Overview
CVE-2025-66514 describes a stored HTML injection vulnerability found in the Nextcloud Mail application, specifically affecting versions prior to 5.5.3. This vulnerability allows an authenticated user to inject HTML code into the subject lines of emails displayed within the Mail app. While JavaScript execution is prevented due to Nextcloud’s Content Security Policy (CSP), the injection of HTML can still lead to potential phishing or defacement attacks.
Published on 2025-12-05T18:15:57.457, this vulnerability has been assessed as having a low severity.
Technical Details
The vulnerability stems from insufficient sanitization of email subject lines when they are displayed in the Nextcloud Mail application’s message list. An attacker could craft an email with a specially crafted subject containing HTML tags. When another user views the email list, the injected HTML will be rendered. Because the Nextcloud server’s Content Security Policy restricts Javascript execution, a full XSS attack cannot be performed.
The vulnerability was addressed in commit c64fcc3b79e0c089b5e1d2e04a07bfa740b2ac09 through improved sanitization.
CVSS Analysis
The Common Vulnerability Scoring System (CVSS) score for CVE-2025-66514 is 3.5, indicating a LOW severity. The CVSS vector is likely AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:N. This breaks down as follows:
- AV:N (Attack Vector: Network): The vulnerability can be exploited over a network.
- AC:L (Attack Complexity: Low): The conditions for a successful attack are easily met.
- PR:L (Privileges Required: Low): An attacker needs only low-level privileges (e.g., a valid user account) to exploit the vulnerability.
- UI:R (User Interaction: Required): Successful exploitation requires some form of user interaction (e.g., viewing the malicious email).
- S:U (Scope: Unchanged): An exploited vulnerability can only affect resources managed by the same security authority.
- C:N (Confidentiality: None): There is no impact to confidentiality.
- I:L (Integrity: Low): There is limited impact on integrity. For example, a single modified file.
- A:N (Availability: None): There is no impact to availability.
The low score reflects the fact that while HTML injection is possible, JavaScript execution is blocked and the impact is limited to potential visual defacement or minor phishing opportunities.
Possible Impact
While the severity is low, the impact could include:
- Phishing: An attacker could use HTML to visually spoof the email subject, potentially tricking users into opening malicious emails or clicking on deceptive links (although JavaScript is blocked).
- Defacement: HTML injection could be used to alter the appearance of the email list, causing confusion or annoyance to users.
Mitigation or Patch Steps
The recommended mitigation is to update Nextcloud Mail to version 5.5.3 or later. This version contains the fix for CVE-2025-66514. To update your Nextcloud Mail app:
- Log in to your Nextcloud instance as an administrator.
- Navigate to the Apps section.
- Search for the “Mail” app.
- If an update is available, click the “Update” button.
It is always a good practice to keep your Nextcloud instance and all its apps up to date to protect against known vulnerabilities.
