CVE-2025-66514 is a stored HTML injection vulnerability classified under CWE-79 (Improper Neutralization of Input During Web Page Generation, commonly known as Cross-site Scripting) affecting the Nextcloud Mail app, a component of the Nextcloud self-hosted productivity platform. The vulnerability 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 fields displayed in the message list. Although this injection could potentially be used to manipulate the user interface or mislead users, the Nextcloud server enforces a strict Content Security Policy (CSP) that blocks JavaScript execution, significantly reducing the risk of typical XSS attacks such as session hijacking or cookie theft. The vulnerability requires the attacker to have authenticated access and some level of user interaction to exploit, as the injected HTML is stored and rendered in the message list. The CVSS v3.1 base score is 3.5, indicating a low severity level, with the vector indicating network attack vector, low attack complexity, requiring privileges, user interaction, and limited impact on integrity only. No known exploits are currently reported in the wild. The vulnerability primarily affects data integrity by allowing HTML injection that could alter the display of email subjects, potentially leading to phishing or social engineering attempts within the trusted environment.

For European organizations, the impact of CVE-2025-66514 is relatively low but non-negligible. The vulnerability could be exploited by malicious insiders or compromised accounts to inject misleading HTML content into email subjects, potentially facilitating phishing or social engineering attacks within the organization. While JavaScript execution is blocked, the altered display could still confuse users or cause reputational damage if used to impersonate trusted communications. Since Nextcloud is widely used in Europe, especially in privacy-conscious countries like Germany, the Netherlands, and France, organizations relying on the Nextcloud Mail app could face targeted attempts to exploit this vulnerability. The impact on confidentiality and availability is minimal; however, the integrity of displayed information is affected. This could undermine user trust and complicate incident response if attackers use the vulnerability to mask malicious emails or communications. The requirement for authenticated access limits exposure to external attackers but does not eliminate risk from insider threats or compromised credentials.

To mitigate CVE-2025-66514, European organizations should immediately upgrade the Nextcloud Mail app to version 5.5.3 or later, where the vulnerability is patched. In addition to patching, organizations should enforce strict user access controls and monitor for unusual user behavior that could indicate attempts to exploit this vulnerability. Implementing multi-factor authentication (MFA) reduces the risk of account compromise, thereby limiting the potential attackers who can exploit this issue. Administrators should review and restrict permissions to the Mail app to minimize the number of users who can inject content. Regularly auditing email subject content and logs for anomalies can help detect exploitation attempts early. Finally, educating users about the risks of phishing and social engineering, even within trusted platforms, is essential to reduce the impact of any injected misleading content.