CVE-2008-5555 in Internet Explorer
Summary
by MITRE
Microsoft Internet Explorer 8.0 Beta 2 relies on the XDomainRequestAllowed HTTP header to authorize data exchange between domains, which allows remote attackers to bypass the product s XSS Filter protection mechanism, and conduct XSS and cross-domain attacks, by injecting this header after a CRLF sequence, related to "XDomainRequest Allowed Injection (XAI)." 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 identified as CVE-2008-5555 represents a critical security flaw in Microsoft Internet Explorer 8.0 Beta 2 that fundamentally undermines the browser's cross-origin security model. This issue stems from the browser's reliance on the XDomainRequestAllowed HTTP header as a mechanism for authorizing cross-domain data exchange, creating an exploitable vector that bypasses the intended security protections. The vulnerability specifically affects the browser's XSS Filter component, which is designed to prevent cross-site scripting attacks by analyzing incoming data for malicious patterns. When an attacker successfully injects the XDomainRequestAllowed header through a carefully crafted CRLF sequence, they can effectively circumvent the filter's protective mechanisms and execute unauthorized cross-domain operations.
The technical exploitation of this vulnerability occurs through a process known as XDomainRequest Allowed Injection (XAI), where malicious actors append the XDomainRequestAllowed header to HTTP responses in a manner that exploits the browser's interpretation of cross-domain communication protocols. This injection technique leverages the fact that Internet Explorer 8.0 Beta 2 does not properly validate or sanitize the presence of this header when it appears after a carriage return line feed sequence, allowing attackers to inject malicious code that would otherwise be blocked by the XSS Filter. The vulnerability demonstrates a fundamental flaw in how the browser handles HTTP headers that control cross-domain access, creating a pathway for attackers to bypass security controls that were specifically implemented to prevent such attacks.
The operational impact of this vulnerability extends beyond simple cross-site scripting attacks, as it enables attackers to perform sophisticated cross-domain data exfiltration and manipulation. By exploiting the XDomainRequestAllowed header injection, threat actors can establish unauthorized communication channels between different domains, potentially accessing sensitive data or performing actions on behalf of users without their knowledge. The vulnerability particularly affects scenarios where users interact with multiple domains within the same browser session, as the injected header can be used to bypass the same-origin policy that normally prevents such unauthorized data exchanges. This creates a significant risk for enterprise environments where users may access both internal and external web applications, as attackers could potentially leverage this vulnerability to move laterally within networks or extract confidential information from trusted domains.
Organizations should implement immediate mitigations including updating to supported versions of Internet Explorer that address this vulnerability, implementing network-level controls to monitor and block suspicious HTTP header injections, and deploying web application firewalls that can detect and prevent CRLF injection attacks. The vulnerability aligns with CWE-1107, which addresses improper neutralization of CRLF sequences in HTTP headers, and corresponds to ATT&CK technique T1213.002, which covers data from information repositories. Security teams must also consider the broader implications of this vulnerability in relation to the browser's security model and ensure that network monitoring systems are configured to detect anomalous HTTP header patterns. Given the vendor's statement that the XSS Filter intentionally does not address every conceivable XSS attack scenario, organizations should not rely solely on browser-based protections and should implement additional layers of defense including proper input validation, content security policies, and regular security assessments to identify similar vulnerabilities in their web applications and browser configurations.