CVE-2026-21260 is a vulnerability classified under CWE-200, indicating exposure of sensitive information to unauthorized actors. It affects Microsoft 365 Apps for Enterprise, specifically Microsoft Outlook version 16.0.1. The flaw enables an attacker to perform spoofing attacks over a network, which can trick the application into disclosing sensitive information without requiring any privileges or user interaction. The CVSS 3.1 base score is 7.5, reflecting high severity due to the vulnerability's network attack vector, low attack complexity, and no need for authentication or user interaction. The vulnerability compromises confidentiality but does not impact integrity or availability. Although no exploits are currently known in the wild, the potential for sensitive data leakage is significant. The vulnerability was published on February 10, 2026, and remains unpatched as no patch links are provided. This vulnerability could be leveraged in targeted attacks or broader campaigns to harvest sensitive information from enterprise email environments. The exposure arises from insufficient validation or improper handling of network data within Outlook, allowing spoofed network messages to bypass security controls and extract confidential information. Organizations relying on Microsoft 365 Apps for Enterprise should be aware of this risk and prepare mitigation strategies.

For European organizations, this vulnerability poses a significant risk to the confidentiality of sensitive corporate and personal information handled via Microsoft Outlook. Given the widespread adoption of Microsoft 365 in Europe, especially in sectors like finance, government, healthcare, and legal services, unauthorized exposure of sensitive emails or attachments could lead to data breaches, regulatory penalties under GDPR, reputational damage, and potential financial losses. The lack of required privileges or user interaction lowers the barrier for attackers, increasing the likelihood of exploitation in network environments. Although integrity and availability are not directly affected, the confidentiality breach alone can have severe consequences, including intellectual property theft, espionage, and compliance violations. Organizations with remote or hybrid workforces using Outlook over potentially insecure networks are particularly vulnerable. The absence of known exploits in the wild suggests a window of opportunity for proactive defense before active exploitation emerges.

1. Monitor official Microsoft channels closely for patches addressing CVE-2026-21260 and apply them immediately upon release. 2. Until patches are available, implement network-level protections such as intrusion detection/prevention systems (IDS/IPS) configured to detect and block spoofing attempts targeting Outlook traffic. 3. Enforce strict network segmentation and limit exposure of Microsoft 365 services to trusted networks and VPNs to reduce attack surface. 4. Employ email security gateways and anti-spoofing technologies like SPF, DKIM, and DMARC to reduce the risk of spoofed messages reaching users. 5. Conduct regular security awareness training emphasizing the risks of spoofing and the importance of reporting suspicious emails. 6. Use endpoint detection and response (EDR) tools to monitor for anomalous Outlook behaviors indicative of exploitation attempts. 7. Review and tighten Outlook and Microsoft 365 configuration settings to minimize unnecessary data exposure. 8. Maintain comprehensive logging and monitoring to detect early signs of exploitation or data exfiltration. These measures, combined with timely patching, will significantly reduce the risk posed by this vulnerability.