CVE-2026-0890 is a vulnerability identified in Mozilla Firefox and Thunderbird affecting versions prior to Firefox 147 and Thunderbird 140.7. The issue resides in the Document Object Model (DOM) implementation specifically within the Copy & Paste and Drag & Drop components. This flaw allows an attacker to spoof content during these user-initiated operations, potentially misleading users by presenting falsified information or content that appears legitimate. The vulnerability is classified under CWE-290, which relates to improper authentication or authorization, indicating that the spoofing can bypass expected content validation or user interface assurances. The CVSS v3.1 base score is 5.4 (medium), with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), user interaction required (UI:R), unchanged scope (S:U), limited confidentiality impact (C:L), no integrity impact (I:N), and limited availability impact (A:L). This means an attacker can exploit the vulnerability remotely without authentication but requires the user to perform an action such as pasting or dragging content. The impact primarily concerns confidentiality, as spoofed content could deceive users into disclosing sensitive information or executing unintended actions. There are no known exploits in the wild at the time of publication, and no official patches have been linked yet, though updates to Firefox 147 and Thunderbird 140.7 or later versions are expected to address the issue. The vulnerability's exploitation does not compromise system integrity or cause denial of service but could facilitate phishing or social engineering attacks by manipulating clipboard or drag-and-drop content. Organizations relying on Firefox and Thunderbird for communication and browsing should be aware of this risk and prepare to apply patches promptly.

For European organizations, this vulnerability poses a moderate risk primarily through social engineering and phishing attacks leveraging spoofed content during copy-paste or drag-and-drop operations. Confidentiality could be compromised if users are tricked into revealing sensitive data or credentials due to deceptive UI elements. Availability and integrity impacts are minimal, but the potential for misinformation or unauthorized data disclosure could disrupt business operations and erode trust. Sectors with high reliance on Firefox and Thunderbird for email and web access, such as finance, government, and critical infrastructure, may face targeted exploitation attempts. The requirement for user interaction limits automated exploitation but does not eliminate risk, especially in environments with less security awareness. The absence of known exploits reduces immediate threat but underscores the need for proactive mitigation. Overall, the impact is moderate but significant enough to warrant attention in European contexts where data protection regulations and cybersecurity standards demand prompt vulnerability management.

1. Upgrade affected Mozilla Firefox and Thunderbird installations to version 147 and 140.7 or later as soon as patches become available. 2. Until patches are applied, consider disabling or restricting drag-and-drop and clipboard operations within enterprise-managed browsers and email clients, especially in high-risk environments. 3. Implement user training focused on recognizing suspicious copy-paste and drag-and-drop behaviors and verifying content authenticity before acting. 4. Deploy endpoint security solutions that monitor and alert on unusual clipboard or drag-and-drop activities. 5. Use browser security policies (e.g., via Group Policy or enterprise management tools) to limit interaction with untrusted web content. 6. Monitor security advisories from Mozilla for updates or workarounds related to this vulnerability. 7. Employ multi-factor authentication and data loss prevention controls to reduce impact if spoofing leads to credential theft or data exposure. 8. Conduct phishing simulations to raise awareness about social engineering risks associated with spoofed content.