CVE-2026-109770: Firefox DOM Web Serial Spoofing Vulnerability

CVE-2026-109770 Overview

CVE-2026-109770 is a spoofing vulnerability in the DOM: Web Serial component used by Mozilla Firefox and Thunderbird. The flaw allows attackers to misrepresent interface elements associated with the Web Serial API, potentially deceiving users during permission or device interaction prompts. Mozilla addressed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. The vulnerability is tracked across multiple Mozilla Foundation Security Advisories including MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110.

Critical Impact
Successful exploitation enables user interface spoofing in the Web Serial API surface, which can mislead users into granting access to serial devices or trusting attacker-controlled content.

Affected Products

Mozilla Firefox versions prior to 158
Mozilla Firefox ESR versions prior to 153.5
Mozilla Thunderbird versions prior to 158 and 153.5

Discovery Timeline

2026-10-13 - CVE CVE-2026-109770 published to NVD
2026-10-13 - Last updated in NVD database

Technical Details for CVE-2026-109770

Vulnerability Analysis

The vulnerability resides in the DOM: Web Serial component, which exposes the Web Serial API to web content. The Web Serial API allows websites to communicate with serial devices such as microcontrollers and peripheral hardware after user consent. A spoofing weakness in this component permits attacker-controlled content to misrepresent trusted UI elements or device metadata presented to users.

Spoofing conditions in browser DOM components typically arise from improper origin isolation, missing visual trust indicators, or inconsistent handling of device identity surfaces. Users relying on these indicators to make security decisions may authorize serial communication with content they would otherwise reject.

Mozilla fixed the issue in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Thunderbird is affected because it shares Gecko platform code with Firefox even though it does not directly expose the Web Serial API by default.

Root Cause

Mozilla has not published granular root-cause detail in the public advisory text. Based on the component designation, the defect involves improper handling within the Web Serial DOM surface that allows presentation of misleading content to the user. Refer to the Bugzilla Report #2059230 once access restrictions lift for implementation specifics.

Attack Vector

Exploitation requires a victim to visit attacker-controlled web content in a vulnerable Firefox or Thunderbird build. The attacker then triggers Web Serial API interactions that render spoofed interface elements. No elevated privileges are required on the target system, and user interaction is limited to visiting the malicious page. See the Mozilla Security Advisory MFSA-2026-109 for vendor-provided guidance.

No verified exploitation code is published for this issue. The vulnerability is described in prose without synthetic proof-of-concept code to avoid fabrication.

Detection Methods for CVE-2026-109770

Indicators of Compromise

Browser process telemetry showing Web Serial API permission prompts originating from unexpected or newly registered domains.
Outbound HTTPS connections from Firefox or Thunderbird to domains hosting unexpected serial-device interaction content.
Endpoint logs showing Firefox or Thunderbird versions below 158 and 153.5 remaining in production after the advisory date.

Detection Strategies

Inventory browser and mail client versions across the fleet and flag any Firefox or Thunderbird installation below the patched builds.
Correlate web proxy logs with user reports of unexpected hardware access prompts in the browser.
Monitor for execution of outdated Mozilla binaries using endpoint telemetry that captures process version metadata.

Monitoring Recommendations

Alert on Firefox and Thunderbird launches where the version string indicates an unpatched release.
Track permission-grant events for Web Serial through browser management policies where supported.
Review phishing and credential-harvesting campaigns that may incorporate Web Serial spoofing to appear as hardware vendor sites.

How to Mitigate CVE-2026-109770

Immediate Actions Required

Upgrade Firefox installations to version 158 or later.
Upgrade Firefox ESR installations to version 153.5 or later.
Upgrade Thunderbird installations to version 158 or 153.5 depending on the deployed branch.
Validate automatic update channels are enabled and reaching endpoints.

Patch Information

Mozilla published fixes in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5. Refer to Mozilla Security Advisory MFSA-2026-106, MFSA-2026-108, MFSA-2026-109, and MFSA-2026-110 for release-specific details.

Workarounds

Disable the Web Serial API via enterprise policy where upgrading immediately is not possible, using the DefaultSerialGuardSetting or equivalent Mozilla policy where supported.
Restrict Firefox and Thunderbird network access to trusted destinations through proxy allowlisting until patched builds are deployed.
Educate users to decline unexpected serial-device permission prompts and report them to the security team.

bash

# Configuration example: enforce Mozilla policy to block Web Serial access
# policies.json placed in Firefox distribution directory
{
  "policies": {
    "Preferences": {
      "dom.serial.enabled": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}