External risk intelligence

Firefox and Thunderbird Use-After-Free Vulnerability in JavaScript WebAssembly Component.

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-2767

This vulnerability affects web browsers and email client applications. These are client-side software programs installed on end-user devices, not internet-facing infrastructure or services. While they connect to the internet to fetch content, they are not reachable public-internet-facing services or gateways.

Use After Free

Mozilla Firefox

before 140.8.0before 148.0

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 exists within the WebAssembly component of Firefox and Thunderbird, specifically a use-after-free flaw in how JavaScript handles memory. This type of issue can potentially allow for significant compromise of affected systems.

Flaw in web technology could allow attackers remote control.
Critical issue in widely used browser and email software.
Confirm exposure and relevance to our specific deployments.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending a specially crafted web page or email to a user. When the user visits the page or opens the email, the vulnerable component in the browser or email client could be triggered, potentially allowing the attacker to execute arbitrary code. This could lead to a compromise of the user's system or data.

No specific user interaction needed.
Triggered via WebAssembly component.
Leads to code execution.

Live Threat

Current exploitation, exposure, and threat context

A use-after-free vulnerability in the WebAssembly component of Firefox and Thunderbird could allow an attacker to execute arbitrary code. This could occur when processing malicious web content or email attachments. The vulnerability impacts the integrity and availability of the affected applications.

Application code and memory integrity.
Processing of untrusted web or email content.
Potential for arbitrary code execution.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability impacts Mozilla Firefox and Thunderbird. The first step is to confirm where these applications are deployed within your environment, assess their reachability, and identify the specific teams or individuals accountable for their management and maintenance, such as end-user computing, application, or platform teams. This will inform a prioritized remediation plan.

Identify accountable application owners.
Verify user exposure and business criticality.
Plan coordinated updates during maintenance windows.

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 WebAssembly component in Firefox and Thunderbird?

WebAssembly is a technology that allows high-performance code to run within browsers and email clients. In Firefox and Thunderbird, this component handles complex tasks and intensive applications directly on your device. It enables web pages and embedded content to perform advanced operations efficiently, acting as a bridge between the software and the computer's underlying memory systems.

How does a use-after-free vulnerability work in CVE-2026-2767?

A use-after-free is a memory management flaw categorized as CWE-416. It occurs when a program continues to use a pointer to a memory location after that memory has been cleared or released. Because the program mistakenly tries to access or modify this 'freed' memory, an attacker can potentially manipulate it to execute unauthorized commands or gain control over the application's processes.

What triggers this vulnerability in the affected applications?

The vulnerability is triggered when the application processes specifically designed web content or email attachments. The browser or email client must interpret this malicious data through the WebAssembly component to initiate the flaw. Simply having the software installed is not enough to trigger the bug; the application must actively load and parse the weaponized content.

Why does Halo Surface Signal classify this as not internet-facing?

Halo Surface Signal notes that Firefox and Thunderbird are client-side applications installed on end-user devices, rather than servers or infrastructure directly exposed to the public internet. While they fetch data from the web, they are not services waiting for incoming network connections. This distinction is important because the risk stems from user-initiated actions, like visiting a site, rather than external probes targeting an open port.

Do I need to update my software to address this issue?

Yes, updating is the primary way to fix this vulnerability. You should identify where Firefox and Thunderbird are installed in your environment and coordinate updates to version 148 or 140.8 ESR. Since this flaw allows for arbitrary code execution when processing malicious content, ensuring all users are on these patched versions is the most effective way to eliminate the risk.

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.