CVE-2008-4066 in Firefox
Summary
by MITRE
Mozilla Firefox 2.0.0.14, and other versions before 2.0.0.17, allows remote attackers to bypass cross-site scripting (XSS) protection mechanisms and conduct XSS attacks via HTML-escaped low surrogate characters that are ignored by the HTML parser, as demonstrated by a "jav�ascript" sequence, aka "HTML escaped low surrogates bug."
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Analysis
by VulDB Data Team • 08/17/2019
The vulnerability identified as CVE-2008-4066 represents a critical flaw in Mozilla Firefox's HTML parsing and XSS protection mechanisms that existed in versions prior to 2.0.0.17. This security issue stems from Firefox's improper handling of Unicode surrogate pairs in HTML-escaped sequences, specifically low surrogate characters that are typically ignored by HTML parsers. The flaw allows attackers to craft malicious payloads that bypass the browser's built-in XSS protection measures by exploiting the way Firefox processes these specific Unicode sequences during HTML parsing operations.
The technical implementation of this vulnerability involves the manipulation of UTF-16 surrogate pairs within HTML-escaped content where low surrogate characters are silently ignored by the HTML parser. When Firefox encounters sequences like "jav�ascript" where the decimal value 56325 represents a low surrogate character, the browser fails to properly sanitize or recognize this as a potential XSS vector. This occurs because the HTML parser processes the escaped sequence and ignores the low surrogate character, effectively transforming the malicious payload into a working XSS exploit. The vulnerability specifically targets the browser's content security policies and input validation mechanisms, allowing attackers to inject malicious JavaScript code that executes within the context of the victim's browser session.
The operational impact of CVE-2008-4066 is significant as it undermines the fundamental security assumptions of web browsers regarding cross-site scripting protection. Attackers can exploit this vulnerability to execute arbitrary JavaScript code in the context of legitimate websites, potentially leading to session hijacking, credential theft, data exfiltration, and other malicious activities. The vulnerability affects not just the targeted browser but also impacts web applications that rely on Firefox's XSS protection mechanisms for user safety. Given that Firefox was widely used at the time, this flaw could have enabled attackers to compromise user sessions across numerous websites, particularly those that did not implement additional client-side security measures beyond the browser's built-in protections.
This vulnerability maps to CWE-79 in the Common Weakness Enumeration catalog, which specifically addresses Cross-Site Scripting flaws in web applications and browsers. The attack pattern aligns with ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript" and T1211 for "Exploitation for Defense Evasion" as attackers can leverage this flaw to execute malicious code while bypassing security controls. The remediation approach for this vulnerability required Firefox developers to update the HTML parser to properly handle Unicode surrogate pairs and ensure that escaped sequences containing low surrogate characters are appropriately sanitized. Organizations should have implemented immediate browser updates and deployed additional security controls including web application firewalls and enhanced input validation to protect against exploitation attempts. The vulnerability also highlighted the importance of comprehensive Unicode handling in security-sensitive applications and the need for thorough testing of international character encoding scenarios in security validation processes.
The broader implications of this vulnerability extend beyond the immediate XSS attack surface to demonstrate the complexity of modern web security challenges. It illustrates how seemingly innocuous parsing behaviors can create security holes when combined with insufficient input sanitization, particularly in applications that must handle international character sets. The flaw serves as a reminder of the importance of considering all possible character encoding scenarios during security design and implementation phases, as well as the necessity of regular security assessments that include Unicode and internationalization testing. Organizations implementing security controls must ensure that their defenses account for such edge cases in parsing and validation logic to prevent similar vulnerabilities from being exploited in other software components.