Asterisk Security

If you are looking to secure your PBX you have several options which can be implemented independently or in combination:

• PBX Configuration – adjust the settings of your PBX to minimize obvious attack surfaces (no longer considered optional – just part of setting up any PBX).
• Perimeter Security – Add software/hardware around your PBX to improve security (one notch above configuration – just part of operating any server).
• Integrated Security – add software which integrates with your specific PBX to improve security (this is what really makes a difference in protecting your PBX).

Note that some recommendations (eg: changing ports, port knocking, etc.) are ideal for small and home office installations, whereas these same recommendations are impractical for large-scale implementations. As well, some recommendations are a great starting point (eg: hardware firewall) but this is no longer sufficient to protect a PBX.

Integrated Security

SecAst

SecAst is an intrusion detection and prevention system designed specifically to protect Asterisk phone systems against intrusion and fraud. SecAst uses a variety of techniques to detect intrusion attempts, halt ongoing attacks, and prevent future attacks. SecAst is available in three editions, including a free edition. SecAst can be downloaded from www.telium.ca or checkout the wiki page ((SecAst (Asterisk Intrusion Detection and Prevention) ))

Fail2Ban

Fail2Ban is a free utility which looks at log files for records of failures (to register, etc.) and then add their source IP to iptables. See security warning regarding fail2ban http://forums.asterisk.org/viewtopic.php?p=159984 Fail2ban is not an intrusion detection/prevention tool, it depends completely on Asterisk to detect and reject an attempt from a hacker.

Perimeter Security

If you are looking to add layers around your PBX with generic protection:

Hardware Firewall

Most Asterisk boxes should be located behind a hardware firewall. Configure the firewall to block traffic from anyone that doesn’t need to connect to you. Allow your VoIP provider, any remote phones/users, and others that may need to connect, but keep the restrictions as tight as possible. If you do have remote users, lock your firewall down to only allow those users to connect if possible, rather than opening it to the entire internet. If you have mobile users this may not be an option, however.

Other services, such as SSH should be blocked by the hardware firewall.

IPtables

IPTables is the generic firewall included with Linux. It is not PBX aware (or intelligent in any way). IPTables is a great add-on to a larger security solution. However, simply adding static rules for every potential source is not a solution. Consider IPTables the basics of preventing someone from getting at your PBX – but it doesn’t / can’t know the actions or intent of someone connecting to your PBX.

Port Knock

Port Knock can be used to provide remote access to your Asterisk machine, opening ports needed to access asterisk services only for your IP, using a predefined sequence of ports to request a temporary on-demand opening of specific ports needed for your remote/dynamic IP. This type of solution is useful for small installations with technically-savvy users (since they have to port knock before every connection) but can effective.

PBX Configuration

If you wish to improve the security of your PBX through basic security configuration

Strong Passwords

All remote users should have strong, alpha-numeric passwords. These should be long. They should NEVER be the same as the username or based on the user’s extension.

Default Context

Your [default] context in extensions.conf should be empty. This context is used when other contexts might not match a peer. You should explicitly refer to a non-default context for remote SIP calls.

sip.conf configuration

In the [general] section, define:

[general]
...
context=bogus
allowguest=no
alwaysauthreject=yes
...

The context line will refer to a context (which you *must define* in extensions.conf!) that should handle “default” SIP calls. This should go to a context with nothing in it (unless you want to play with them – then, if you have the bandwidth, feel free to play a recording or such!). Obviously, if you want to accept anonymous SIP calls, send them to the proper context (but *never* a context that can dial out!).

The “allowguest” line disables anonymous SIP calls to your PBX. Some SIP providers connect as a guest user, however, so this may be inappropriate for your situation. Also, if you want to accept anonymous SIP calls, this line would block them, so you wouldn’t want that. But it is listed here because it is the safest configuration.

The “alwaysauthreject” line is important. This causes a hacker to get the same response from your PBX when they try to guess passwords whether or not they guessed a valid username. This also has the side-effect of making poorly written scanning scripts (the vast majority of hacker scripts seem to be poorly written) take fewer resources on your Asterisk box, as even if they scan a valid username, they’ll think it doesn’t exist.

In addition to these, verify that all peers listed in sip.conf are valid and have strong passwords.

iax.conf

(I’m hoping others will fill in this section – I block IAX)

Dialplan

Do your users need to be able to dial internationally? If not, make sure your dialplan blocks international calls (in the US, these calls start with a 011 typically, although some countries “look like” US numbers – so also block calls to area codes that don’t correspond to areas you call). You may be able to request your provider also blocks international calls. If you only have SOME users that need to call internationally, place them in a different context than the rest of your users.

Logs, CDR

Review your logs and CDR at least daily. Even one day of illegitimate calls can add up to tons of money quickly.

References

Security in a complex piece of software like Asterisk is not a simple thing. Help us collect information on the subject:

• Asterisk Security White Paper: A white paper written by Zone24x7 Inc about how to configure Asterisk to be secure
• Astricon Europe Powerpoint presentation about asterisk security and stability
• SIP security: What security functions are implemented for SIP in asterisk
• coding security: Any thoughts on secure coding, buffer overflows etc?
• Firewalls and Asterisk: What ports are involved and how do I set up a firewall to protect Asterisk?
• Linux security and Asterisk: Any special considerations when installing your Linux platform?
• IAX security
• MGCP security
• ISDN – pri and bri – security
• Ranch Networks Configuration for MIDCOM with IP-PBX Asterisk
• Dialplan security – What to consider when setting up a dialplan in extensions.conf
• Asterisk security through geographic IP address restriction
• NetSec version of Asterisk v1.2.2: This release of Asterisk contains support for network security devices manufactured by Ranch Networks, Inc., using their MIDCOM interface library. You will need the companion libmidcom-0.1.0.tar.gz file to build the library. Contact Ranch Networks’ support department for assistance in building and configuring MIDCOM support.

Articles

• Good paper that also includes short section on Midcom: NIST: Security Considerations for Voice Over IP Systems
• Nerd Vittles – Avoiding the $100,000 Phone Bill: A Primer on Asterisk Security
• Smartvox article – Tips for better security in your Asterisk server (and FreePBX) config.

See also

• ((SecAst (Asterisk Intrusion Detection and Prevention) ))
• Asterisk administration
• Asterisk encryption