CVE-1999-0347 in Internet Explorer
Summary
by MITRE
Internet Explorer 4.01 allows remote attackers to read local files and spoof web pages via a "%01" character in an "about:" Javascript URL, which causes Internet Explorer to use the domain specified after the character.
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Analysis
by VulDB Data Team • 02/27/2025
This vulnerability in Internet Explorer 4.01 represents a critical security flaw that enables remote attackers to perform unauthorized file access and web page spoofing through manipulation of javascript URLs. The issue specifically manifests when the "%01" character is included in an "about:" javascript URL, causing the browser to interpret and utilize the domain specified after this character in an unexpected manner. This behavior creates a pathway for attackers to bypass normal security restrictions and access local system resources that should remain protected from remote web content.
The technical mechanism behind this vulnerability involves the improper handling of special characters within URL parsing logic. When Internet Explorer encounters the "%01" sequence in an "about:" URL, it fails to properly validate or sanitize the input, leading to a situation where the browser interprets the subsequent domain information as part of the execution context rather than as part of the URL structure. This parsing error creates a window for attackers to craft malicious URLs that can access local files through the browser's security model, effectively allowing arbitrary file read operations from the victim's system.
The operational impact of this vulnerability extends beyond simple file access, as it also enables sophisticated web page spoofing attacks. Attackers can exploit this flaw to create convincing fake web pages that appear to originate from legitimate domains, making phishing and social engineering attacks more effective. The ability to read local files combined with domain spoofing capabilities creates a dangerous combination that can lead to information disclosure, credential theft, and further exploitation of the victim's system. This vulnerability directly violates fundamental web security principles and demonstrates the importance of proper input validation in browser implementations.
This vulnerability maps to CWE-20, which describes improper input validation, and represents a classic case of insufficient sanitization of user-supplied data. The issue also aligns with ATT&CK technique T1190, which covers exploitation of web applications through input validation flaws. Organizations affected by this vulnerability would experience significant security risks including potential data breaches, unauthorized system access, and compromised user privacy. The vulnerability's exploitation requires minimal technical skill, making it particularly dangerous as it can be leveraged by attackers with limited expertise.
Mitigation strategies for this vulnerability include immediate patching of Internet Explorer 4.01 installations, implementation of proper URL validation in web applications, and deployment of network-based security controls to monitor and block suspicious URL patterns. Browser vendors should implement robust input sanitization mechanisms and conduct thorough testing of URL parsing logic to prevent similar issues. Additionally, organizations should consider implementing web application firewalls and content filtering solutions to detect and prevent exploitation attempts. The vulnerability underscores the critical importance of maintaining up-to-date browser software and implementing defense-in-depth strategies to protect against legacy security flaws that may remain unpatched in older systems.