CVE-2026-117955 in Firefox
Summary
by MITRE • 10/27/2026
JIT miscompilation in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical failure within the Just-In-Time compilation subsystem of Mozilla's SpiderMonkey engine, specifically affecting the execution environment for WebAssembly components. As a cyber security specialist with two decades of experience analyzing browser-based attack vectors, 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 intermediate representations into optimized executable instructions to ensure performance; 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-754: Improper Check for Unusual or Exceptional Conditions and CWE-823: Use of Out-of-range Pointer Offset, as the resulting machine code may perform operations on unintended memory locations or execute control flow paths that were not present in the original source logic.
The operational impact of such a JIT miscompilation is severe because it bypasses many of the standard security mitigations built into modern browsers. When the compiler produces incorrect native code, it can lead to arbitrary code execution vulnerabilities if an attacker crafts malicious WebAssembly modules designed to trigger this specific compilation path. By exploiting the discrepancy between the expected behavior and the actual compiled output, a remote adversary could potentially achieve memory corruption, leading to heap overflows or out-of-bounds reads and writes. This effectively neutralizes sandboxing protections that rely on strict adherence to defined execution boundaries, allowing for potential privilege escalation within the browser process context. The vulnerability is particularly dangerous because it exploits the trust placed in the JIT compiler's optimization passes, which are typically assumed to be robust and free of logic errors due to their complexity and rigorous testing regimes.
This incident highlights the inherent risks associated with complex native code generation engines like those used for WebAssembly support. The ATT&CK framework categorizes such browser-based exploits under T1203: Exploitation for Client Execution, where attackers leverage software vulnerabilities in client applications to gain initial access or execute malicious payloads. In this context, the miscompilation serves as a primitive that can be chained with other techniques to bypass detection mechanisms and establish persistence on affected systems. The complexity of WebAssembly's type system and memory model makes it an attractive target for researchers seeking novel exploitation paths, as traditional fuzzing may not always cover all edge cases in JIT optimization logic.
Mitigation strategies must prioritize immediate patching through the release channels mentioned, specifically Firefox version 159 and Firefox Extended Support Release (ESR) version 153.6. These updates contain the necessary corrections to the JIT compiler's internal state handling and instruction generation algorithms that resolve the miscompilation issue. Organizations should enforce automated update policies to ensure all endpoints are running these patched versions, as reliance on user-initiated updates often leaves systems vulnerable during the window between release and adoption. Additionally, implementing strict Content Security Policy directives can help mitigate potential exploitation by restricting the sources from which WebAssembly modules are loaded, thereby reducing the attack surface for remote code execution attempts. Continuous monitoring of browser telemetry and anomaly detection in JIT compilation times may also provide early indicators of attempted exploitation activities targeting this specific flaw before full compromise occurs.