CVE-2019-11744 in Firefox
Summary
by MITRE
Some HTML elements, such as <title> and <textarea>, can contain literal angle brackets without treating them as markup. It is possible to pass a literal closing tag to .innerHTML on these elements, and subsequent content after that will be parsed as if it were outside the tag. This can lead to XSS if a site does not filter user input as strictly for these elements as it does for other elements. This vulnerability affects Firefox < 69, Thunderbird < 68.1, Thunderbird < 60.9, Firefox ESR < 60.9, and Firefox ESR < 68.1.
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Analysis
by VulDB Data Team • 11/26/2025
This vulnerability represents a cross-site scripting weakness in Mozilla Firefox and Thunderbird browsers that stems from improper handling of HTML element parsing. The flaw exists in how these applications process certain HTML elements such as title and textarea tags when using the innerHTML property. Unlike other HTML elements where angle brackets are automatically treated as markup delimiters, elements like title and textarea can contain literal angle brackets without triggering markup parsing. This behavior creates a parsing inconsistency that attackers can exploit to bypass security filters.
The technical implementation of this vulnerability occurs when malicious input containing a literal closing tag is passed to the innerHTML property of affected HTML elements. When Firefox processes this input, it fails to properly sanitize the content, allowing subsequent HTML content that follows the literal closing tag to be parsed as if it were outside the original element. This parsing anomaly effectively breaks out of the intended HTML context and can inject malicious code into the page. The vulnerability specifically affects versions prior to Firefox 69, Thunderbird 68.1, and various ESR releases, indicating it was present in multiple browser products and their extended support releases.
The operational impact of this vulnerability is significant for web applications that rely on user input sanitization and may not apply the same level of filtering to title and textarea elements as they do to other HTML constructs. Attackers can leverage this weakness to inject malicious scripts that execute in the context of the victim's browser session, potentially leading to session hijacking, data theft, or redirection to malicious sites. The vulnerability demonstrates a classic case of insufficient input validation and sanitization, where the application assumes that all HTML elements should be treated uniformly in terms of markup interpretation. This flaw can be particularly dangerous in web applications that dynamically generate content using user-provided data without proper contextual escaping.
Mitigation strategies should focus on implementing comprehensive input sanitization that treats all HTML elements uniformly regardless of their type. Security measures must include strict filtering of user input before insertion into any HTML element, particularly those that can contain literal angle brackets. Organizations should update to patched versions of affected browsers immediately, as the vulnerability exists in multiple product lines including Firefox, Thunderbird, and their ESR versions. Additionally, developers should implement Content Security Policy headers and utilize proper HTML escaping techniques to prevent exploitation. This vulnerability aligns with CWE-79 (Cross-site Scripting) and can be mapped to ATT&CK technique T1203 (Exploitation for Client Execution) in threat modeling frameworks. The issue highlights the importance of consistent security controls across all HTML elements and demonstrates why comprehensive security testing is essential for web applications.