External risk intelligence

Firefox JavaScript Engine JIT Miscompilation Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-16355

This vulnerability is located within the JavaScript engine of a web browser. Exploitation requires a user to navigate to a malicious site or execute specific code within the browser application itself. It is a client-side component and not a network-accessible service, gateway, or public-facing infrastructure that would be exposed to the internet independently.

Mozilla Firefox

before 115.38.0before 153.0.0128.1.0 to before 140.13.0before 140.13.0141.0 to before 153.0

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in the JavaScript engine of the Firefox browser, related to JIT miscompilation. This issue, if exploited, could allow for significant compromise of affected systems, as indicated by its high severity score. While the main concern is confirming relevance and exposure, leadership should be aware of such critical flaws impacting widely used software.

Flaw in browser's code execution.
High severity, potential widespread impact.
Confirm relevance and assess exposure.

Attack Path

How an attacker could exploit the issue

An attacker could deliver a specially crafted web page or document that, when opened by a user, triggers a Just-In-Time (JIT) miscompilation flaw within the browser's JavaScript engine. Successful exploitation could allow an attacker to compromise the confidentiality, integrity, and availability of the affected system.

No special access needed.
Triggered by processing malicious code.
Risks system compromise.

Live Threat

Current exploitation, exposure, and threat context

The JavaScript Engine's JIT component could be affected by miscompilation, potentially allowing for the execution of arbitrary code when supported by the advisory. This could impact the integrity and availability of the affected system.

JIT component data.
Malicious website interaction.
Arbitrary code execution.

Operational Fix

Recommended remediation, mitigation, and detection steps

Real-World Ownership

This vulnerability impacts the JavaScript engine within Firefox. Initial triage should focus on identifying all Firefox instances across the environment, assessing their reachability, and determining business criticality. The platform or application owner, in coordination with the security team, should then confirm the exposure and prioritize remediation based on risk, considering available vendor fixes and the operational impact of updates.

Platform/Application owners should lead.
Verify Firefox instances and business criticality.
Plan updates based on risk assessment.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the Firefox JavaScript engine and the JIT component?

Firefox is a widely used web browser. Its JavaScript engine is the core software responsible for interpreting and running JavaScript code on websites. The Just-In-Time (JIT) component is a performance-optimizing feature that translates JavaScript into machine code on the fly to help websites load and run faster, significantly improving your browsing experience.

What does CWE-843 mean for CVE-2026-16355?

CWE-843 is a weakness class describing a Type Confusion vulnerability. In the context of this CVE, it means the browser's JavaScript engine incorrectly processes the data types used during JIT compilation. Because the engine gets confused about what kind of data it is handling, an attacker might be able to trick the browser into performing unintended actions, potentially compromising the system.

How is this JIT miscompilation triggered?

This flaw is triggered when a user visits a malicious website or opens a specially crafted document that forces the browser to process harmful code. It is not triggered by standard network traffic or simply having the browser open. The code must interact with the browser's JIT compiler, meaning the vulnerability requires active user interaction with malicious content to initiate the miscompilation process.

Is my system at risk according to Halo Surface Signal?

Halo Surface Signal notes that this is a client-side browser component rather than a standalone, network-accessible server. Because this vulnerability exists inside the browser, it is not a public-facing infrastructure service that can be attacked remotely without user interaction. You should focus on endpoints where users actively browse the web, rather than network gateways.

Do I need to update my Firefox version?

Yes. Since this is a critical flaw in how your browser handles code, you should prioritize updating to the fixed versions mentioned in the official security advisory. Start by identifying all Firefox instances in your environment, then proceed with the standard update process provided by the vendor to ensure your browser is running the secure, patched code.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.