External risk intelligence

Firefox and Thunderbird JIT Use-After-Free Vulnerability.

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-2764

This vulnerability affects the JIT component of a web browser and email client. While these applications interact with internet content, the vulnerability itself exists within client-side software on an end-user device, which does not constitute a public-internet-facing service or reachable infrastructure.

Use After Free

Mozilla Firefox

before 115.33.0before 148.0128.0 to before 140.8.0before 140.8.0

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

This critical vulnerability in the JavaScript Engine of Firefox and Thunderbird could allow an attacker to remotely execute code. Given the widespread use of these applications, understanding the potential exposure is important. The primary concern is confirming if our specific environments are affected and, if so, the extent of that exposure.

Flaw in browser/email code allows remote execution.
Affects widely used Mozilla software.
Confirm relevance and exposure for your organization.

Attack Path

How an attacker could exploit the issue

Attackers can exploit a vulnerability in the JIT component of Firefox and Thunderbird by tricking a user into visiting a malicious webpage or opening a specially crafted email. This interaction triggers a miscompilation in the JavaScript engine, leading to a use-after-free condition. If successful, this could allow an attacker to execute arbitrary code and compromise the user's system.

No authentication or user interaction needed.
Malicious web page or email triggers bug.
Arbitrary code execution and system compromise.

Live Threat

Current exploitation, exposure, and threat context

A vulnerability in the JavaScript Engine's JIT component could allow an attacker to compromise the integrity and availability of affected applications, potentially impacting user data and service behavior. This risk exists when users interact with malicious content through vulnerable versions of Firefox or Thunderbird.

Application code and data.
Malicious content interaction.
Application instability or data corruption.

Operational Fix

Recommended remediation, mitigation, and detection steps

Security and application teams should focus on identifying all deployments of Firefox and Thunderbird, confirming their reachability and criticality, and then identifying the respective owners. This initial assessment will inform the remediation planning based on the identified risks and potential business impact.

Identify affected application owners.
Verify exposure and business criticality.
Plan remediation by risk.

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 role of the JIT component in Firefox and Thunderbird?

The Just-In-Time (JIT) compiler is a core part of the JavaScript engine in Firefox and Thunderbird. Its job is to improve performance by translating JavaScript code into machine code while the application is running. By optimizing this execution process, it helps web pages and complex web-based content in emails load and run smoothly.

What does CWE-416 mean for CVE-2026-2764?

CWE-416 represents a 'Use-After-Free' weakness. In this CVE, a miscompilation error in the JIT component causes the program to attempt to use memory that has already been deallocated or freed. Because the system can no longer track this memory correctly, an attacker may be able to manipulate it to run unauthorized code on the host system.

How is this vulnerability triggered by an attacker?

An attacker triggers this flaw by luring a user into interacting with malicious content, such as visiting a specially crafted webpage or opening a malicious email. The bug does not trigger through background network scanning or by simply having the software installed; it requires the application's engine to process the specific, deceptive content that exploits the JIT miscompilation.

Is my organization at risk according to Halo Surface Signal?

Halo Surface Signal identifies this as a client-side risk. Since the vulnerability resides within end-user applications like browsers and email clients rather than on public-internet-facing servers, it is classified as very unlikely to be reachable infrastructure. The primary concern is individual user devices rather than public-facing services.

What should I do first to address this issue?

Your first step is to inventory all deployments of Firefox and Thunderbird within your environment to identify systems running unpatched versions. Once you have a clear list of affected assets and their owners, prioritize updating those applications to the latest secure versions provided by Mozilla to eliminate the vulnerability.

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.