CVE-2016-9904 in Firefox
Summary
by MITRE
An attacker could use a JavaScript Map/Set timing attack to determine whether an atom is used by another compartment/zone in specific contexts. This could be used to leak information, such as usernames embedded in JavaScript code, across websites. 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
This vulnerability represents a sophisticated timing attack that exploits the internal implementation details of JavaScript engine compartmentalization in Mozilla Firefox and Thunderbird applications. The flaw resides in how the JavaScript Map and Set data structures handle atom storage and compartmentalization, creating a side-channel attack vector that allows adversaries to infer information about shared resources across different execution contexts. The vulnerability specifically targets the underlying atom table management system where atoms represent string identifiers used throughout JavaScript code execution, making it particularly dangerous for applications that process user-provided content or maintain complex cross-origin interactions. The timing discrepancies occur when the JavaScript engine attempts to access atoms that may or may not be present in different compartments, creating measurable differences in execution time that can be exploited to determine atom usage patterns across zones.
The technical implementation of this vulnerability leverages the fundamental architectural design of Firefox's JavaScript engine, which employs compartmentalization to isolate different execution contexts and prevent cross-site scripting attacks. However, the timing variations in Map and Set operations create a covert channel that reveals whether specific atoms exist in other compartments, effectively bypassing the intended security boundaries. This attack pattern aligns with the common vulnerability classification CWE-203: "Information Exposure Through Visualization of Error Messages" and more specifically relates to CWE-347: "Improper Verification of Cryptographic Signature" in terms of how timing variations can expose information about internal system states. The vulnerability is particularly concerning because it operates at a level below traditional security boundaries, making it difficult to detect through conventional monitoring approaches and exploiting the inherent timing characteristics of JavaScript engine implementations rather than direct code injection or memory corruption techniques.
The operational impact of this vulnerability extends beyond simple information disclosure, as it enables sophisticated cross-site attacks that can extract sensitive information embedded within JavaScript code structures. Attackers can use this timing attack to determine whether specific usernames, identifiers, or other sensitive data exists in different compartments, potentially leading to account enumeration, user identification, and targeted attacks against specific users. The attack requires careful measurement and analysis of execution timing differences but can be automated to systematically extract information from target applications. This vulnerability particularly affects web applications that rely heavily on JavaScript Map and Set operations, or those that process complex user data that may be stored as atoms in the JavaScript engine's internal structures. The implications are severe for applications that handle sensitive user information, as the timing attack can be used to gradually reconstruct user data without direct access to the underlying resources.
Mitigation strategies for this vulnerability require both immediate application updates and architectural considerations to prevent similar timing attacks from compromising system security. The primary and most effective mitigation is upgrading to versions that contain the patched JavaScript engine implementation, specifically Firefox 50.1, Firefox ESR 45.6, and Thunderbird 45.6, which address the timing variations in atom access patterns. Organizations should also implement additional defensive measures such as introducing random delays in JavaScript operations to prevent precise timing measurements, disabling or restricting Map and Set operations in sensitive contexts, and implementing comprehensive monitoring for unusual timing patterns in JavaScript execution. The vulnerability highlights the importance of considering side-channel attacks in security design and aligns with the ATT&CK framework's T1059.007: "Command and Scripting Interpreter: JavaScript" and T1082: "System Information Discovery" tactics, as it enables attackers to gather system information through indirect means rather than traditional direct attacks. Security teams should also consider implementing application-level protections against timing-based information leakage and establish protocols for monitoring JavaScript engine behavior for anomalous timing characteristics that could indicate exploitation attempts.