External risk intelligence

Firefox and Thunderbird Sandbox Escape Vulnerability in Web Serial Component

CVE advisorySeverity: CRITICAL (CVSS 9.6)

CVE-2026-109745

This vulnerability is located within the DOM Web Serial component of a web browser. Exploitation requires a user to navigate to a malicious site or interact with specific web content, making it a client-side execution issue rather than a service or appliance that is independently reachable or exposed on the public internet.

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 Web Serial component of certain browsers, specifically related to uninitialized memory within the DOM. This issue could allow for sandbox escapes, potentially impacting the confidentiality, integrity, and availability of affected systems if exploited. The primary concern at this time is to confirm if your environment utilizes the affected technology and assess any potential exposure.

Browser component allows escaping its sandbox.
Confirms exposure and relevance of this browser issue.
Focus on confirming relevance and potential exposure.

Attack Path

How an attacker could exploit the issue

An attacker could lure a user to a malicious website, which then exploits an uninitialized memory flaw within the browser's Web Serial component. Successfully triggering this vulnerability could allow an attacker to break out of the browser's sandbox.

Entry requires user interaction with a malicious site.
Vulnerability is triggered by the Web Serial component.
Risk includes sandbox escape with high impact.

Live Threat

Current exploitation, exposure, and threat context

A sandbox escape in the Web Serial component could allow an attacker to gain elevated privileges when a user visits a specially crafted website. This could potentially affect system data and user data by allowing the attacker to perform actions with higher privileges than intended.

System data and user data.
User visits malicious website.
May lead to elevated privileges.

Operational Fix

Recommended remediation, mitigation, and detection steps

Real-world remediation for this sandbox escape vulnerability will likely involve coordinated efforts between application owners, platform teams, and potentially network/security teams to identify all instances of the affected software. The first practical step is to inventory where this software is deployed, assess its reachability and business criticality, and then assign ownership for the remediation effort. Planning the update process, considering user impact and maintenance windows, will be crucial.

Application and platform teams own remediation.
Verify affected software deployment and reachability.
Plan and execute approved updates.

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 Web Serial component in Firefox and Thunderbird?

The Web Serial API is a browser feature that allows web applications to communicate directly with serial devices, such as microcontrollers or 3D printers, connected to your computer. By enabling this interaction, the browser acts as a bridge between the internet and physical hardware. The affected component manages this data exchange within the browser's DOM.

What does CWE-908 mean for CVE-2026-109745?

CWE-908 refers to the use of uninitialized memory. In this case, the Web Serial component fails to properly clear memory before it is accessed. An attacker can leverage this programming oversight to manipulate the browser's internal state, potentially escaping the security sandbox that normally isolates web content from your underlying operating system.

How is the CVE-2026-109745 vulnerability triggered?

An attacker must lure a user into visiting a specially crafted malicious website that interacts with the Web Serial component. The vulnerability does not trigger simply by having the browser installed or running; it requires active user interaction with malicious web content to initiate the flawed memory access.

Is this CVE-2026-109745 a network-facing threat?

Halo Surface Signal notes this is a client-side execution issue, not a service or appliance independently reachable on the public internet. Because it requires user interaction, the risk is tied to the browsing habits of users rather than direct network exposure of the software itself.

How should I respond to this vulnerability?

Your first step is to inventory all systems running the affected versions of Firefox or Thunderbird. Once identified, prioritize these systems for an update to the patched versions—Firefox 158 or ESR 153.5 and their corresponding Thunderbird releases—to ensure the memory handling flaw is corrected.

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.