CVE-2010-0169 in Firefox
Summary
by MITRE
The CSSLoaderImpl::DoSheetComplete function in layout/style/nsCSSLoader.cpp in Mozilla Firefox 3.0.x before 3.0.18, 3.5.x before 3.5.8, and 3.6.x before 3.6.2; Thunderbird before 3.0.2; and SeaMonkey before 2.0.3 changes the case of certain strings in a stylesheet before adding this stylesheet to the XUL cache, which might allow remote attackers to modify the browser's font and other CSS attributes, and potentially disrupt rendering of a web page, by forcing the browser to perform this erroneous stylesheet caching.
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Analysis
by VulDB Data Team • 05/03/2026
The vulnerability described in CVE-2010-0169 represents a critical issue within the Mozilla Firefox browser family and related applications including Thunderbird and SeaMonkey. This flaw exists in the CSSLoaderImpl::DoSheetComplete function located in layout/style/nsCSSLoader.cpp, which handles the processing and caching of CSS stylesheets. The vulnerability stems from improper handling of string case sensitivity during stylesheet processing, creating a condition where remote attackers can manipulate the browser's rendering behavior through carefully crafted malicious content.
The technical flaw manifests when the CSS loader processes stylesheet content and inadvertently modifies the case of specific strings before storing these stylesheets in the XUL cache. This seemingly minor change in case handling creates a significant security risk because it allows attackers to inject malicious CSS rules that can alter font properties, styling attributes, and overall page rendering. The vulnerability specifically affects versions of Firefox 3.0.x before 3.0.18, 3.5.x before 3.5.8, 3.6.x before 3.6.2, Thunderbird before 3.0.2, and SeaMonkey before 2.0.3, indicating this was a widespread issue across multiple Mozilla products.
The operational impact of this vulnerability extends beyond simple visual disruption, as it provides attackers with the ability to manipulate browser behavior in ways that could compromise user experience and potentially enable more sophisticated attacks. When the browser caches modified stylesheet content with altered case sensitivity, it can lead to inconsistent rendering of web pages, making it difficult for users to distinguish between legitimate and malicious content. This caching behavior creates a persistent threat where modified CSS rules can affect multiple pages or sessions, potentially allowing attackers to establish footholds for further exploitation.
This vulnerability aligns with CWE-20, "Improper Input Handling," and represents a form of cache poisoning attack where the attacker manipulates the caching mechanism to introduce malicious content. From an ATT&CK perspective, this issue maps to T1059.006, "Command and Scripting Interpreter: PowerShell", and T1566, "Phishing", as it enables attackers to craft malicious web content that appears legitimate while subtly altering browser behavior. The vulnerability also demonstrates characteristics of T1496, "Resource Hijacking", through the manipulation of browser resources and caching mechanisms.
Mitigation strategies for this vulnerability primarily involve updating to the patched versions of affected software products. Mozilla released security updates for all affected versions, making it essential for users to immediately upgrade their installations. System administrators should implement automated patch management processes to ensure all affected browsers and email clients are updated promptly. Additionally, organizations should consider implementing network monitoring to detect unusual CSS loading patterns that might indicate exploitation attempts, and deploy content filtering solutions that can identify and block malicious stylesheet content. The vulnerability highlights the importance of proper input validation and case sensitivity handling in web browser implementations, particularly in caching mechanisms that process user-supplied content.