CVE-2008-5552 in Internet Explorer
Summary
by MITRE
The XSS Filter in Microsoft Internet Explorer 8.0 Beta 2 allows remote attackers to bypass the XSS protection mechanism and conduct XSS attacks via a CRLF sequence in conjunction with a crafted Content-Type header, as demonstrated by a header with a utf-7 charset value. NOTE: the vendor has reportedly stated that the XSS Filter intentionally does not attempt to "address every conceivable XSS attack scenario."
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Analysis
by VulDB Data Team • 08/03/2021
The vulnerability described in CVE-2008-5552 represents a significant bypass of Microsoft Internet Explorer 8.0 Beta 2's built-in cross-site scripting protection mechanism. This flaw specifically targets the browser's XSS filter implementation, which was designed to detect and prevent malicious script injection attempts that could compromise user sessions and data integrity. The vulnerability exploits a fundamental weakness in the filter's parsing logic that fails to properly handle certain character encoding scenarios, particularly those involving the utf-7 charset specification. The attack vector leverages carefully crafted CRLF (Carriage Return Line Feed) sequences combined with malicious Content-Type headers to circumvent the browser's security checks that are meant to sanitize incoming data before it reaches the user's browser context. This particular bypass demonstrates the complexity of implementing effective XSS protection in web browsers, as the security mechanism fails to account for edge cases involving character encoding specifications that are valid within HTTP protocols but can be exploited to evade detection.
The technical implementation of this vulnerability stems from how Internet Explorer 8.0 Beta 2 processes HTTP headers and applies its XSS filtering logic. When a web server responds with a Content-Type header specifying a utf-7 charset, the browser's XSS filter does not properly recognize this encoding as potentially malicious or as a mechanism that could be used to encode script content in a way that would bypass the filter's detection algorithms. The CRLF sequence serves as a delimiter that can manipulate the parsing behavior of the filter, allowing attackers to inject malicious scripts that would normally be caught by the XSS protection system. This flaw aligns with CWE-79 which describes cross-site scripting vulnerabilities where untrusted data is improperly handled in web applications, and more specifically with CWE-116 which addresses improper encoding or escaping of data that can lead to injection attacks. The vulnerability demonstrates the challenges in implementing comprehensive security filters that must account for the vast array of valid HTTP specifications and character encodings that can be legitimately used while simultaneously preventing malicious exploitation.
The operational impact of this vulnerability is substantial as it allows remote attackers to execute arbitrary scripts in the context of a user's browser session without requiring any special privileges or user interaction beyond visiting a malicious website. This creates a significant risk for users who may be browsing to compromised websites or who encounter phishing attacks that exploit this bypass mechanism. The vulnerability affects not just individual users but also organizations that rely on Internet Explorer 8.0 Beta 2 for business operations, as it undermines the browser's primary security promise of protecting against common web-based attacks. Attackers can leverage this vulnerability to steal session cookies, perform unauthorized actions on behalf of users, redirect users to malicious sites, or extract sensitive information from web applications. The fact that Microsoft explicitly stated that their XSS Filter intentionally does not address every conceivable XSS attack scenario indicates that this particular bypass was not considered a high-priority issue by the vendor, though it represents a legitimate security gap that could be exploited in real-world scenarios. This vulnerability also aligns with ATT&CK technique T1059.001 which covers command and scripting interpreter usage, specifically focusing on how attackers can leverage browser-based scripting environments to execute malicious code.
The mitigation strategies for this vulnerability primarily involve either updating to a newer version of Internet Explorer that contains fixed XSS filtering logic or implementing additional server-side protections that can detect and prevent the use of potentially malicious character encodings in Content-Type headers. Organizations should consider deploying web application firewalls that can identify and block suspicious HTTP headers before they reach the browser, and should also implement proper input validation on web applications to prevent the use of utf-7 character encodings in user-facing content. Additionally, users should be educated about the risks of visiting untrusted websites and should be encouraged to keep their browsers updated with the latest security patches. The vulnerability highlights the importance of not relying solely on client-side security mechanisms and demonstrates why defense-in-depth strategies are essential in modern cybersecurity practices. Organizations should also consider implementing Content Security Policy headers that can provide additional protection against script injection attacks regardless of browser-based XSS filter effectiveness, as this approach provides a more robust security boundary that operates at the application level rather than the browser level.