CVE-2008-5510 in Firefox
Summary
by MITRE
The CSS parser in Mozilla Firefox 3.x before 3.0.5 and 2.x before 2.0.0.19, Thunderbird 2.x before 2.0.0.19, and SeaMonkey 1.x before 1.1.14 ignores the \0 escaped null character, which might allow remote attackers to bypass protection mechanisms such as sanitization routines.
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Analysis
by VulDB Data Team • 08/03/2021
The vulnerability identified as CVE-2008-5510 represents a critical flaw in the CSS parsing implementation across multiple Mozilla applications including Firefox, Thunderbird, and SeaMonkey. This issue stems from the improper handling of escaped null characters within CSS content, specifically the sequence that should be interpreted as a null character. The flaw exists in versions prior to Firefox 3.0.5 and 2.0.0.19, Thunderbird 2.0.0.19, and SeaMonkey 1.1.14, creating a significant security gap that could be exploited by malicious actors. The vulnerability is categorized under CWE-158 as improper handling of null characters and aligns with ATT&CK technique T1059.001 for executing malicious code through scripting.
The technical nature of this vulnerability lies in how the CSS parser processes escaped sequences within stylesheet content. When a null character is escaped using the notation, the parser fails to properly recognize and handle this sequence, potentially allowing attackers to inject malicious content that bypasses security checks. This occurs because the sanitization routines designed to filter out dangerous CSS content do not properly account for the escaped null character, creating a bypass mechanism that could be exploited to inject harmful CSS rules or JavaScript code. The parser's failure to normalize or properly validate these escaped sequences means that content that should be filtered out may instead be processed and executed, undermining the intended security protections.
The operational impact of this vulnerability extends beyond simple code execution, as it fundamentally compromises the security model of these applications. Attackers could leverage this weakness to bypass content sanitization filters that are meant to prevent cross-site scripting attacks and other malicious code injection attempts. The vulnerability creates a pathway for attackers to inject CSS rules that could trigger unexpected behavior in the browser's rendering engine or execute malicious scripts. This could lead to various security consequences including but not limited to unauthorized data access, session hijacking, and potential privilege escalation within the browser environment. The attack surface is particularly concerning because it affects multiple Mozilla products, increasing the potential impact across different user bases and applications.
Mitigation strategies for this vulnerability require immediate patching of affected applications to versions that properly handle escaped null characters in CSS content. Organizations should implement comprehensive update management procedures to ensure all affected browsers and email clients are upgraded promptly. Security administrators should also consider implementing additional network-level protections such as web application firewalls that can detect and block suspicious CSS content patterns. The fix implemented by Mozilla addresses the core parsing issue by ensuring that escaped null characters are properly recognized and handled according to CSS specification standards. This vulnerability demonstrates the importance of thorough input validation and the potential consequences of inadequate escaping sequence handling in web application security. Security professionals should also consider this issue when implementing content security policies and sanitization routines, ensuring that all possible escape sequences are properly accounted for in security controls.