OpenWrt security hardening

Good news, OpenWrt has reasonable security by default.

If you are inexperienced in hardening and firewall and web security, there is no need to worry, OpenWrt is hardened by default in a sufficient way, such that non-experienced muggles can use it right away, without being worried.

...with one important single exception:

You need to set a password on your OpenWrt root account. The root account is the default OpenWrt admin account on your device. The next chapter will show you how to do this.

This page also contains some general information about security of OpenWrt and what you should do in general, to keep your router in a properly secured state.

Set the root password using web interface.

1. Navigate to LuCI → System → Administration → Router Password.
2. Enter the new password in the Router Password section.
3. Click Save & Apply a the bottom of the page.

You can also set the root password using command-line interface.

passwd

Enable password prompt for TTY and serial console.

uci set system.@system[0].ttylogin="1"
uci commit system
service system restart

Authentication for OpenWrt TTY and serial console is disabled by default. Using TTY and serial console requires physical access to the device. You can reduce the attack surface by enabling authentication.

Note that hardware attacks on serial console pins are also possible. However, it requires physical access, time and skills.

Single user mode is available through GRUB and allows to boot without password. An attacker is then able to change root password and reboot. A solution would be to lock-down OpenWrt booloader process, to make sure that booting in Linux single user mode is impossible. This has to be discussed and this is not yet documented.

If you have 8MB flash or more and share your home network with other people, it is good practice to activate HTTPS for your LuCI web interface.

If you don't ever use your LuCI web interface at all, you can disable the uHTTPd webserver.

...and that is a very bad idea.

Treat your root account with some sane respect.

Do what every major company does with the root accounts of their Linux servers:

• Stay away from admin access (SSH and web interface), when you don't need it
• Close/Log off your root admin sessions once your are done administrating (not 8h later)
• Only connect as root, when really in the need for administration
• Don't share your root password with others
• Don't share your root password with others, even if they promise some hot skateboarding penguins pictures in return

Congratulations that you do not have to share precious bandwidth with others, but you still need to set a root password.

Any web site you call from a browser in your home network (e.g. those that promise hot skateboarding penguins pictures) could easily use so called cross-site request forgery to access web interface of your OpenWrt device, without you noticing it and then do evil things there.

If no root password is set, such malicious sites could manipulate your OpenWrt device in a way that you won't like. So just go and set a password on your root account now.

Handle firewall rules with care:

• Do not expose services on the WAN Internet port, if you do not understand the security implications. Automatic scanners of evil fources and script kids will find any open port on your WAN side sometimes within minutes and will then run extensive intrusion software suits on such open ports, probing a lot of attack vectors without any manual effort. The Internet is permanently being scanned for careless people.
• if you want to access home services while being on the road, consider using a WireGuard VPN instead of opening service-related ports publically on the WAN side.
• Unfortunately a lot of online games have lots of “recommended settings” to permanently open various port ranges for best gaming experience. Before blindly following these practices, check first, if any server connection problems are due to a double NAT situation of cascaded routers at your home.
• Always use reasonable comments, when you add your own customized firewall rules (e.g. “...that's the rule that a random nice guy on the Internet asked me to add, promising me some really hot skateboarding penguins pictures in return...”)

If you have already performed various firewall changes on your OpenWrt device and now lost overview of your custom rules, you can always reset all your OpenWrt settings back to the to the initial default (see trouble shooting section).

Not so fast...

Did you notice that even OpenWrt firmware gets updated from time to time?

As with your former vendor firmware, you should check regularly, whether OpenWrt has released new firmware and apply these updates to your device. There is even a configuration backup and restore feature, such that you do not have to start from scratch after each update.

As with the firmware you should also keep an eye on the custom packages you install. There are several hundreds of optional packages. Not all security problems of those packages get addressed by OpenWrt system upgrades, but instead require you to manually upgrade the packages as well.

If you are using custom packages, you should run a opkg update; opkg list-upgradable from time to time. This shows your installed packages that have available updates. You then install package upgrades manually by running opkg upgrade <package>. Note that not every listed package upgrade is due to security issues, it can also be a harmless bug fix or feature extension.

An update will continue to use your existing service configuration, but for critical OpenWrt environments, a manual config backup never hurts as safety precaution before upgrading packages...

Note: OpenWrt uses a read-only root file system plus a differential extension partition for all package installs and upgrades. When wanting to maximize usage of your precious flash space, it tends to be a better approach, to applying up-to-date OpenWrt firmware and then reinstall your packages instead of only upgrading packages, when expecting larger volumes of upgrades.

OpenWrt devices have 2-4 common services running, which kind of mark high-value targets for malware (even when only available in your LAN-zone): Any harmless looking web site, you have visited in your browser, could use cross site request forgery tricks, abusing an unpatched security flaw in one of these services. This could lead to malicious malware redirect attacks where website redirects to a malware site and so on.

These high-value services in particular are:

• The webserver running LuCI (based on Lua) for OpenWrt web interface access
• The dropbear SSH server for OpenWrt command-line admin access
• The SFTP deamon for GUI file explorer admin access (only if manually activated, it's not there by default)
• Samba SMB share to provide user network file shares (only if manually activated, it's not there by default)

It is up to your personal responsibility, to counter such weak points on your OpenWrt device(s):

• Set a root password
• Keep your OpenWrt firmware up to date
• When you have Samba and/or SFTP activated manually: check regularly, if there are package upgrade available for Samba and SFTP and apply those upgrades