CVE-2003-1025 in Internet Explorer
Summary
by MITRE
Internet Explorer 5.01 through 6 SP1 allows remote attackers to spoof the domain of a URL via a "%01" character before an @ sign in the user@domain portion of the URL, which hides the rest of the URL, including the real site, in the address bar, aka the "Improper URL Canonicalization Vulnerability."
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Analysis
by VulDB Data Team • 03/18/2025
The CVE-2003-1025 vulnerability represents a significant web browser security flaw in Internet Explorer versions 5.01 through 6 Service Pack 1 that exploited improper URL canonicalization mechanisms. This vulnerability falls under the CWE-180 category of Improper Neutralization of Input During URL Canonicalization, which specifically addresses how web browsers handle URL parsing and normalization processes. The flaw enabled attackers to manipulate URL display in the browser's address bar by inserting a null character %01 before the symbol in the userdomain portion of URLs, creating a deceptive user experience that could mask malicious destinations.
The technical implementation of this vulnerability exploited the way Internet Explorer processed URL components during canonicalization, where the %01 character was interpreted as a null byte that effectively truncated the URL parsing process. When users encountered a malformed URL containing this sequence, the browser would display only the portion of the URL that appeared before the null character, concealing the actual destination domain and potentially hiding malicious sites behind seemingly legitimate addresses. This behavior occurred because the browser's URL parser failed to properly handle the null byte character in the authentication portion of the URL, causing the canonicalization process to terminate prematurely.
The operational impact of this vulnerability was substantial as it enabled sophisticated phishing attacks and social engineering campaigns that could deceive users into trusting malicious websites. Attackers could craft URLs that appeared to lead to trusted domains while actually directing users to harmful destinations, bypassing traditional URL validation mechanisms. This vulnerability specifically targeted the user trust model of web browsers, where users rely on address bar information to verify website authenticity. The attack vector was particularly dangerous because it required no special privileges or complex exploitation techniques, making it accessible to attackers with minimal technical expertise while providing maximum impact for deception purposes.
Security professionals should note that this vulnerability demonstrates the critical importance of proper input validation and URL canonicalization in web browsers, aligning with ATT&CK technique T1071.004 for application layer protocol: DNS. Organizations should ensure that all web browsers are updated to current versions with proper URL handling mechanisms, as Microsoft addressed this vulnerability through service pack updates and subsequent browser releases. The remediation process involves implementing proper URL parsing routines that treat null characters and special sequences appropriately during canonicalization, ensuring that the displayed URL accurately reflects the actual destination. Additionally, user education regarding URL verification and awareness of suspicious address bar behavior remains crucial in mitigating the risks associated with this type of deception vulnerability.