CVE-2008-5551 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 by injecting data at two different positions within an HTML document, related to STYLE elements and the CSS expression property, aka a "double injection."
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Analysis
by VulDB Data Team • 05/24/2025
The vulnerability described in CVE-2008-5551 represents a critical flaw in Microsoft Internet Explorer 8.0 Beta 2's cross-site scripting protection mechanism. This issue specifically targets the browser's XSS filter implementation, which was designed to prevent malicious script execution by sanitizing user input and blocking suspicious content patterns. The vulnerability exploits a fundamental weakness in the filter's logic that fails to properly handle certain injection scenarios, allowing attackers to bypass the security protections that should normally prevent cross-site scripting attacks.
The technical flaw manifests through a sophisticated double injection technique that leverages the structure of HTML documents, particularly focusing on STYLE elements and CSS expression properties. When malicious data is injected at two distinct positions within an HTML document, the XSS filter's detection algorithms fail to recognize the combined threat pattern, enabling attackers to execute malicious scripts without triggering the browser's security mechanisms. This vulnerability operates at the intersection of HTML parsing, CSS interpretation, and security filter logic, creating a complex attack vector that bypasses traditional input validation approaches.
The operational impact of this vulnerability is significant for users of the affected browser version, as it completely undermines the intended XSS protection capabilities that were being introduced in Internet Explorer 8.0. Attackers can now successfully execute persistent cross-site scripting attacks against users who have not yet updated to patched versions, potentially leading to session hijacking, credential theft, data exfiltration, and full system compromise. The vulnerability affects the core security model of the browser, making it particularly dangerous for enterprise environments where users may not immediately receive security updates.
This vulnerability aligns with CWE-79, which specifically addresses cross-site scripting flaws in software applications, and demonstrates the complexity of implementing effective input sanitization in web browsers. The attack pattern referenced in the MITRE ATT&CK framework falls under T1059.007 for command and scripting interpreter, specifically targeting browser-based execution environments. The double injection technique represents an advanced evasion method that exploits gaps in security filter implementations, highlighting the ongoing arms race between security researchers and attackers who continuously develop new bypass techniques. Organizations should implement immediate mitigations including browser updates, enhanced network monitoring, and user education about phishing and social engineering attacks that could exploit this vulnerability.
The remediation approach requires immediate deployment of Microsoft security updates that address the XSS filter bypass mechanism. Security teams should also consider implementing additional network-level protections such as web application firewalls and content security policies to provide defense-in-depth measures. Regular security assessments should focus on identifying similar filter bypass vulnerabilities in other browser components and web applications, as this type of vulnerability demonstrates the inherent difficulty in creating comprehensive security solutions for complex web environments.