CVE-2026-109721 in Firefox
Summary
by MITRE • 10/13/2026
Incorrect boundary conditions in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified security flaw resides within the Just-In-Time compilation subsystem of the Mozilla SpiderMonkey JavaScript engine, specifically concerning incorrect handling of boundary conditions during code optimization or execution. In complex web applications that rely heavily on dynamic script evaluation, such as single-page applications or real-time data processing tools, the JIT compiler is responsible for translating high-level JavaScript instructions into optimized machine code to ensure performance efficiency. When this component fails to properly validate array indices, buffer sizes, or loop limits before generating native code, it creates a scenario where memory access checks are bypassed or miscalculated by the optimizer. This type of logical error allows an attacker who can inject malicious scripts into a victim's browser session to trigger out-of-bounds reads or writes within the engine's internal data structures.
From a technical perspective, this vulnerability aligns with CWE-125 Out-of-bounds Read and potentially CWE-787 Out-of-bounds Write depending on whether the malformed boundary condition leads to reading sensitive memory contents or overwriting adjacent heap allocations. The exploitation of such flaws typically involves crafting specific JavaScript payloads that force the JIT compiler into a state where it assumes certain constraints are met, thereby skipping necessary safety checks during the generation of machine code instructions. This can result in arbitrary read capabilities if the vulnerability allows accessing uninitialized memory or stack data, which may contain cryptographic keys, session tokens, or other sensitive information. In more severe cases involving write operations, an attacker could achieve arbitrary code execution by corrupting function pointers or object headers within the JavaScript heap, effectively gaining control over the browser process and potentially compromising the underlying operating system through further exploitation chains.
The operational impact of this vulnerability is significant for users relying on Firefox to access untrusted web content, particularly in enterprise environments where browsers are used as gateways to internal applications that may not have robust security headers or sandboxing measures. An attacker could leverage this flaw to perform cross-site scripting attacks with elevated privileges, bypass same-origin policies by leaking data from other tabs or windows through side-channel effects, or execute remote code if the vulnerability is chained with other browser-specific bugs. The risk is heightened in scenarios where users frequently interact with dynamic content generated by third-party scripts, such as advertising networks, analytics providers, or collaborative web tools that parse user input without sufficient sanitization before passing it to the JavaScript engine for execution.
To mitigate these risks, organizations and individual users must ensure that Mozilla Firefox is updated to version 158 or later, including the Extended Support Release variant at version 153.5, which incorporate patches addressing this specific boundary condition logic error in the JIT compiler. Beyond software updates, administrators should enforce strict content security policies within their web applications to limit the sources from which scripts can be loaded and executed. Implementing browser-level sandboxing features where available and disabling unnecessary JavaScript execution for untrusted domains can further reduce the attack surface. Additionally, regular vulnerability assessments and penetration testing focused on client-side logic flaws will help identify similar implementation errors in custom web applications before they can be exploited by adversaries leveraging techniques documented under MITRE ATT&CK ID T1059.007 for command and script interpretation via JavaScript.