CVE-2026-117890 in Firefox
Summary
by MITRE • 10/27/2026
JIT miscompilation in the JavaScript Engine: JIT 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 the browser's 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 and streamline control flow. A miscompilation occurs when these optimizations incorrectly transform the source logic, resulting in generated machine code that behaves differently than the original interpreted script would have. Such errors are particularly insidious because they often bypass standard runtime checks, allowing flawed instructions to execute with full privileges granted to the JavaScript sandbox environment.
From a technical perspective, JIT miscompilations typically stem from incorrect assumptions made during the optimization phase regarding variable types or memory states. For instance, if the compiler incorrectly determines that a specific variable holds an integer value when it actually contains a pointer or object reference, subsequent arithmetic operations may be performed on invalid data structures. This can lead to out-of-bounds memory accesses, use-after-free conditions, or arbitrary code execution scenarios where the attacker controls the flow of execution through crafted JavaScript payloads. The complexity of JIT engines means that edge cases in type inference or loop unrolling optimizations are frequent sources of such defects, requiring rigorous fuzzing and formal verification methods to detect before deployment.
The operational impact of this vulnerability is severe, as it undermines the fundamental security model of the browser known as same-origin policy isolation. By exploiting a miscompilation, an attacker hosting malicious content on a controlled domain could potentially escape the JavaScript sandbox and interact with sensitive internal components or other tabs running under different origins. This escalation can lead to remote code execution on the victim's system, allowing for data exfiltration, installation of persistent malware, or complete compromise of the user's digital identity. Given that browsers are often the primary gateway to enterprise networks and personal data, such vulnerabilities pose a significant risk to both individual privacy and organizational security postures.
To mitigate this class of vulnerability, developers rely on continuous integration pipelines that include extensive fuzzing campaigns targeting the JIT engine's edge cases. Static analysis tools and formal verification techniques are increasingly employed to mathematically prove the correctness of optimization passes before they are merged into the main codebase. For end-users, immediate patching is essential; this specific issue was resolved in Firefox version 159 and Firefox Extended Support Release (ESR) version 153.6. Administrators managing enterprise environments should ensure that all client systems are updated to these versions or later to eliminate the attack surface associated with JIT miscompilations. Additionally, enabling advanced security features such as Content Security Policy headers can provide an additional layer of defense by restricting the types of scripts and resources that can be executed within the browser context.
This vulnerability aligns with Common Weakness Enumeration (CWE) category CWE-754, which covers improper check for unusual or exceptional conditions, often leading to logic errors in compiled code. It also relates closely to CWE-120, buffer overflows, if the miscompilation results in incorrect memory access patterns. In terms of adversary tactics, this flaw supports techniques described in the MITRE ATT&CK framework under Execution and Defense Evasion, specifically allowing attackers to bypass application sandboxing mechanisms through native code exploitation. The resolution highlights the ongoing challenge of maintaining correctness in highly optimized runtime environments where performance gains must not compromise security guarantees.