CVE-2008-5556 in Internet Explorer
Summary
by MITRE
** DISPUTED ** The XSS Filter in Microsoft Internet Explorer 8.0 Beta 2 does not recognize attack patterns designed to operate against web pages that are encoded with utf-7, which allows remote attackers to bypass the XSS protection mechanism and conduct XSS attacks by injecting crafted utf-7 content. NOTE: the vendor reportedly disputes this issue, stating "Behaviour is by design."
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Analysis
by VulDB Data Team • 08/07/2024
The vulnerability described in CVE-2008-5556 represents a significant bypass of cross-site scripting protection mechanisms within Microsoft Internet Explorer 8.0 Beta 2. This issue specifically targets the browser's XSS filter implementation, which is designed to prevent malicious scripts from executing when users encounter crafted web content. The flaw occurs when web pages are encoded using UTF-7 character encoding, a legacy encoding standard that was originally designed for email applications but has found limited use in web contexts. The XSS filter's failure to properly recognize and neutralize attack patterns within UTF-7 encoded content creates a pathway for attackers to inject malicious scripts that would otherwise be detected and blocked by the browser's security mechanisms.
The technical nature of this vulnerability stems from the browser's inability to properly parse and sanitize UTF-7 encoded content within its XSS protection framework. UTF-7 encoding uses a specific mechanism where certain characters are represented using a combination of ASCII characters and escape sequences that can be interpreted as valid HTML or script content. When Internet Explorer processes web pages encoded in UTF-7, the XSS filter fails to recognize that certain encoded sequences represent malicious payloads rather than legitimate content. This parsing failure allows attackers to craft payloads that exploit the difference between how the browser interprets UTF-7 encoded content versus how the XSS filter analyzes the same content for potential threats. The vulnerability specifically affects the browser's security model and demonstrates a critical gap in how legacy encoding standards are handled within modern security frameworks.
The operational impact of this vulnerability extends beyond simple script execution, as it represents a fundamental flaw in the browser's security architecture that could enable attackers to perform a wide range of malicious activities. When successful, attackers can bypass the browser's built-in XSS protection mechanisms and inject scripts that can steal user session cookies, redirect users to malicious sites, or perform actions on behalf of authenticated users. This type of vulnerability is particularly concerning in enterprise environments where users may encounter untrusted web content from various sources, and the bypass of security mechanisms could lead to widespread compromise of user accounts and sensitive data. The vulnerability's classification under CWE-79 (Cross-site Scripting) highlights its core nature as an input validation and output encoding issue that has been present in the browser's security model for several years.
The vendor's response stating "Behaviour is by design" reflects a complex security decision that demonstrates the challenges of balancing legacy compatibility with modern security requirements. This response indicates that Microsoft may have intentionally designed the browser to handle UTF-7 content in a way that prioritizes compatibility over security in certain edge cases. However, this design decision creates a significant security gap that could be exploited by threat actors who understand the intricacies of character encoding and browser behavior. From an ATT&CK framework perspective, this vulnerability maps to techniques involving command and control communication, credential access, and privilege escalation through web-based attacks. The security implications extend to organizations that may have relied on the browser's XSS protection as a defense-in-depth mechanism, potentially leaving their users vulnerable to sophisticated attack campaigns that leverage encoding-based bypass techniques. Organizations should consider implementing additional security controls such as web application firewalls and content security policies to mitigate the risk associated with this vulnerability, while also recognizing that the root cause lies in the browser's handling of legacy encoding standards.