CVE-2016-9895 in Firefox
Summary
by MITRE
Event handlers on "marquee" elements were executed despite a strict Content Security Policy (CSP) that disallowed inline JavaScript. This vulnerability affects Firefox < 50.1, Firefox ESR < 45.6, and Thunderbird < 45.6.
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Analysis
by VulDB Data Team • 11/25/2025
The vulnerability described in CVE-2016-9895 represents a critical security flaw in Mozilla Firefox and Thunderbird browsers that undermines the fundamental security mechanism of Content Security Policy. This issue specifically targets the handling of marquee elements, which are deprecated html tags used for scrolling text displays. The flaw allows malicious actors to bypass CSP restrictions that are designed to prevent execution of inline JavaScript code, thereby creating a vector for cross-site scripting attacks. The vulnerability exists because the browser's event handler processing for marquee elements does not properly respect the CSP directives that should block inline script execution.
The technical implementation of this vulnerability stems from how Firefox processes the marquee element's event attributes. When a web page contains a marquee element with event handlers such as onmouseover, onmouseout, or onstart, the browser's parser fails to properly validate these event attributes against the active CSP policy. This occurs because the CSP enforcement mechanism does not adequately inspect the event handler attributes within marquee elements, allowing potentially malicious JavaScript code embedded in these attributes to execute regardless of the CSP configuration. The flaw specifically affects versions prior to Firefox 50.1 and Thunderbird 45.6, indicating that the issue was present in the browser's rendering engine's handling of deprecated html elements.
The operational impact of this vulnerability is significant as it provides attackers with a bypass mechanism for one of the most important web security controls. Security administrators who implement CSP policies to protect against XSS attacks may find their defenses compromised if users encounter pages containing marquee elements with malicious event handlers. This vulnerability particularly affects environments where CSP is used as a primary defense against script injection attacks, as it allows attackers to execute arbitrary JavaScript code that would normally be blocked by CSP. The issue becomes more dangerous in contexts where users may encounter untrusted content or when browsers are used in corporate environments where CSP policies are strictly enforced.
This vulnerability aligns with CWE-15 (Improper Neutralization of Data within a Security Policy) and represents a failure in security policy enforcement mechanisms. The flaw demonstrates how deprecated html elements can introduce security gaps in modern browsers, as the event handling system for these elements was not properly updated to respect contemporary security controls. From an ATT&CK framework perspective, this vulnerability maps to T1059.007 (Command and Scripting Interpreter: JavaScript) and T1211 (Exploitation for Defense Evasion) as it enables attackers to execute JavaScript code while circumventing defense mechanisms. The vulnerability also relates to T1566 (Phishing) as attackers can craft malicious pages that exploit this weakness to deliver JavaScript payloads. Organizations should prioritize patching affected versions and consider implementing additional monitoring for suspicious marquee element usage in web applications. The fix implemented by Mozilla involved modifying the event handler processing logic to ensure that CSP policies are properly enforced even for deprecated html elements like marquee, thereby restoring the intended security posture of the browser's content security controls.