CVE-2008-5553 in Internet Explorer
Summary
by MITRE
The XSS Filter in Microsoft Internet Explorer 8.0 Beta 2 disables itself upon encountering a certain X-XSS-Protection HTTP header, which allows remote attackers to bypass the XSS protection mechanism and conduct XSS attacks by injecting this header after a CRLF sequence. 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-2008-5553 represents a critical flaw in Microsoft Internet Explorer 8.0 Beta 2's cross-site scripting protection mechanism. This issue stems from the browser's implementation of the X-XSS-Protection HTTP header which is designed to enable or disable the built-in XSS filtering capabilities. The vulnerability occurs when a malicious attacker can inject a specially crafted X-XSS-Protection header that contains a carriage return line feed sequence followed by specific values that cause the browser's security filter to disable itself entirely. This behavior creates a dangerous bypass condition where the intended protection mechanisms are effectively neutralized, allowing attackers to inject malicious scripts that would otherwise be detected and blocked by the browser's XSS filter. The flaw operates at the HTTP response level where the browser's security policy is evaluated before the content is rendered, making it particularly effective for bypassing client-side protections.
The technical implementation of this vulnerability exploits the way Internet Explorer 8.0 Beta 2 processes HTTP headers and evaluates their content for security purposes. When the browser encounters an X-XSS-Protection header that contains a CRLF sequence followed by specific values, the parsing logic incorrectly interprets this as a signal to disable the XSS protection mechanism. This parsing error stems from improper header validation and processing within the browser's security subsystem. The vulnerability specifically targets the interaction between the HTTP response header processing and the XSS filter activation logic, where the presence of certain character sequences causes the security filter to transition from an active state to a disabled state without proper validation of the header's legitimacy. This represents a classic case of improper input validation and header parsing that leads to security policy bypass.
The operational impact of CVE-2008-5553 is significant as it allows remote attackers to completely circumvent the XSS protection that was specifically implemented to defend against cross-site scripting attacks. This vulnerability enables attackers to inject malicious JavaScript code into web applications that would normally be blocked by the browser's built-in security mechanisms. The attack vector is particularly concerning because it can be executed through HTTP response manipulation, meaning that attackers can exploit this vulnerability even when the target application itself is not directly compromised. The bypass affects all web applications that rely on Internet Explorer 8.0 Beta 2's XSS protection, potentially allowing attackers to perform session hijacking, cookie theft, and other malicious activities that depend on executing arbitrary JavaScript code within the victim's browser context. This vulnerability undermines the fundamental security assumptions that users and developers make about the effectiveness of client-side XSS protection mechanisms.
The technical flaw in this vulnerability aligns with CWE-116, which describes improper encoding or escaping of output, and CWE-20, which covers input validation issues. This vulnerability also maps to ATT&CK technique T1211, which involves exploitation of weaknesses in web application security controls, and T1566, which covers social engineering techniques that can be enhanced by bypassing security mechanisms. The vulnerability demonstrates a critical design flaw in how the browser's security components interact with HTTP response headers, specifically in the parsing and validation logic that determines when to activate or deactivate security filters. The vendor's statement that the XSS filter intentionally does not attempt to address every conceivable XSS attack scenario indicates that this was a known limitation in the implementation rather than an accidental flaw, suggesting that the security model was intentionally designed to be incomplete. This design decision, while potentially intentional, created a dangerous gap in security coverage that malicious actors could exploit to bypass protection mechanisms.
Mitigation strategies for this vulnerability should focus on both server-side and client-side controls. Server administrators should avoid sending the vulnerable X-XSS-Protection headers that can trigger the bypass condition, and should implement proper header validation to prevent injection of CRLF sequences in HTTP response headers. Web application developers should implement additional layers of protection including proper input sanitization, output encoding, and Content Security Policy headers that provide defense-in-depth against XSS attacks. Browser vendors should implement more robust header parsing logic that prevents malformed headers from triggering security filter disabling conditions. Organizations should consider deploying web application firewalls that can detect and block suspicious header injection patterns, and should regularly update their browser software to ensure they have the latest security patches. The vulnerability highlights the importance of comprehensive security testing that includes edge cases in header processing and the need for security mechanisms to be resilient against malformed input that could be used to bypass protection controls.