Hardening and security guidance

Nextcloud aims to ship with secure defaults that do not need to get modified by administrators. However, in some cases some additional security hardening can be applied in scenarios were the administrator has complete control over the Nextcloud instance. This page assumes that you run Nextcloud Server on Apache2 in a Linux environment.

Note

Nextcloud will warn you in the administration interface if some critical security-relevant options are missing. However, it is still up to the server administrator to review and maintain system security.

Passwords

Storage of access tokens

Upon successful authentication, Nextcloud issues an access token that clients will use for all future HTTP requests. This access token uniquely identifies a user and should not be stored on any system other than the client requesting it. The user password is also stored encrypted in the Nextcloud database. For encryption of the password, the token and an instance-specific secret is used.

Leakage of the access token can have negative security consequences. Depending on the data access by the actor, the risk here is different:

• An actor with access to only the access token can impersonate users and login as them.

• An actor with access to the access token, the Nextcloud config file, and the Nextcloud database can decrypt user passwords stored in the database.

Limit on password length

Nextcloud uses the bcrypt algorithm, and thus for security and performance reasons, e.g. Denial of Service as CPU demand increases exponentially, it only verifies the first 72 characters of passwords. This applies to all passwords that you use in Nextcloud: user passwords, passwords on link shares, and passwords on external shares.

Operating system

Give PHP read access to /dev/urandom

Nextcloud uses a RFC 4086 (“Randomness Requirements for Security”) compliant mixer to generate cryptographically secure pseudo-random numbers. This means that when generating a random number Nextcloud will request multiple random numbers from different sources and derive from these the final random number.

The random number generation also tries to request random numbers from /dev/urandom, thus it is highly recommended to configure your setup in such a way that PHP is able to read random data from it.

Note

When having an open_basedir configured within your php.ini file, make sure to include /dev/urandom.

Enable hardening modules such as SELinux

It is highly recommended to enable hardening modules such as SELinux where possible. See SELinux configuration to learn more about SELinux.

Deployment

Place data directory outside of the web root

It is highly recommended to place your data directory outside of the Web root (i.e. outside of /var/www). It is easiest to do this on a new installation.

Disable preview image generation

Nextcloud is able to generate preview images of common filetypes such as images or text files. By default the preview generation for some file types that we consider secure enough for deployment is enabled. However, administrators should be aware that these previews are generated using PHP libraries written in C which might be vulnerable to attack vectors.

For high security deployments we recommend disabling the preview generation by setting the enable_previews switch to false in config.php. As an administrator you are also able to manage which preview providers are enabled by modifying the enabledPreviewProviders option switch.

Disable Debug Mode

Verify that debug is false in your config.php. The default is false in new installations (or when not specified). It should not be enabled in production environments or outside of targeted troubleshooting situations. When enabled, things like server-wide WebDAV collection listings are permitted. It is intended for local development and usage in controlled environments only.

Use HTTPS

Using Nextcloud without using an encrypted HTTPS connection opens up your server to a man-in-the-middle (MITM) attack, and risks the interception of user data and passwords. It is a best practice, and highly recommended, to always use HTTPS on production servers, and to never allow unencrypted HTTP.

How to setup HTTPS on your Web server depends on your setup; please consult the documentation for your HTTP server. The following examples are for Apache.

Redirect all unencrypted traffic to HTTPS

To redirect all HTTP traffic to HTTPS administrators are encouraged to issue a permanent redirect using the 301 status code. When using Apache this can be achieved by a setting such as the following in the Apache VirtualHosts configuration:

<VirtualHost *:80>
   ServerName cloud.nextcloud.com
   Redirect permanent / https://cloud.nextcloud.com/
</VirtualHost>

Enable HTTP Strict Transport Security

While redirecting all traffic to HTTPS is good, it may not completely prevent man-in-the-middle attacks. Thus administrators are encouraged to set the HTTP Strict Transport Security header, which instructs browsers to not allow any connection to the Nextcloud instance using HTTP, and it attempts to prevent site visitors from bypassing invalid certificate warnings.

This can be achieved by setting the following settings within the Apache VirtualHost file:

<VirtualHost *:443>
  ServerName cloud.nextcloud.com
    <IfModule mod_headers.c>
      Header always set Strict-Transport-Security "max-age=15552000; includeSubDomains"
    </IfModule>
 </VirtualHost>

Warning

We recommend the additional setting ; preload to be added to that header. Then the domain will be added to a hardcoded list that is shipped with all major browsers and enforce HTTPS upon those domains. See the HSTS preload website for more information. Due to the policy of this list you need to add it to the above example for yourself once you are sure that this is what you want. Removing the domain from this list could take some months until it reaches all installed browsers.

This example configuration will make all subdomains only accessible via HTTPS. If you have subdomains not accessible via HTTPS, remove includeSubDomains.

This requires the mod_headers extension in Apache.

Proper SSL configuration

Default SSL configurations by Web servers are often not state-of-the-art, and require fine-tuning for an optimal performance and security experience. The available SSL ciphers and options depend completely on your environment and thus giving a generic recommendation is not really possible.

We recommend using the Mozilla SSL Configuration Generator to generate a suitable configuration suited for your environment. To verify your configuration you can use the free Web TLS Profiler service. This service gives detailed error messages, if your server’s TLS settings deviate from the Mozilla Configuration. Another useful tool to check your server’s TLS configuration is the free Qualys SSL Labs Test which provides general information about the TLS settings.

Also ensure that HTTP compression is disabled to mitigate the BREACH attack.

Restrict admin actions to a specific range of IP addresses

Configure allowed_admin_ranges in config.php to restrict the admin actions to trusted IP ranges.

This can be achieved with this kind of setting, usually using private IP ranges:

'allowed_admin_ranges' => [
  '127.0.0.1/8',
  '192.168.0.0/16',
  'fd00::/8',
],

All requests originating from IP addresses outside of these ranges will not be able to execute admin actions.

Administrators connected from untrusted IP addresses will be able to use Nextcloud, but all admin specific actions will be hidden.

Use a dedicated domain for Nextcloud

Administrators are encouraged to install Nextcloud on a dedicated domain such as cloud.domain.tld instead of domain.tld to gain all the benefits offered by the Same-Origin-Policy.

Ensure that your Nextcloud instance is installed in a DMZ

As Nextcloud supports features such as Federated File Sharing we do not consider Server Side Request Forgery (SSRF) part of our threat model. In fact, given all our external storage adapters this can be considered a feature and not a vulnerability.

This means that a user on your Nextcloud instance could probe whether other hosts are accessible from the Nextcloud network. If you do not want this you need to ensure that your Nextcloud is properly installed in a segregated network and proper firewall rules are in place.

Serve security related headers by the Web server

Basic security headers are served by Nextcloud already in a default environment. These include:

• X-Content-Type-Options: nosniff

  • Instructs some browsers to not sniff the mimetype of files. This is used for example to prevent browsers from interpreting text files as JavaScript.

• X-XSS-Protection: 1; mode=block

  • Instructs browsers to enable their browser side Cross-Site-Scripting filter.

• X-Robots-Tag: noindex, nofollow

  • Instructs search machines to not index these pages and not follow any links there.

• X-Frame-Options: SAMEORIGIN

  • Prevents embedding of the Nextcloud instance within an iframe from other domains to prevent Clickjacking and other similar attacks.

• Referrer-Policy: no-referrer

  • The default no-referrer policy instructs the browser not to send referrer information along with requests to any origin.

These headers are hard-coded into the Nextcloud server, and need no intervention by the server administrator.

For optimal security, administrators are encouraged to serve these basic HTTP headers by the Web server to enforce them on response. To do this Apache has to be configured to use the .htaccess file and the following Apache modules need to be enabled:

• mod_headers

• mod_env

Administrators can verify whether this security change is active by accessing a static resource served by the Web server and verify that the above mentioned security headers are shipped.

Connections to remote servers

Some functionalities require the Nextcloud server to be able to connect remote systems via https/443. This pragraph also includes the data which is being transmitted to the Nextcloud GmbH. Depending on your server setup, these are the possible connections:

• nextcloud.com, startpage.com, eff.org, edri.org

  • optional (config)

  • for checking the internet connection

• cloud.nextcloud.com

  • used for enterprise license monitoring

  • submitted data: subscription key, user count

• updates.nextcloud.com

  • to check for available Nextcloud server updates

  • submitted data: server version, subscription key, install time, instance id, instance size

• apps.nextcloud.com

  • to check for available apps and their updates

  • submitted data: subscription key

• github.com, objects.githubusercontent.com

  • to download Nextcloud standard apps

• push-notifications.nextcloud.com

  • sending push notifications to mobile clients

  • submitted data: unique device identifier, public key, push token

• pushfeed.nextcloud.com

  • optional

  • checking for updates to be shown in the Nextcloud Announcements app

• lookup.nextcloud.com

  • optional

  • for updating and lookups to the federated sharing addressbook

  • submitted data: pending

• surveyserver.nextcloud.com

  • optional

  • if the admin has agreed to share anonymized server data

  • submitted data: statistical data. see here for the detailed field list

• Any remote Nextcloud server that is connected with federated sharing

Setup fail2ban

Exposing your server to the internet will inevitably lead to the exposure of the services running on the internet-exposed ports to brute force login attempts.

This guide will enable blocking of the originating IP addresses at an operating system level, so the webserver, PHP and the database do not need to handle this unnecessary traffic at all.

Nextcloud prerequisites

Nextcloud logs failed login attempts in nextcloud.log with log level 2, so you need to define a loglevel of 2 or less in config.php.

Make sure your nextcloud.log is writeable by your webserver user, possibly by defining a correct logfilemode in config.php.

Perform a bad login attempt and check whether it does get logged to nextcloud.log.

Note that audit.log (if enabled) currently only logs successful logins and cannot be used.

Fail2ban introduction

Fail2ban is a service that uses iptables to automatically drop connections for a pre-defined amount of time from IPs that continuously failed to authenticate to the configured services.

In order to setup fail2ban, you first need to download and install it on your server. Downloads for several distributions can be found on fail2ban download page. It is often available from most distributions’ package managers (e.g. apt-get).

The standard path for fail2ban’s configuration is /etc/fail2ban.

Setup a filter and a jail for Nextcloud

A filter defines regex rules to identify when users fail to authenticate on Nextcloud’s user interface, WebDAV, or use an untrusted domain to access the server.

Create a file in /etc/fail2ban/filter.d named nextcloud.conf with the following contents:

[Definition]
_groupsre = (?:(?:,?\s*"\w+":(?:"[^"]+"|\w+))*)
failregex = ^\{%(_groupsre)s,?\s*"remoteAddr":"<HOST>"%(_groupsre)s,?\s*"message":"Login failed:
            ^\{%(_groupsre)s,?\s*"remoteAddr":"<HOST>"%(_groupsre)s,?\s*"message":"Two-factor challenge failed:
            ^\{%(_groupsre)s,?\s*"remoteAddr":"<HOST>"%(_groupsre)s,?\s*"message":"Trusted domain error.
datepattern = ,?\s*"time"\s*:\s*"%%Y-%%m-%%d[T ]%%H:%%M:%%S(%%z)?"

The jail file defines how to handle the failed authentication attempts found by the Nextcloud filter.

Create a file in /etc/fail2ban/jail.d named nextcloud.local with the following contents:

[nextcloud]
backend = auto
enabled = true
port = 80,443
protocol = tcp
filter = nextcloud
maxretry = 3
bantime = 86400
findtime = 43200
logpath = /path/to/data/directory/nextcloud.log

Ensure to replace logpath with your installation’s nextcloud.log location. If you are using ports other than 80 and 443 for your Web server you should replace those too. The bantime and findtime are defined in seconds.

Restart the fail2ban service. You can check the status of your Nextcloud jail by running:

fail2ban-client status nextcloud

If you need to unban certain IP addresses (1.2.3.4 in this example), you may do so by issuing:

fail2ban-client unban 1.2.3.4

There may be scenarios where you want to more permantently ban certain IP addresses that repeatedly generate bad login attempts (or other attacks) by using fail2ban’s recidive feature.