CVE-2026-21521 is a vulnerability classified under CWE-150, which pertains to improper neutralization of escape, meta, or control sequences. Specifically, it affects Microsoft 365 Word Copilot, a feature integrated into Microsoft Word that leverages AI to assist users. The vulnerability arises because the Copilot component fails to properly sanitize certain control sequences embedded in user inputs or documents. This improper neutralization can be exploited by an attacker who crafts malicious content that, when processed by Word Copilot, causes unauthorized disclosure of sensitive information over the network. The attack vector is remote network-based (AV:N), requires no privileges (PR:N), but does require user interaction (UI:R), such as opening or interacting with a malicious document. The scope is changed (S:C), meaning the vulnerability can affect resources beyond the initially vulnerable component. The impact is high on confidentiality (C:H), with no impact on integrity (I:N) or availability (A:N). The CVSS 3.1 score of 7.4 reflects these factors. Although no public exploits are known yet, the vulnerability's nature suggests that attackers could leverage it to exfiltrate data from affected systems. The lack of available patches at the time of publication means organizations must rely on interim mitigations until official fixes are released.

For European organizations, the primary impact of CVE-2026-21521 is the potential unauthorized disclosure of sensitive or confidential information processed through Microsoft 365 Word Copilot. This could include corporate documents, intellectual property, or personal data, leading to data breaches and compliance violations under regulations such as GDPR. Since Microsoft 365 is widely adopted across Europe, especially in sectors like finance, government, and professional services, the risk of data leakage is significant. The vulnerability does not affect system integrity or availability, so it is less likely to cause service disruptions but poses a serious confidentiality risk. Attackers exploiting this flaw could gain access to sensitive data remotely without needing elevated privileges, increasing the threat level. The requirement for user interaction means phishing or social engineering could be used to trigger the exploit, emphasizing the need for user awareness. The lack of known exploits currently provides a window for proactive defense, but the vulnerability’s characteristics suggest it could be weaponized quickly once exploit code becomes available.

1. Monitor Microsoft security advisories closely and apply official patches or updates for Microsoft 365 Word Copilot as soon as they are released. 2. Implement network segmentation and restrict outbound network traffic from endpoints running Microsoft 365 to limit unauthorized data exfiltration. 3. Employ advanced email and document filtering solutions to detect and block malicious documents that could exploit this vulnerability. 4. Conduct targeted user awareness training focusing on the risks of interacting with unsolicited or suspicious documents, emphasizing the need to verify document sources. 5. Use endpoint detection and response (EDR) tools to monitor for anomalous behaviors related to Microsoft Word processes, particularly network connections initiated by Word Copilot. 6. Consider temporarily disabling or restricting the use of Word Copilot in high-risk environments until patches are applied. 7. Enforce strict access controls and data loss prevention (DLP) policies to detect and prevent unauthorized data transmissions. 8. Maintain up-to-date backups and incident response plans to quickly address any potential data breach resulting from exploitation.