External risk intelligence

Firefox and Thunderbird Use-After-Free Vulnerability in JavaScript GC Component

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-2795

This vulnerability exists within the JavaScript engine component of client-side software (Firefox and Thunderbird). Such components are processed locally on the user's device and do not represent a public-facing network service, edge gateway, or internet-accessible API that would be exposed to the public internet.

Use After Free

Mozilla Firefox

before 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 has been identified in the JavaScript engine used by Mozilla products like Firefox and Thunderbird. This flaw could allow for significant compromise of affected systems, potentially impacting confidentiality, integrity, and availability of data. The primary concern is to confirm if these products are in use and if any exposure exists.

Use-after-free bug in JavaScript engine.
Critical flaw impacts client-side software.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by directing a victim to a specially crafted web page or document that leverages the vulnerable JavaScript garbage collection component. If successful, this could lead to arbitrary code execution with the privileges of the application.

No specific entry conditions are mentioned.
Triggered by visiting a malicious site.
Allows arbitrary code execution.

Live Threat

Current exploitation, exposure, and threat context

A use-after-free vulnerability in the JavaScript garbage collection component could allow an attacker to execute arbitrary code when a user visits a malicious website or opens a crafted file, potentially impacting the integrity and availability of the affected application.

Application code execution.
Triggered by malicious content.
Compromise of application data.

Operational Fix

Recommended remediation, mitigation, and detection steps

Given that this vulnerability is in the JavaScript engine of client-side applications like Firefox and Thunderbird, ownership likely falls to teams managing end-user computing, endpoint security, or application packaging and deployment, working in conjunction with vendor management if commercial support is involved. The first practical move is to inventory all instances of affected software, determine business criticality and network exposure, and identify the accountable owner for each deployment.

Identify affected applications and owners.
Verify business criticality and exposure.
Plan risk-based remediation.

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 Firefox and Thunderbird?

These are common client-side software applications. Firefox is a web browser used for navigating the internet, and Thunderbird is an email client used for managing communications. Both rely on a shared internal JavaScript engine to process code, scripts, and web content.

What does use-after-free mean in CVE-2026-2795?

This is a memory management flaw classified as CWE-416. It occurs when a program continues to use a pointer to a memory location after that memory has been cleared or reclaimed by the system's garbage collector. Because the application no longer "owns" that memory, it can lead to unstable behavior or allow unauthorized code to manipulate the application's processes.

How is this vulnerability triggered?

The flaw is triggered when a user interacts with malicious content, such as navigating to a specially crafted website or opening a compromised document. Simply having the software installed on a device does not trigger the bug; it requires the user to access external, weaponized JavaScript content that forces the application into the error state.

Do I need to worry about this if I use these apps?

According to Halo Surface Signal, this vulnerability is very unlikely to be exposed to public-facing network services. Because it resides in client-side software, it typically affects local devices rather than servers. However, it remains a risk for end-users who might be directed to malicious web content, making it a priority for organizations managing employee devices.

Is there a first step to take for this software?

Start by identifying all computers and systems in your environment that have Firefox or Thunderbird installed. Once you have an inventory, verify your current version numbers. If you are running any version older than 148, plan to update to the latest release provided by Mozilla to apply the necessary memory management fixes.

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.