CVE-2026-16378: Firefox DOM Copy & Paste Vulnerability

CVE-2026-16378 Overview

CVE-2026-16378 is a high-severity vulnerability affecting the Document Object Model (DOM) Copy & Paste and Drag & Drop component in Mozilla Firefox and Thunderbird. Mozilla addressed the issue in Firefox 153 and Thunderbird 153. The flaw is classified under [CWE-20] Improper Input Validation and is remotely exploitable across the network without authentication or user interaction. Successful exploitation impacts confidentiality of data handled by the browser's clipboard and drag-and-drop subsystems. Mozilla published details in security advisories MFSA-2026-68 and MFSA-2026-71, along with the associated bug tracker entry.

Critical Impact
Remote, unauthenticated attackers can trigger the flaw over the network to compromise the confidentiality of browser data flowing through the DOM Copy & Paste and Drag & Drop component.

Affected Products

Mozilla Firefox versions prior to 153
Mozilla Thunderbird versions prior to 153
Mozilla DOM: Copy & Paste and Drag & Drop component

Discovery Timeline

2026-07-21 - CVE-2026-16378 published to NVD
2026-07-22 - Last updated in NVD database

Technical Details for CVE-2026-16378

Vulnerability Analysis

The vulnerability resides in Mozilla's DOM Copy & Paste and Drag & Drop component. This subsystem handles clipboard reads, writes, and drag-and-drop data transfers between web content and the browser. The flaw allows a remote attacker to influence how data is processed during these operations without any user interaction. The vulnerability is categorized as an input validation weakness [CWE-20], where untrusted input reaching clipboard or drag-and-drop APIs is not sufficiently constrained. The result is an impact on confidentiality, consistent with information leakage from the browser context. Mozilla resolved the issue in Firefox 153 and Thunderbird 153 through the fixes referenced in Mozilla Security Advisory MFSA-2026-68 and Mozilla Security Advisory MFSA-2026-71.

Root Cause

The root cause is improper input validation within the DOM Copy & Paste and Drag & Drop component. The component fails to enforce sufficient constraints on data crossing origin or trust boundaries during clipboard and drag operations, per the Mozilla Bug Report #2038868.

Attack Vector

Exploitation requires a victim to load attacker-controlled web content in a vulnerable Firefox or Thunderbird build. The attack proceeds over the network, without privileges and without user interaction, targeting the confidentiality of data available to the DOM clipboard and drag-and-drop APIs. No public proof-of-concept exploit or in-the-wild exploitation has been reported at publication time.

No verified exploit code is publicly available. Refer to Mozilla Bug Report #2038868 for technical implementation details.

Detection Methods for CVE-2026-16378

Indicators of Compromise

Firefox or Thunderbird client versions reporting a build earlier than 153 in HTTP User-Agent headers or endpoint inventory data.
Unexpected outbound requests from browser processes following interaction with untrusted pages that invoke clipboard or drag-and-drop APIs.
Web pages invoking navigator.clipboard.read, navigator.clipboard.readText, or DataTransfer APIs from untrusted origins.

Detection Strategies

Inventory Firefox and Thunderbird installations across managed endpoints and flag versions below 153 for prioritized remediation.
Monitor endpoint process telemetry for firefox.exe and thunderbird.exe spawning unusual child processes or making anomalous network connections after visiting untrusted web content.
Correlate proxy logs to identify browser sessions loading pages that abuse clipboard or drag-and-drop APIs across cross-origin boundaries.

Monitoring Recommendations

Track Mozilla security advisories MFSA-2026-68 and MFSA-2026-71 for any follow-up fixes or related CVEs.
Enable browser telemetry and crash reporting to surface anomalous behavior in the DOM component.
Alert on endpoints running out-of-date Firefox or Thunderbird versions after the patch release window closes.

How to Mitigate CVE-2026-16378

Immediate Actions Required

Upgrade Mozilla Firefox to version 153 or later on all managed endpoints.
Upgrade Mozilla Thunderbird to version 153 or later, including Firefox ESR and Thunderbird ESR builds that incorporate the fix.
Validate that automatic browser updates are enabled and functioning across the enterprise fleet.

Patch Information

Mozilla addressed CVE-2026-16378 in Firefox 153 and Thunderbird 153. Details of the fix are published in Mozilla Security Advisory MFSA-2026-68 and Mozilla Security Advisory MFSA-2026-71. The underlying defect is tracked in Mozilla Bug Report #2038868.

Workarounds

Restrict access to untrusted websites through web filtering or DNS-based controls until patching is complete.
Disable clipboard read permissions for untrusted origins using enterprise browser policies where operationally acceptable.
Educate users to avoid interacting with drag-and-drop prompts on unfamiliar websites during the remediation window.

bash

# Verify installed Firefox version on Linux endpoints
firefox --version

# Verify installed Thunderbird version on Linux endpoints
thunderbird --version

# Windows: query installed version via registry
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion