CVE-2025-66514 is a stored cross-site scripting (XSS) vulnerability classified under CWE-79, found in the Nextcloud Mail app, a component of the Nextcloud self-hosted productivity platform. The flaw exists in versions from 5.2.0-beta.1 up to but not including 5.5.3. It allows an authenticated user to inject arbitrary HTML content into the email subject list displayed by the Mail app. While the injection is stored, the Nextcloud server enforces a Content Security Policy (CSP) that blocks JavaScript execution, mitigating the risk of full script-based attacks. However, the injected HTML could still be used to manipulate the user interface or conduct phishing attacks by altering the appearance of email subjects or embedding misleading links. Exploitation requires the attacker to have valid credentials and for a user to interact with the maliciously crafted email subject. The vulnerability does not affect confidentiality or availability but impacts data integrity within the application context. The CVSS 3.1 base score is 3.5 (low severity), reflecting the limited impact and exploitation complexity. No public exploits have been reported, and no patches are linked in the advisory, but upgrading to version 5.5.3 or later is recommended to remediate the issue.

For European organizations, the impact of CVE-2025-66514 is primarily on the integrity of displayed email subject data within Nextcloud Mail. Attackers with valid credentials could manipulate email subjects to mislead users, potentially facilitating phishing or social engineering attacks. Although JavaScript execution is blocked, the altered UI could still cause confusion or indirect compromise of user trust. This could be particularly concerning for organizations relying heavily on Nextcloud for internal communication and collaboration, including government agencies, financial institutions, and enterprises with strict data integrity requirements. The vulnerability does not compromise confidentiality or availability, limiting its overall risk. However, the presence of stored HTML injection in a widely used self-hosted platform could be leveraged as part of a multi-stage attack or combined with other vulnerabilities. Organizations with large user bases or those in regulated sectors should prioritize remediation to maintain trust and compliance.

To mitigate CVE-2025-66514, European organizations should: 1) Upgrade Nextcloud Mail to version 5.5.3 or later, where the vulnerability is fixed. 2) Enforce strict access controls to limit authenticated user permissions, reducing the risk of malicious input injection. 3) Review and strengthen Content Security Policies to ensure robust blocking of script execution and other potentially harmful content. 4) Educate users about phishing risks and encourage vigilance when interacting with email subjects or links within Nextcloud Mail. 5) Monitor logs and user activity for unusual behavior indicative of exploitation attempts. 6) Consider deploying web application firewalls (WAFs) with rules tailored to detect and block HTML injection patterns in Nextcloud Mail traffic. 7) Regularly audit and sanitize user-generated content inputs beyond the Mail app to prevent similar injection issues. These steps go beyond generic advice by focusing on access control, user awareness, and layered defenses specific to the Nextcloud environment.