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Taking a Fresh Look at the Asterisk, FreePBX, and Incredible PBX Security Models

About once a year, we try to shine the spotlight on Asterisk® security in hopes of saving lots of organizations and individuals a little bit (or a lot) of money. In light of last week’s major security lapse in the Asterisk® dialplan of those using FreePBX® since the Asterisk@Home days, now seemed like a good time for a review. As we’ve noted before, the problem with open source phone systems is they’re open source phone systems. So the bad guys can figure out how they work just like the good guys. Unfortunately, some of the bad guys are paying particular attention to Asterisk and FreePBX so it behooves all of us to remain vigilant and patch vulnerabilities quickly. The FreePBX Devs have done an admirable job in responding quickly to this issue.

Last week’s vulnerability involves the call transfer methodology that has been incorporated into FreePBX-based Asterisk servers for at least a decade. In a nutshell, it allows an internal or outside caller or called party to transfer a call using touchtones instead of a dedicated transfer button or hook flash. ## performs a blind transfer while *2 sets up an attended transfer where the person transferring the call can actually talk to the transfer recipient before executing the call transfer. Some of our foreign friends used this *2 methodology to initiate calls to Asterisk servers and then to transfer those calls to expensive destinations while the other party to the call listened to music on hold. Worse yet, it could be performed within an answering IVR on some servers so the administrator never knew the call transfer took place other than reviewing the call detail records. As with some previous vulnerabilities, this one had lain dormant since the inception of call transfer technology in Asterisk. The default settings in FreePBX permitted outside calling or called parties to initiate transfers using these feature codes. We’re reminded of a similar vulnerability that used to exist in many Asterisk voicemail systems that allowed callers to dialout to another number from within the voicemail system.

We hope to persuade you today that allowing transfer of calls using touch tones is a very bad idea to begin with. Even when you don’t get a surprise phone bill, it often results in unanticipated consequences such as depicted in this video shared on DSL Reports:


Here’s how you can protect any server that uses all or some of the FreePBX GUI. First, be aware that the FreePBX developers are working on a rewrite of the Core component in versions 13 and 12. The fix would limit use of this technology to those on the internal side of a PBX. In other words, remote callers would be blocked from calling into an Asterisk server and transferring themselves to a phone on a cruise ship sailing in the Indian Ocean. In the meantime, issuing the following commands will patch things up:

mysql -uroot -ppassw0rd asterisk -e "update freepbx_settings set value = 'tr' where keyword = 'DIAL_OPTIONS' limit 1"
mysql -uroot -ppassw0rd asterisk -e "update freepbx_settings set value = '' where keyword = 'TRUNK_OPTIONS' limit 1"
amportal a r

For those using Incredible PBX™, the Automatic Update Utility will patch your server the next time you log in as root.

Olle Johansson has been one of the primary shakers and movers when it comes to educating folks on Asterisk security and inspiring developers to do a better job designing these systems. If you didn’t attend AstriCon 2013 and haven’t watched the Security Master Class, put these videos on your Bucket List. They’re all free and well worth your time.

When we began building out Incredible PBX on other platforms several years ago, we decided it was an opportune time to revisit our Asterisk security model and make it as bullet-proof as possible given the number of people now deploying Asterisk servers in the cloud. As a practical matter, there are no hardware-based firewalls to protect you with many of the cloud-based systems. So you literally live or die based upon the strength of your own software-based security model.

As in the past, security is all about layers of protection. A bundle of sticks is harder to break than a single stick. There now are Incredible PBX builds for CentOS, Scientific Linux, Ubuntu 14, and the latest Raspbian 8 for the Raspberry Pi 2 and 3. All of these releases include the new Incredible PBX security model. Here’s how it works…

The 7 Security Layers include the following, and we will go into the details below:

  1. Preconfigured IPtables Linux Firewall
  2. Preconfigured Travelin’ Man 3 WhiteLists
  3. Randomized Port Knocker for Remote Access
  4. TM4 WhiteListing by Telephone (optional)
  5. Fail2Ban
  6. Randomized Ultra-Secure Passwords
  7. Automatic Security Updates & Bug Fixes

1. IPtables Linux Firewall. Yes, we’ve had IPtables in place with PBX in a Flash for many years. And, yes, it was partially locked down in previous Incredible PBX releases if you chose to deploy Travelin’ Man 3. Now it’s automatically installed AND locked down, period. As installed, the new Incredible PBX limits login access to your server to those on your private LAN (if any) and anyone logging in from the server’s public or private IP address and the public IP address of the desktop machine used to install the Incredible PBX software. If you or your users need access from other computers or phones, those addresses can be added quickly using either the Travelin’ Man 3 tools (add-ip and add-fqdn) or using the Port Knocker application running on your desktop or smartphone. All you need is your randomized 3 codes for the knock. You can also enable a remote IP address by telephone. Keep reading!

2. Travelin’ Man 3 WhiteLists. As in the past, many of the major SIP providers have been whitelisted in the default setup so that you can quickly add new service without worrying about firewall access. These are providers that we’ve used over the years. The preconfigured providers include Vitelity (outbound1.vitelity.net and inbound1.vitelity.net), Google Voice (talk.google.com), VoIP.ms (city.voip.ms), DIDforsale (, CallCentric (callcentric.com), and also VoIPStreet.com (chi-out.voipstreet.com plus chi-in.voipstreet.com), Les.net (did.voip.les.net), Future-Nine, AxVoice (magnum.axvoice.com), SIP2SIP (proxy.sipthor.net), VoIPMyWay (sip.voipwelcome.com), Obivoice/Vestalink (sms.intelafone.com), Teliax, and IPkall. You are, of course, free to add other providers or users using the whitelist tools being provided. add-ip lets you add an IP address to your whitelist. add-fqdn lets you add a fully-qualified domain name to your whitelist. del-acct lets you remove an entry from your whitelist. Because FQDNs cause problems with IPtables if the FQDN happens to be invalid or non-functional, we’ve provided a customized iptables-restart tool which will filter out bad FQDNs and start up IPtables without the problematic entries.

Be advised that whitelist entries created with PortKnocker are stored in RAM, not in your IPtables file. These RAM entries will get blown out of the water whenever your system is restarted OR if IPtables is restarted. Stated another way, PortKnocker should be used as a stopgap tool to get new IP addresses qualified quickly. If these addresses need access for more than a few hours, then the Travelin’ Man 3 tools should be used to add them to your IPtables whitelist. If your whitelist setup includes dynamic IP addresses, be aware that using ipchecker in a cron job to test for changing dynamic IP addresses will remove PortKnocker whitelist RAM entries whenever an IP address change triggers an iptables-restart.

For more detail on Travelin’ Man 3, review our original tutorial.

3. PortKnocker WhiteListing. We’ve previously written about PortKnocker so we won’t repeat the article here. Simply stated, it lets you knock on three ports on a host machine in the proper order to gain access. If you get the timing and sequence right, the IP address from which you knocked gets whitelisted for access to the server… with appropriate admin or root passwords, of course. The knocking can be accomplished with either a command line tool or an iOS or Android app using your smartphone or tablet. As noted above, it’s a terrific stopgap tool to let you or your users gain quick access to your server. For the reasons we’ve documented, don’t forget that it’s a stopgap tool. Don’t use it as a replacement for Travelin’ Man 3 whitelists unless you don’t plan to deploy dynamic IP address automatic updating. Just to repeat, PortKnocker whitelists get destroyed whenever IPtables is restarted or your server is rebooted. You’ve been warned.

4. TM4 WhiteListing by Telephone. Newer releases of Incredible PBX are preconfigured with ODBC support for telephony applications. One worth mentioning is our new Travelin’ Man 4 utility which lets a remote user dial into a dedicated DID and register an IP address to be whitelisted on the server. Within a couple minutes, the user will be sent an email confirming that the IP address has been whitelisted and remote access is now enabled. For phone systems and administrators supporting hundreds of remote users, this new feature will be a welcome addition. It can be configured in a couple minutes by following the Installation instructions in the Travelin’ Man 4 tutorial. Unlike PortKnocker, whitelisted IP addresses added with TM4 are permanent until modified by the remote user or deleted by the administrator.

5. Fail2Ban. We’ve never been a big fan of Fail2Ban which scans your logs and blacklists IP addresses after several failed attempts to log in or register with SSH or Apache or Asterisk. The reason is because of documented cases where attacks from powerful servers (think: Amazon) completely overpower a machine and delay execution of Fail2Ban log scanning until tens of thousands of registration attempts have been launched. The FreePBX folks are working on a methodology to move failed login attempts to a separate (smaller) log which would go a long way toward eliminating the log scanning bottleneck. In the the meantime, Fail2Ban is included, and it works when it works. But don’t count on it as your only security layer.

6. Randomized Passwords. With the new security model described above, we’ve dispensed with Apache security to protect FreePBX® access. These new Incredible PBX releases rely upon the FreePBX security model which uses encrypted passwords stored in MySQL or MariaDB. As part of the installation process, Incredible PBX randomizes ALL FreePBX passwords including those for the default 701 extension as well as the admin password. When your new Incredible PBX install completes, the most important things to remember are your (randomized) FreePBX admin password AND the (randomized) 3 ports required for Port Knocker access. Put them in a safe place. Sooner or later, you’ll need them. You can review your PortKnocker settings in /root/knock.FAQ. We’ve also included admin-pw-change in the /root folder for those that are too lazy to heed our advice. With the new security model, there is no way to look up your admin password. All you can do is change it… assuming you haven’t also forgotten your root password. 😉

7. Automatic Update Service. All new Incredible PBX builds include an automatic update service to provide security patches and bug fixes whenever you log into your server as root. It saved you just last week! If you don’t want the updates for some reason, you can delete the /root/update* file from your server. If the cost of maintaining this service becomes prohibitive, we may implement a pay-for-service fee, but it presently is supported by voluntary contributions from our users. It has worked extremely well and provided a vehicle for pushing out updates that affect the reliability and security of your server.

A Word About IPv6. Sooner or later Internet Protocol version 6 will be upon us because of the exhaustion of IPv4 IP addresses. Incredible PBX is IPv6-aware and IPtables has been configured to support it as well. As deployed, outbound IPv6 is not restricted. Inbound access is limited to localhost. You, of course, are free to modify it in any way desired. Be advised that disabling IPv6 localhost inbound access will block access to the FreePBX GUI. Don’t ask us how we know. 🙂

Originally published: Monday, April 18, 2016

Need help with Asterisk? Visit the PBX in a Flash Forum.


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