CVE-2026-0890: Firefox DOM Spoofing Vulnerability

CVE-2026-0890 Overview

CVE-2026-0890 is a spoofing vulnerability in Mozilla Firefox's DOM Copy & Paste and Drag & Drop component. This vulnerability allows attackers to potentially manipulate content during clipboard operations, enabling spoofing attacks that could deceive users into interacting with malicious content. The vulnerability stems from Authentication Bypass by Spoofing (CWE-290), where the browser fails to properly validate or sanitize content during DOM-level copy/paste and drag/drop operations.

Critical Impact

Attackers could leverage this spoofing vulnerability to manipulate pasted or dropped content in the DOM, potentially leading to user deception, phishing attacks, or unauthorized actions through crafted malicious web content.

Affected Products

• Firefox < 147
• Firefox ESR < 140.7

Discovery Timeline

• 2026-01-13 - CVE CVE-2026-0890 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2026-0890

Vulnerability Analysis

This vulnerability affects the DOM-level handling of Copy & Paste and Drag & Drop operations within Mozilla Firefox. The issue resides in how the browser processes and validates content during these clipboard-related interactions. When users copy, paste, or drag content between different contexts (such as from a malicious webpage to a trusted input field), the browser may fail to properly authenticate or validate the source and integrity of the transferred data.

The network-accessible nature of this vulnerability means attackers can exploit it through specially crafted web pages without requiring any user authentication. The attack requires minimal complexity, making it relatively straightforward for threat actors to weaponize.

Root Cause

The root cause is classified under CWE-290 (Authentication Bypass by Spoofing). This weakness occurs when the application performs a check on the source of data but allows an attacker to spoof the source to bypass the protection mechanism. In this case, the Firefox DOM Copy & Paste and Drag & Drop component does not adequately verify the authenticity of content during clipboard operations, allowing malicious content to masquerade as legitimate data.

Attack Vector

The attack vector is network-based, meaning exploitation can occur remotely through malicious web content. An attacker could craft a webpage containing specially designed elements that, when copied or dragged by a user, produce unexpected or spoofed results when pasted or dropped. This could be used to:

• Inject misleading content into user inputs
• Bypass security indicators or origin checks
• Deceive users about the nature or source of pasted content
• Potentially facilitate phishing or social engineering attacks

The vulnerability can be triggered through normal user interactions with clipboard operations, making it particularly concerning for scenarios where users frequently copy content between web applications.

Detection Methods for CVE-2026-0890

Indicators of Compromise

• Unusual clipboard-related JavaScript activity on web pages, particularly involving DOM manipulation during paste or drop events
• Web pages attempting to intercept or modify clipboard data through event handlers
• Unexpected content appearing in input fields after paste or drop operations
• Browser console errors related to clipboard permission or DOM manipulation

Detection Strategies

• Monitor for suspicious JavaScript patterns that register handlers on paste, drop, or clipboardchange events
• Implement network-level detection for known malicious pages exploiting this vulnerability
• Use browser extension-based monitoring to detect anomalous clipboard interactions
• Review web application logs for unusual input patterns that may indicate spoofed content injection

Monitoring Recommendations

• Enable enhanced logging for clipboard-related browser events in enterprise environments
• Deploy endpoint detection solutions capable of monitoring browser process behavior
• Establish baseline clipboard usage patterns to identify anomalous activity
• Monitor security advisories from Mozilla for additional indicators and detection guidance

How to Mitigate CVE-2026-0890

Immediate Actions Required

• Update Firefox to version 147 or later immediately
• Update Firefox ESR to version 140.7 or later immediately
• Review and audit web applications for potential exposure to clipboard-based spoofing attacks
• Educate users about the risks of copying content from untrusted sources

Patch Information

Mozilla has addressed this vulnerability in Firefox 147 and Firefox ESR 140.7. Organizations should prioritize updating all Firefox installations to these patched versions. For detailed patch information, refer to the official Mozilla Security Advisories:

• Mozilla Security Advisory MFSA-2026-01
• Mozilla Security Advisory MFSA-2026-03

Additional technical details can be found in the Mozilla Bug Report.

Workarounds

• Disable JavaScript on untrusted websites as a temporary measure to prevent exploitation
• Use browser policies to restrict clipboard access for untrusted domains
• Consider using browser containers or profiles to isolate sensitive browsing activities
• Implement Content Security Policy (CSP) headers on internal web applications to limit clipboard API abuse

# Firefox enterprise policy to enhance clipboard security (policies.json)
{
  "policies": {
    "Preferences": {
      "dom.event.clipboardevents.enabled": false
    }
  }
}