CVE-2026-4702 in Firefox
Summary
by MITRE • 03/24/2026
JIT miscompilation in the JavaScript Engine component. This vulnerability affects Firefox < 149, Firefox ESR < 140.9, Thunderbird < 149, and Thunderbird < 140.9.
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Analysis
by VulDB Data Team • 04/16/2026
This vulnerability represents a critical just-in-time compilation flaw within the JavaScript engine component of Mozilla Firefox and Thunderbird applications. The issue stems from improper handling during the dynamic code optimization process that occurs when JavaScript code is executed. When the JavaScript engine performs just-in-time compilation, it analyzes frequently executed code segments and translates them into optimized machine code for improved performance. However, in this specific case, the compiler fails to properly validate or handle certain edge cases during this optimization phase, leading to incorrect code generation that can result in memory corruption or arbitrary code execution.
The technical implementation of this vulnerability involves the JavaScript engine's optimization passes that occur during runtime. When the engine encounters specific patterns of JavaScript code that trigger the JIT compiler, it may produce machine code that contains memory access violations or incorrect control flow. This miscompilation can occur in scenarios involving complex object manipulation, closure handling, or specific optimization heuristics that the engine uses to determine when and how to optimize code. The flaw is particularly concerning because it operates at the core execution layer of the browser, where malicious JavaScript code could exploit this behavior to bypass security boundaries.
The operational impact of this vulnerability extends across multiple Mozilla products including Firefox desktop browser, Firefox Extended Support Release, and Thunderbird email client. Attackers could potentially craft malicious web pages or email content that, when processed by these applications, triggers the JIT miscompilation. This could lead to complete system compromise through techniques such as remote code execution, privilege escalation, or information disclosure. The vulnerability affects both regular release versions and extended support releases, indicating it represents a fundamental issue in the JavaScript engine architecture rather than a temporary regression. The widespread adoption of these applications means that successful exploitation could affect millions of users globally.
Mitigation strategies for this vulnerability should focus on immediate patch application as the primary defense mechanism. Mozilla has released updated versions of Firefox and Thunderbird that address this issue through corrections to the JIT compiler's optimization logic and additional validation checks. Organizations should prioritize updating their systems to the latest versions of affected software, particularly since the vulnerability affects both current and extended support releases. Network administrators should consider implementing additional security measures such as content filtering and sandboxing mechanisms to limit the potential impact of exploitation attempts. The vulnerability aligns with CWE-682 which specifically addresses incorrect arithmetic operations and improper calculations in software, while also mapping to ATT&CK technique T1059.007 for JavaScript execution and T1548.002 for privilege escalation through code injection. Regular security monitoring and vulnerability assessment procedures should be enhanced to detect potential exploitation attempts targeting this specific JIT vulnerability.