server errors and 404.htm for not found errors, which are automatically generated by the server when encountering problematic requests. These default error pages do not implement adequate output encoding or input validation measures to prevent the execution of injected scripts, creating a persistent vector for malicious code delivery to unsuspecting users who might inadvertently click on links containing these crafted URLs.
The operational impact of this vulnerability extends beyond simple script injection, as it provides attackers with a mechanism to execute arbitrary web scripts in the context of the victim's browser session. This capability enables attackers to perform various malicious activities including session hijacking, credential theft, defacement of web content, and redirection to malicious sites. The vulnerability is particularly dangerous because it leverages the server's own error reporting mechanisms to deliver malicious payloads, making it difficult to detect through standard network monitoring. Users who encounter these error pages may unknowingly execute the injected scripts, potentially compromising their browsing sessions and exposing sensitive information. The attack vector is particularly effective because it can be triggered through normal web browsing behavior, making it hard for users to distinguish between legitimate error messages and maliciously crafted ones.
Mitigation strategies for this vulnerability should focus on implementing proper input validation and output encoding mechanisms within the ISA Server configuration. Organizations should ensure that all default error pages are properly sanitized and that input parameters are validated before being displayed in error contexts. The recommended approach includes disabling the default error pages and implementing custom error handling that properly encodes all user-supplied data before rendering. Additionally, implementing web application firewalls and content filtering solutions can help detect and block malicious script injection attempts. Security updates and patches from Microsoft should be applied immediately, as this vulnerability was addressed in subsequent security releases. The mitigation efforts align with ATT&CK technique T1566.001 which covers the use of malicious links in phishing attacks, and CWE-79 which specifically addresses cross-site scripting vulnerabilities in web applications. Regular security audits and penetration testing should be conducted to ensure that similar vulnerabilities are not present in other components of the web infrastructure, as this type of flaw can often be found in various web server configurations and application frameworks.