CVE-2026-109720 in Firefox
Summary
by MITRE • 10/13/2026
JIT miscompilation in the JavaScript: WebAssembly 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 vulnerability represents a critical failure within the Just-In-Time compilation subsystem of Mozilla's SpiderMonkey engine, specifically affecting the execution context for WebAssembly components. As a cyber security specialist with two decades of experience analyzing browser-based attack surfaces, it is evident that this miscompilation stems from an incorrect translation of high-level WebAssembly bytecode into native machine code during the optimization phase. The Just-In-Time compiler is responsible for converting abstract instructions into optimized processor-specific operations to ensure performance efficiency; however, in this instance, a logical error or state management flaw caused the compiler to generate invalid or semantically incorrect assembly instructions. This type of defect falls squarely under CWE-471, which describes an omission of necessary normalization, and more critically, it aligns with CWE-823, where object reference is used without proper pointer arithmetic checks, leading to memory corruption when the flawed code executes.
The operational impact of this vulnerability is severe because WebAssembly operates at a low level close to the hardware, bypassing many of the safety guarantees inherent in standard JavaScript execution. When the JIT compiler produces incorrect machine code for a WebAssembly module, it can lead to out-of-bounds memory accesses, type confusion errors, or arbitrary read and write primitives within the browser process's memory space. An attacker could craft a malicious WebAssembly component that triggers this specific miscompilation path, potentially allowing them to execute arbitrary code with the privileges of the user running the browser. This capability effectively neutralizes the sandboxing mechanisms designed to isolate web content from the host operating system, creating a direct pathway for remote code execution attacks against end-users visiting compromised or maliciously constructed websites that utilize WebAssembly technologies.
From an offensive security perspective, this vulnerability maps directly to MITRE ATT&CK technique T1203, which covers Exploitation for Client Execution, as it involves leveraging a software flaw in client-side applications to achieve code execution. Furthermore, the underlying mechanism likely relates to CWE-787, out-of-bounds write, or CWE-416, use after free, depending on how the miscompiled instructions interact with heap memory structures during runtime. The severity is amplified by the increasing adoption of WebAssembly for performance-critical web applications, including gaming, video editing, and complex data processing tools in browsers. Because the flaw exists within a core component used to interpret binary modules, it affects not only simple scripts but also sophisticated application logic that relies on precise memory layout and execution flow integrity.
Mitigation strategies must prioritize immediate patching as outlined by Mozilla's release cycle for Firefox 158 and Firefox ESR 153.5. For enterprise environments relying on the Extended Support Release, administrators should verify that all endpoints are updated to version 153.5 or later to ensure the JIT compiler logic is corrected. In addition to patching, organizations should implement strict Content Security Policy headers to restrict the loading of untrusted WebAssembly modules and disable unnecessary browser features if they are not required for business operations. Developers integrating WebAssembly into their applications should also consider implementing additional runtime checks or using sandboxed execution environments where possible to limit the blast radius in case a zero-day vulnerability is discovered before patches can be deployed across all user bases. Continuous monitoring of browser update notifications and automated patch management systems are essential to maintaining this security posture against evolving JIT-based exploits.