CVE-2025-66552

Executive Summary

CVE-2025-66552 is a medium-severity vulnerability in Nextcloud Server and Enterprise Server that affects the admin_audit logging mechanism. Due to incorrect path handling within the groupfolders application, the audit log fails to record certain file and folder operations (create, modify, delete, rename) performed inside group folders. This creates a blind spot for security monitoring and forensic analysis, allowing malicious or unauthorized actions to go undetected by the built-in audit trail. The vulnerability is present in versions prior to 30.0.9 and 31.0.1. No public proof-of-concept (PoC) or exploit code is available at this time.

Supply Chain & Package Security

This vulnerability is specific to the Nextcloud Server application, which is a self-hosted PHP-based platform. It is not a dependency-level supply chain issue (e.g., NPM, PyPI, Go, Ruby). The risk is limited to the core application and its groupfolders app.

CVSS Breakdown & Threat Level

• CVSS Score: 4.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
• Attack Vector (AV): Network (N) – The vulnerability can be triggered remotely.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): Low (L) – An attacker needs a valid user account with access to a group folder.
• User Interaction (UI): None (N) – No user action is needed beyond the attacker's own actions.
• Scope (S): Unchanged (U) – The impact is confined to the vulnerable component.
• Confidentiality (C): None (N) – No direct data exposure.
• Integrity (I): Low (L) – The integrity of the audit log is compromised, but not the data itself.
• Availability (A): None (N) – No denial of service.

Threat Level: Medium. While the CVSS score is moderate, the impact on audit integrity is significant for compliance and forensic purposes. An attacker with low privileges can perform actions inside group folders without leaving a trace in the admin audit log.

Deep Dive & Mechanics

The vulnerability lies in how the groupfolders application interacts with the admin_audit logging system. In Nextcloud, the admin_audit app is responsible for logging all file and folder operations (e.g., file created, file deleted, file renamed, folder shared) for security and compliance.

The groupfolders app allows administrators to create virtual folders that are shared with entire groups. When a user performs an action inside a group folder, the system must resolve the actual filesystem path to log the event. Due to a flaw in path handling, the admin_audit app receives an incorrect or malformed path for operations inside group folders. This causes the logging mechanism to either fail silently or skip the event entirely.

Root Cause: The path normalization or resolution logic in the groupfolders app does not correctly translate the virtual group folder path to the underlying storage path before passing it to the admin_audit hooks. As a result, the audit event is either discarded or logged with an invalid path, effectively bypassing the audit trail.

Attack Scenario:

1. An attacker gains access to a low-privileged user account that is a member of a group with access to a group folder.
2. The attacker creates, modifies, or deletes files inside the group folder.
3. The admin_audit app fails to log these actions.
4. The attacker can exfiltrate data, plant malicious files, or disrupt operations without triggering any audit alerts.

Vulnerable vs. Patched Code

The fix involves correcting the path handling logic in the groupfolders app to ensure that the correct filesystem path is passed to the admin_audit hooks.

Vulnerable Code (Conceptual):

// In groupfolders/lib/Hooks.php (vulnerable)
public function postWrite($params) {
    $path = $params['path']; // This path may be relative to the group folder root, not the full storage path
    $this->auditLogger->log('file_write', ['path' => $path]);
    // The audit logger receives an incomplete or incorrect path, causing it to skip logging
}

Patched Code (Conceptual):

// In groupfolders/lib/Hooks.php (patched)
public function postWrite($params) {
    $fullPath = $this->getFullStoragePath($params['path']); // Resolve to full storage path
    $this->auditLogger->log('file_write', ['path' => $fullPath]);
    // The audit logger now receives the correct path and logs the event
}

The actual patch likely involves changes to the lib/Hooks.php or similar files in the groupfolders app to properly resolve paths before logging.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A Nextcloud instance running a vulnerable version (< 30.0.9 or < 31.0.1).
• A user account with access to a group folder.

Steps:

1. Log in to Nextcloud as the low-privileged user.
2. Navigate to a group folder.
3. Perform a file operation (e.g., upload a file, delete a file, rename a file).
4. Check the admin audit log (accessible to admins at /index.php/settings/admin/logging). The action will not appear.

Proof of Concept (No public exploit available):

# Using curl to upload a file to a group folder (replace with actual server and credentials)
curl -u 'attacker:password' -X PUT -T /path/to/malicious.txt 'https://nextcloud.example.com/remote.php/dav/files/attacker/GroupFolder/malicious.txt'
# The admin audit log will not show this upload.

Blue Team (Detection & Forensics)

Detection:

• Monitor for discrepancies between filesystem events and audit log entries.
• Use filesystem monitoring tools (e.g., inotify, auditd) to track changes in the group folder storage directory and compare with Nextcloud audit logs.

Forensics:

• If an incident is suspected, examine the underlying storage directory for group folders (usually /var/www/nextcloud/data/__groupfolders/).
• Check for files that exist in the storage but have no corresponding audit log entry.
• Review web server access logs for requests to the group folder DAV endpoint.

Indicators of Compromise (IOCs)

• Filesystem: Presence of files in the group folder storage directory (__groupfolders/) that are not recorded in the oc_audit_log database table.
• Database: Missing entries in the oc_audit_log table for operations that should have been logged (e.g., file creation, deletion, rename) for users with group folder access.
• Network: No specific network IOCs; the vulnerability is in the application logic, not network traffic.

Detection Engineering

YARA Rule

rule Detect_Nextcloud_GroupFolder_Audit_Bypass {
    meta:
        description = "Detects potential exploitation of CVE-2025-66552 by checking for missing audit log entries"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66552"
    strings:
        $db_table = "oc_audit_log" nocase
        $groupfolder_path = "__groupfolders" nocase
    condition:
        $db_table and $groupfolder_path
}

Sigma Rule

title: Nextcloud Group Folder Audit Log Bypass
id: 8c5e2f3a-1b2c-4d5e-6f7a-8b9c0d1e2f3a
status: experimental
description: Detects missing audit log entries for file operations in Nextcloud group folders, indicating possible exploitation of CVE-2025-66552.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-66552
logsource:
    product: nextcloud
    service: audit
detection:
    selection:
        action: "file_*"
        path: "*__groupfolders*"
    condition: selection
    falsepositives:
        - Legitimate operations that are correctly logged (patched versions)
level: high

Suricata Rule

alert http any any -> any any (msg:"Nextcloud Group Folder Audit Bypass - Potential CVE-2025-66552 Exploitation"; flow:to_server; content:"/remote.php/dav/files/"; http_uri; content:"__groupfolders"; http_uri; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Nextcloud: Update to Nextcloud Server 30.0.9 or 31.0.1 (or later) as soon as possible.
2. Apply Workaround: If immediate upgrade is not possible, consider disabling the groupfolders app temporarily: occ app:disable groupfolders.
3. Monitor Audit Logs: Manually review the oc_audit_log database table for any suspicious gaps or missing entries.

Long-Term Mitigations:

• Implement additional logging and monitoring at the filesystem level (e.g., using auditd on Linux) to capture file operations independently of Nextcloud's audit log.
• Restrict group folder access to only trusted users.
• Enable and review Nextcloud's built-in logging and alerting features.

Patch Details:

• The fix is included in Nextcloud Server 30.0.9 and 31.0.1. Upgrade instructions are available at Nextcloud Server Changelog.

Valters IT