CVE-2008-4788 in Internet Explorer
Summary
by MITRE
Microsoft Internet Explorer 6 omits high-bit URL-encoded characters when displaying the address bar, which allows remote attackers to spoof the address bar via a URL with a domain name that differs from an important domain name only in these characters, as demonstrated by using exam%A9ple.com to spoof example.com, aka MSRC ticket MSRC7900.
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Analysis
by VulDB Data Team • 08/02/2021
This vulnerability in Microsoft Internet Explorer 6 represents a sophisticated domain name spoofing attack that exploits a fundamental flaw in URL decoding and display mechanisms. The issue occurs when the browser processes high-bit URL-encoded characters in domain names, specifically failing to properly render certain encoded characters in the address bar display. This creates a situation where an attacker can craft a URL using characters that appear identical or nearly identical to legitimate domain names but are actually different at the bit level, enabling successful phishing attacks. The vulnerability is particularly dangerous because it operates at the user interface level where trust is established, making it difficult for users to distinguish between legitimate and malicious sites.
The technical flaw stems from how Internet Explorer 6 handles UTF-8 encoding in URLs, specifically when processing characters in the range of 128-255. When a user navigates to a URL containing high-bit encoded characters, the browser's address bar rendering engine does not properly decode these characters for display purposes, instead showing the encoded versions or omitting them entirely. This creates a discrepancy between the actual URL being accessed and what the user sees in the address bar. The attack vector demonstrates using exam%A9ple.com where %A9 represents the copyright symbol encoded in hexadecimal, which when displayed in the address bar may appear visually identical to example.com, thus deceiving users into believing they are visiting a legitimate domain.
The operational impact of this vulnerability is significant within the context of web security and user trust models. Attackers can exploit this weakness to create convincing phishing pages that appear to be legitimate websites, potentially stealing credentials, personal information, or financial data. The vulnerability aligns with CWE-183, which addresses "Permissive List of Allowed Inputs" and specifically relates to improper handling of encoded data in web applications. This weakness enables attackers to bypass security measures that rely on visual verification of domain names, undermining the fundamental security principle that users should be able to trust the address bar as an indicator of site authenticity. The attack pattern corresponds to techniques described in the ATT&CK framework under T1566, which covers "Phishing" and specifically "Spearphishing Attachment" and "Spearphishing Link" tactics.
Mitigation strategies for this vulnerability require both immediate and long-term approaches. The most effective immediate solution is to upgrade from Internet Explorer 6 to a modern browser that properly handles URL encoding and display. Microsoft released patches for this vulnerability as part of their regular security updates, but the underlying architectural issue highlights the need for better input validation and encoding handling in web browsers. Organizations should implement additional security measures such as URL filtering, content security policies, and user education about recognizing potential spoofing attempts. The vulnerability also emphasizes the importance of proper security testing for internationalized domain names and encoding handling, as outlined in various security standards and best practices for web application security. Regular security assessments should include testing for similar encoding-related vulnerabilities that could enable similar spoofing attacks.