CVE-2023-48239
📋 TL;DR
This vulnerability in Nextcloud Server allows a malicious user to update any personal or global external storage configuration, making those storage locations inaccessible to all other users. It affects Nextcloud Server versions 25.0.0 through 25.0.12, 26.0.0 through 26.0.7, and 27.0.0 through 27.1.2, as well as Nextcloud Enterprise Server versions 20.0.0 through 20.0.14.15, 21.0.0 through 21.0.9.12, 22.2.0 through 22.2.10.14, 23.0.0 through 23.0.12.11, 24.0.0 through 24.0.12.7, 25.0.0 through 25.0.12, 26.0.0 through 26.0.7, and 27.0.0 through 27.1.2.
💻 Affected Systems
- Nextcloud Server
- Nextcloud Enterprise Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
A malicious insider or compromised account could disable all external storage connections, causing widespread data access disruption and potential business impact.
Likely Case
An authenticated malicious user disables external storage for targeted users or groups, causing localized data access issues.
If Mitigated
With proper access controls and monitoring, impact is limited to temporary storage access disruption until configuration is restored.
🎯 Exploit Status
Exploitation requires authenticated user access. The vulnerability is in improper access control for external storage management.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Nextcloud Server: 25.0.13, 26.0.8, 27.1.3; Nextcloud Enterprise Server: 20.0.14.16, 21.0.9.13, 22.2.10.15, 23.0.12.12, 24.0.12.8, 25.0.13, 26.0.8, 27.1.3
Vendor Advisory: https://github.com/nextcloud/security-advisories/security/advisories/GHSA-f962-hw26-g267
Restart Required: Yes
Instructions:
1. Backup your Nextcloud installation and database. 2. Update to the patched version using your package manager or manual download. 3. Run occ upgrade via command line. 4. Restart web server and PHP-FPM services.
🔧 Temporary Workarounds
Disable files_external app
linuxDisables the external storage functionality entirely, preventing exploitation but also making external storage inaccessible.
sudo -u www-data php occ app:disable files_external
🧯 If You Can't Patch
- Restrict user permissions to minimize who can access external storage management
- Implement additional monitoring for external storage configuration changes
🔍 How to Verify
Check if Vulnerable:
Check Nextcloud version via occ command or admin interface. If version falls within affected ranges and files_external app is enabled, system is vulnerable.Check Version:
sudo -u www-data php occ statusVerify Fix Applied:
Verify version is updated to patched version and test external storage functionality remains accessible to authorized users only.📡 Detection & Monitoring
Log Indicators:
- Unexpected external storage configuration changes
- Multiple users reporting storage access issues
- files_external app activity from unauthorized users
Network Indicators:
- Increased failed storage connection attempts
- Unusual patterns in storage API calls
SIEM Query:
source="nextcloud.log" AND ("files_external" OR "external storage") AND ("update" OR "modify" OR "configure")🔗 References
- https://github.com/nextcloud/security-advisories/security/advisories/GHSA-f962-hw26-g267
- https://github.com/nextcloud/server/pull/41123
- https://hackerone.com/reports/2212627
- https://github.com/nextcloud/security-advisories/security/advisories/GHSA-f962-hw26-g267
- https://github.com/nextcloud/server/pull/41123
- https://hackerone.com/reports/2212627
