CVE-2026-21521 is a vulnerability classified under CWE-150, which involves improper neutralization of escape, meta, or control sequences within Microsoft 365 Word Copilot. This flaw arises when the Copilot component fails to properly sanitize or neutralize certain control sequences embedded in documents or inputs it processes. As a result, an attacker can craft malicious content that, when processed by Word Copilot, causes unauthorized disclosure of sensitive information over the network. The vulnerability does not require any privileges and can be triggered with user interaction, such as opening or interacting with a malicious document. The CVSS 3.1 base score of 7.4 reflects a high severity, with network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The scope is changed (S:C), indicating that exploitation affects resources beyond the vulnerable component. The impact is high on confidentiality (C:H), with no impact on integrity or availability. No patches or known exploits are currently available, but the vulnerability is publicly disclosed and should be addressed promptly. This vulnerability is particularly concerning given the widespread deployment of Microsoft 365 Word Copilot in enterprise environments, where sensitive documents are routinely handled. Attackers could leverage this flaw to exfiltrate confidential data, potentially leading to data breaches or exposure of intellectual property.

The primary impact of CVE-2026-21521 is unauthorized disclosure of sensitive information, which can lead to significant confidentiality breaches. Organizations relying on Microsoft 365 Word Copilot for document creation and collaboration are at risk of data leakage if attackers exploit this vulnerability. Since the attack requires user interaction, phishing or social engineering campaigns could be used to deliver malicious documents. The scope change means that the vulnerability could affect other components or systems beyond Word Copilot, potentially amplifying the impact. Although integrity and availability are not affected, the loss of confidentiality can result in regulatory penalties, reputational damage, and loss of competitive advantage. The lack of patches increases the window of exposure, making proactive mitigation critical. Enterprises with high-value intellectual property, sensitive personal data, or regulated information are particularly vulnerable. The global nature of Microsoft 365 usage means that this threat has a broad potential impact across industries and geographies.

1. Implement strict email and document filtering to block or quarantine suspicious documents containing unusual control sequences or macros before they reach end users. 2. Educate users to avoid opening documents from untrusted or unknown sources, emphasizing the risk of social engineering attacks exploiting this vulnerability. 3. Monitor network traffic for unusual outbound connections or data exfiltration attempts originating from Microsoft 365 Word Copilot processes. 4. Employ endpoint detection and response (EDR) solutions to detect anomalous behavior related to document processing and network communications. 5. Restrict or disable the use of Word Copilot features that process external or untrusted content until a patch is available. 6. Maintain up-to-date backups of critical documents to mitigate potential secondary impacts. 7. Stay informed on vendor advisories and apply security patches promptly once released. 8. Consider deploying network segmentation to limit the exposure of sensitive systems that use Microsoft 365 Word Copilot. 9. Use data loss prevention (DLP) tools to detect and block unauthorized transmission of sensitive information. 10. Collaborate with Microsoft support channels for guidance and early access to mitigations or patches.