CVE-2019-9816 in Firefox
Summary
by MITRE
A possible vulnerability exists where type confusion can occur when manipulating JavaScript objects in object groups, allowing for the bypassing of security checks within these groups. *Note: this vulnerability has only been demonstrated with UnboxedObjects, which are disabled by default on all supported releases.*. This vulnerability affects Thunderbird < 60.7, Firefox < 67, and Firefox ESR < 60.7.
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Analysis
by VulDB Data Team • 03/06/2025
This vulnerability represents a critical type confusion issue within the JavaScript engine of Mozilla Firefox and Thunderbird browsers. The flaw occurs during the manipulation of JavaScript objects within object groups, specifically when dealing with UnboxedObjects which are disabled by default in all supported versions. The vulnerability stems from improper handling of object type information during runtime operations, creating conditions where the JavaScript engine may incorrectly interpret object types leading to security bypass scenarios. This type confusion vulnerability falls under the CWE-843 category of "Access of Resource Using Incompatible Type" and aligns with ATT&CK technique T1059.007 for JavaScript execution.
The technical implementation of this vulnerability involves the JavaScript engine's object group management system where objects are grouped together for performance optimization. When UnboxedObjects are enabled, the engine attempts to optimize memory layout for certain object types, but the type checking mechanisms fail to properly validate object state transitions. This allows malicious code to manipulate object properties in ways that circumvent expected type constraints, potentially enabling attackers to execute arbitrary code or bypass security controls. The vulnerability specifically affects the garbage collection and memory management routines where object type information is not consistently validated.
The operational impact of this vulnerability is significant as it could allow remote code execution when a victim visits a malicious website or opens a specially crafted email in Thunderbird. Attackers could leverage this type confusion to bypass security checks and execute malicious JavaScript code with the privileges of the browser user. The vulnerability's exploitation requires the specific enabling of UnboxedObjects which are disabled by default, but security researchers note that in certain complex attack scenarios, attackers might be able to force the enabling of these objects through other means. This makes the vulnerability particularly dangerous as it could be exploited in zero-day attacks against users who have not explicitly disabled these features.
Mitigation strategies for this vulnerability involve ensuring that all affected browsers and email clients are updated to the latest versions where the issue has been patched. System administrators should enforce security policies that disable UnboxedObjects in browser configurations, as these objects are not required for normal browsing operations. The patch implementations typically involve strengthening type validation checks in the JavaScript engine's object management routines and ensuring proper memory layout validation during object group operations. Organizations should also consider implementing network-level protections such as content filtering and web application firewalls to detect and block exploitation attempts. Regular security updates and browser hardening practices are essential to prevent exploitation of this type of vulnerability, which represents a fundamental flaw in the JavaScript engine's type safety mechanisms.