External risk intelligence

Firefox and Thunderbird Widget Use-After-Free Sandbox Escape

CVE advisorySeverity: CRITICAL (CVSS 9.6)

CVE-2026-100818

This vulnerability affects web browsers and email clients (Firefox and Thunderbird). These are client-side applications that run on end-user devices, not internet-facing services, gateways, or servers. While they interact with the internet, they are not reachable as public network services or infrastructure, making their specific attack surface local-only to the user's machine.

Use After Free

Mozilla Firefox

before 140.17.0141.0 to before 153.4.0154.0.0 to before 157.0.0154.0 to before 157.0

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 vulnerability involves a flaw in how certain Mozilla applications handle memory, potentially allowing unauthorized actions if a user encounters a malicious element. The primary concern is to confirm if these specific applications are in use and, if so, to verify the exposure.

Browser and email software flaw.
Matters if our teams use affected products.
Confirm relevance and exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by tricking a user into visiting a malicious website or opening a specially crafted email. This action would trigger a use-after-free error within the Widget component of affected Mozilla applications. If successful, this could allow the attacker to escape the application's sandbox and gain elevated privileges.

Requires user interaction via a malicious site or email.
Triggered by interacting with a vulnerable Widget component.
Allows sandbox escape and potential system compromise.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability, a sandbox escape within the Widget: Gtk component, could allow an attacker to affect the behavior of the application and potentially gain unauthorized access to system resources when a user interacts with a specially crafted file or website. The risk exists in supported versions of Firefox and Thunderbird when encountering malicious content.

Application and system data.
Via malicious files or websites.
Unauthorized access to system resources.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability impacts Mozilla Firefox and Thunderbird, meaning that application owners and potentially end-user device administrators need to coordinate remediation efforts. The first practical step is to identify all instances of these applications across the environment, determine their reachability and business criticality, and then prioritize actions based on risk.

Application owners should prioritize remediation.
Verify application reachability and criticality.
Plan 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 Firefox and Thunderbird in this context?

These are popular client-side software applications used for web browsing and managing email communications. They rely on complex internal components, such as the Widget: Gtk subsystem, to render interface elements, manage windowing, and handle user interactions with web content or message displays on your local device.

What does use-after-free mean for CVE-2026-100818?

This is a memory management error classified as CWE-416. It occurs when a program continues to use a memory location after it has been cleared or freed. Because the software mistakenly believes the memory is still valid, an attacker can manipulate this state to bypass security boundaries, which in this case leads to a sandbox escape.

How is the CVE-2026-100818 sandbox escape triggered?

An attacker must trick a user into interacting with malicious content, such as visiting a compromised website or opening a specially crafted email. Simply having the software installed is not enough to trigger the flaw; it requires the user to actively load the problematic element that interacts with the vulnerable Widget: Gtk component.

Why is Halo Surface Signal labeling this vulnerability as Very unlikely?

Halo Surface Signal notes that while Firefox and Thunderbird interact with the internet, they are end-user applications rather than public-facing servers. Because they reside on local devices and are not reachable as open network services or infrastructure, the attack surface is effectively limited to the user's specific machine environment.

What should I do if I use Firefox or Thunderbird?

Begin by identifying all installed instances of Firefox and Thunderbird within your environment to understand your current footprint. Once located, verify if your specific versions fall within the affected ranges provided in the advisory and prioritize updating these applications to the patched versions as part of your standard maintenance procedures.

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.