CVE-2002-2031 in Internet Explorer
Summary
by MITRE
Internet Explorer 5.0, 5.0.1 and 5.5 with JavaScript execution enabled allows remote attackers to determine the existence of arbitrary files via a script tag with a src parameter that references a non-JavaScript file, then using the onError event handler to monitor the results.
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Analysis
by VulDB Data Team • 06/14/2025
This vulnerability exists in Microsoft Internet Explorer versions 5.0, 5.0.1, and 5.5 when JavaScript execution is enabled, representing a significant information disclosure flaw that allows remote attackers to perform file system reconnaissance. The vulnerability stems from how Internet Explorer handles script loading errors when referencing non-JavaScript files through the script tag's src attribute. When a malicious attacker supplies a script tag with a src parameter pointing to a non-JavaScript file, the browser attempts to load and execute the content as JavaScript, which fails. The onError event handler then captures this failure, providing a mechanism for attackers to determine whether specific files exist on the target system. This occurs because the browser's error handling behavior differs between cases where a file exists but cannot be executed as JavaScript versus cases where a file does not exist at all.
The technical implementation of this vulnerability leverages the asynchronous nature of script loading in Internet Explorer's JavaScript engine. When a script tag references a non-JavaScript file, the browser's JavaScript interpreter attempts to parse and execute the content as JavaScript code. If the file exists but contains non-JavaScript content, the parsing fails, triggering the onError handler. However, if the file does not exist, the error handling mechanism behaves differently, creating distinguishable error states that attackers can observe and correlate with file existence. This flaw essentially creates a timing-based side-channel attack vector that reveals information about the target system's file structure. The vulnerability is classified under CWE-200 as "Information Disclosure" and represents a classic case of improper error handling that exposes system state information.
The operational impact of this vulnerability is substantial as it enables attackers to perform reconnaissance activities without requiring direct system access or authentication. An attacker can systematically test for the existence of various files on the target system by crafting script tags with different file paths and monitoring the error responses. This capability allows for mapping of the target file system, potentially revealing sensitive files, configuration data, or system artifacts that could be exploited in subsequent attacks. The vulnerability is particularly dangerous in environments where sensitive files might be accessible through predictable paths, as it provides a method for attackers to identify and target these files. The attack can be executed entirely through web-based means, making it accessible to remote adversaries and potentially automatable for large-scale reconnaissance operations.
Mitigation strategies for this vulnerability focus on both browser-level and application-level protections. Microsoft addressed this issue through security updates and patches that modified how Internet Explorer handles script loading errors, ensuring consistent error responses regardless of file existence. Organizations should implement comprehensive patch management programs to ensure all affected Internet Explorer versions are updated promptly. Additionally, administrators can configure browser security settings to disable JavaScript execution in contexts where it is not required, though this may impact legitimate web applications. Network-level protections such as web application firewalls can help detect and block malicious script tag patterns, while proper file system permissions and access controls can limit the exposure of sensitive files. This vulnerability demonstrates the importance of proper error handling in web browsers and the need for consistent security practices across all components of web application infrastructure. The attack pattern aligns with techniques described in the attack tree framework where reconnaissance precedes exploitation, making this vulnerability particularly concerning for organizations that do not maintain up-to-date security patches.