CVE-2025-66545

Executive Summary

CVE-2025-66545 is a vulnerability in Nextcloud Groupfolders, an application that provides admin-configured folders shared by everyone in a group or team. The vulnerability allows a user with read-only permission to restore a file from the trash bin. This is a violation of the intended access control, as read-only users should not be able to perform write operations like restoring files. The vulnerability has a low CVSS score of 3.5, indicating a limited impact, but it still represents a security boundary bypass that could lead to data integrity issues.

Supply Chain & Package Security

This vulnerability is specific to the Nextcloud ecosystem, which is a self-hosted file sync and sharing platform. It is not a package manager (NPM, PyPI, Go, Ruby) vulnerability, but rather a server application vulnerability. The Nextcloud Groupfolders app is typically installed via the Nextcloud app store or manually deployed. Administrators should ensure they are using a patched version of the app.

CVSS Breakdown & Threat Level

• CVSS Score: 3.5 (Low)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) - The attacker must have a valid user account with read-only permissions on the group folder.
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: Low (I:L) - The attacker can restore a file, potentially overwriting or undoing a deletion.
• Availability Impact: None (A:N)

Threat Level: Low. The vulnerability requires an authenticated user with read-only access. The impact is limited to restoring files from the trash, which does not directly expose sensitive data or allow code execution.

Deep Dive & Mechanics

The Nextcloud Groupfolders app implements access control to ensure that users with read-only permissions cannot modify files. However, the trash bin restoration functionality was not properly protected. When a file is deleted from a group folder, it is moved to the trash bin. The restore operation should be restricted to users with write or delete permissions, but due to a missing permission check, a read-only user can trigger the restore.

The vulnerability exists in the trash bin handling code. The application likely checks permissions for the initial delete operation but fails to re-validate permissions when the restore action is performed. This allows a read-only user to send a restore request, which the server processes without verifying that the user has the necessary write permissions.

Vulnerable vs. Patched Code

The vulnerable code likely lacks a permission check in the restore function. The patched code adds a check to ensure the user has write or delete permissions before allowing the restore.

Vulnerable Code (Conceptual):

// In the restore function
public function restore($fileId) {
    $file = $this->trashManager->getFile($fileId);
    // Missing permission check
    $this->trashManager->restore($file);
}

Patched Code (Conceptual):

// In the restore function
public function restore($fileId) {
    $file = $this->trashManager->getFile($fileId);
    // Added permission check
    if (!$this->groupfoldersManager->hasWritePermission($file->getFolderId(), $this->userId)) {
        throw new \Exception('Permission denied');
    }
    $this->trashManager->restore($file);
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• A Nextcloud instance with Groupfolders app installed (vulnerable version).
• A user account with read-only permissions on a group folder.
• A file that has been deleted from that group folder (exists in the trash bin).

Exploitation Steps:

1. Log in as the read-only user.
2. Navigate to the trash bin of the group folder.
3. Identify a deleted file.
4. Send a restore request (e.g., via the Nextcloud WebDAV interface or API).
5. The file is restored, bypassing the intended permission restriction.

Example API Request (using curl):

curl -X POST 'https://nextcloud.example.com/ocs/v2.php/apps/groupfolders/trash/restore' \
  -H 'OCS-APIRequest: true' \
  -H 'Content-Type: application/json' \
  -d '{"folderId": 123, "fileId": 456}' \
  -u 'readonlyuser:password'

Blue Team

Detection:

• Monitor for restore operations on group folders by users with read-only permissions.
• Look for unusual patterns in trash bin activity, such as a high volume of restore requests from a single user.
• Review logs for permission denied errors that may indicate attempted exploitation.

Prevention:

• Apply the patch immediately.
• Implement additional monitoring on the trash bin restore endpoint.
• Use Web Application Firewall (WAF) rules to block suspicious restore requests.

Indicators of Compromise (IOCs)

• Log Entries: Successful restore operations by users with read-only permissions.
• API Calls: POST requests to /ocs/v2.php/apps/groupfolders/trash/restore from unexpected users.
• File Changes: Files that were previously deleted reappearing in the group folder without a corresponding delete or upload event.

Detection Engineering

YARA Rule

rule Detect_CVE_2025_66545_Exploit {
    meta:
        description = "Detects attempts to exploit CVE-2025-66545 in Nextcloud Groupfolders"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66545"
    strings:
        $s1 = "groupfolders/trash/restore" ascii wide nocase
        $s2 = "restore" ascii wide nocase
    condition:
        all of them
}

Sigma Rule

title: Nextcloud Groupfolders Trash Restore by Read-Only User
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects a restore operation on a group folder trash bin by a user with read-only permissions.
author: Valters IT
date: 2025-04-10
references:
    - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-66545
logsource:
    category: application
    product: nextcloud
detection:
    selection:
        event: "restore"
        permission: "read-only"
    condition: selection
falsepositives:
    - Legitimate restore by a user whose permissions were recently changed.
level: medium

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-66545 Nextcloud Groupfolders Trash Restore Exploit";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/ocs/v2.php/apps/groupfolders/trash/restore";
    http_uri;
    classtype:attempted-user;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade Nextcloud Groupfolders to version 14.0.11, 15.3.12, 16.0.15, 17.0.14, 18.1.8, 19.1.8, or 20.1.2 (or later).
2. Verify: After patching, test that read-only users can no longer restore files from the trash bin.
3. Audit: Review logs for any signs of exploitation.

Long-Term Mitigations:

• Implement the principle of least privilege for all users.
• Regularly review and audit user permissions.
• Enable detailed logging for all file operations.
• Consider using a Web Application Firewall (WAF) to monitor and block suspicious API calls.

Workarounds (if patching is not immediately possible):

• Disable the trash bin for group folders.
• Manually remove the restore permission from read-only users in the database (not recommended).

Valters IT
Securing Digital Frontiers