CVE-2026-109722 in Firefox
Summary
by MITRE • 10/13/2026
JIT miscompilation in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified vulnerability represents a critical failure within the Just-In-Time compilation subsystem of the Mozilla JavaScript engine, specifically manifesting as a miscompilation error. In modern web browsers, the JIT compiler is responsible for translating high-level JavaScript code into optimized machine code to ensure rapid execution and responsive user experiences. This process involves complex static analysis, type inference, and optimization passes designed to eliminate redundant operations or streamline control flow. However, in this specific instance, a logical flaw within these optimization heuristics caused the generated native code to deviate from the semantic behavior defined by the ECMAScript specification. Such miscompilations are particularly insidious because they do not typically result in immediate crashes but rather produce incorrect computational results or unexpected control flow transitions that can be exploited by malicious actors to bypass security checks or manipulate application state in unintended ways.
From a technical perspective, JIT miscompilation vulnerabilities often stem from edge cases in type inference where the compiler incorrectly assumes variable types remain constant across certain execution paths, leading to invalid optimizations such as aggressive unboxing of values or incorrect bounds checking elimination. When these assumptions are violated at runtime due to dynamic JavaScript features like prototype pollution or late binding, the resulting machine code may access memory locations outside their intended boundaries or execute instructions that were not logically part of the source script. This creates a discrepancy between what the developer wrote and what the browser executes, effectively creating a logic flaw in the security boundary of the sandboxed environment. The impact can range from data leakage if sensitive variables are exposed due to incorrect access control checks, to arbitrary code execution if memory safety constraints like buffer bounds are incorrectly optimized away by the JIT compiler.
The operational impact of this vulnerability is significant for users relying on Firefox and its derivatives for secure web browsing. An attacker could craft a malicious webpage containing specially designed JavaScript that triggers the miscompilation scenario during normal page load or interaction. By exploiting the incorrect code generation, an adversary might achieve sandbox escape, allowing them to interact with host system resources beyond the browser's restricted environment. This aligns closely with CWE-471, which describes modification of assumed-immutability data, and potentially CWE-823 if it leads to out-of-bounds access due to incorrect optimization assumptions. Furthermore, in the context of the MITRE ATT&CK framework, this vulnerability facilitates techniques related to Execution through Client-side Scripting (T1059) and potentially Privilege Escalation (T1068) if the sandbox boundary is breached successfully. The severity is heightened by the fact that JIT engines are heavily optimized for performance, meaning these flaws can be triggered with minimal resource overhead, making them attractive targets for drive-by download attacks or malicious ad networks.
To mitigate this risk, users must ensure they have applied the updates included in Firefox version 158, as well as the extended support release versions 115.43 and 153.5. These releases contain patches that correct the flawed logic within the JIT compiler's optimization passes, ensuring that type assumptions are validated more rigorously before applying aggressive optimizations. For enterprise environments utilizing Firefox ESR, it is imperative to verify that automatic update mechanisms are enabled or manually deploy these specific patch levels across all endpoints. Additionally, security teams should monitor for any emerging exploits targeting this specific miscompilation pattern and consider implementing content security policies that restrict the execution of inline scripts where possible, thereby reducing the attack surface available to potential adversaries attempting to leverage JIT bugs. Continuous monitoring of browser vendor advisories remains essential as new vulnerabilities in core engine components are frequently discovered and patched.