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Sleep Like a Baby: 20 Failsafe Tips to Enhance Asterisk PBX Security

We often tell the tale of the early Asterisk@Home days when almost every server was configured with no firewall, unlimited web access, and a 201 extension with a password of either 201 or 1234. What could possibly go wrong? Remember this Monday morning newspaper headline? "Small business gets $120,000 phone bill after hackers attack VoIP phone." News.com.au ran this story back in 2009: "Criminals hacked into an Internet phone system and used it to make 11,000 international calls in just 46 hours… 115,000 international mobile calls were made… over a six month period."

Much has changed over the past ten years in Asterisk® Land. And, to get everyone in the football mood, today we want to do a little sofa quarterbacking and take a fresh look at security applying some 20-20 hindsight to everything we’ve all learned over the years. Whether you’re running PBX in a Flash or Incredible PBX in your basement or on a virtual machine in the cloud somewhere, security matters and the checklist that follows hopefully will assist everyone in tightening up your systems so that you or your company aren’t the next headline waiting to happen.

PBX in a Flash Security Alert: Run upgrade-programs then upgrade-fixes to secure your server today!

1. Review PIAF Security Alerts Daily. We devote a lot of time to making sure PBX in a Flash and Incredible PBX are secure. But stuff happens! For privacy and security reasons, we don’t push fixes to your server. You have to go get them. If you never see the alerts, our attention to security is for naught. Here are 3 Easy Ways to Keep Informed:

  1. Subscribe to the PBX in a Flash RSS Security Feed
  2. Follow @NerdUno on Twitter
  3. Review the RSS Feed in the PIAF Dashboard with a browser

Every security alert has a link to a solution. Finally, visit the PIAF Forums and click on the What’s New link. It only takes a minute to scan the list for security issues.

2. Hardware-Based Firewall Protection. Unless your PBX is operating on a shared server in the cloud, always run it on a private LAN behind a hardware-based firewall with no Internet port exposure. The one exception would be for those with remote telephone extensions, and we’ll get to that in a minute. The cheapest consumer grade router/firewall provides more security for your server than all of the other security mechanisms combined. Use it!

3. The Linux iptables Firewall. All PBX in a Flash and Incredible PBX servers have the iptables firewall in place. With PBX in a Flash, you have to configure it yourself unless you deploy Travelin’ Man 3. With Incredible PBX, iptables is preconfigured if you opt to install Travelin’ Man 3 as part of the installation process. It doesn’t do much good to have iptables if it’s not functioning. So check it regularly and especially after rebooting your server. On CentOS-based systems, issue the command: iptables -nL. On the Raspberry Pi, type: iptables-save. You should see a list with a lot of permitted IP addresses for preferred providers. If not, restart iptables and then check it again. To restart iptables on CentOS: service iptables restart. On the Raspberry Pi, issue the command: iptables-restore /etc/network/iptables. If you discover that your iptables firewall was not functioning and you’re running PBX in a Flash or Travelin’ Man 3, a security alert has been issued to address the problem. You can get the security fix here.

4. IP Address Filtering. Even with remote phones and dynamic IP addresses, it often is relatively easy to narrow down the range of permissible IP addresses that should have access to your server. With the Linux iptables firewall, you can implement dynamic DNS FQDNs for your remote users. With many hardware-based firewalls, you can’t. But often you can limit remote access to a range of IP addresses. A little protection is still better than none. With a hardware-based firewall, these IP address ranges usually can be changed via web access to your firewall. The minute it takes to make necessary changes is well worth the effort. Just make sure your hardware-based firewall has a long password with upper and lower case letters as well as numbers and non-alphanumeric characters if your firewall supports them.

5. Fail2Ban Access Monitoring. On PBX in a Flash and CentOS-based Incredible PBX servers, fail2ban is activated to limit access attempts to protected resources such as SIP extensions, SSH, and Apache. It is not infallible particularly in this age of megaservers such as Amazon’s S3 service. Because fail2ban reads your logs looking for failed login attempts, it can be defeated with powerful servers attempting thousands of access attempts simultaneously because fail2ban never gets sufficient Linux resources to read logs and block access. It’s better than nothing, but not by much.

6. Deploy WhiteLists for Remote Access. If your server is in the Cloud (meaning it is directly exposed to the Internet) or if you have remote extensions directly connected to your server, your primary line of defense against the bad guys is your iptables firewall. We’ve tried many designs with the objective of letting the good guys in while keeping the bad guys out. The one failsafe solution is IP address WhiteLists. What this means is, if an IP address is listed as safe in iptables, then connections to certain resources from that IP address are permitted. Otherwise, your server remains invisible to the outside world. We have a couple of tools to assist you in setting this up. Travelin’ Man 2 lets authorized users manage their remote IP addresses themselves through a simple browser interface to your server. Travelin’ Man 3 lets a system administrator manage remote IP addresses using both permitted IP addresses and fully-qualified domain names. In the case of remote users with dynamic IP addresses, DynDNS management tools can be deployed on Macs, Windows machines, and Android devices to automatically update FQDNs used in conjunction with Travelin’ Man 3. As noted previously, a security alert has been issued with Travelin’ Man 3. You can get the security fix here.

7. Remote Access with User Agent Knocking. A new approach to remote user access uses a derivative of the original Sunshine Networks port knock utility. With jeffmac’s new design, you define a customized "User Agent" string on your remote phones and then define iptables rules that permit access from SIP devices that attempt server connections using one of these obscure user agent strings. Here’s how to deploy it. To use this approach you’ll need remote phones that permit customization of the user agent string or that have sufficiently obscure, predefined user agent strings that wouldn’t lend themselves to dictionary-style, brute force hacking attempts by the bad guys.

8: Implement VPNs for PBX Systems. There are install scripts for PBX in a Flash to deploy a NeoRouter VPN or a PPTP VPN. Either or both of them can be installed and configured in minutes! VPNs provide an incredibly simple way to interconnect PBX systems worldwide and assure secure communications between these interconnected systems. Encourage remote users to deploy softphones on their Windows and Mac machines, and use secure, VPN access to connect to your server using these softphones.

9. Don’t Use ‘Normal Ports’ for Internet Access. Think of network and PBX security as a shell game. You want to do as many things differently as possible to make it as difficult as possible for the bad guys to figure out what you’ve done. Read that last sentence again. It’s important! With a hardware-based firewall, this is easy. dLink routers call them Virtual Servers. Other routers have similar functionality. Here is a typical entry:

HTTP TCP 22/2319 Allow All Always
This entry redirects a specified port to a different port for Internet access. Don’t do this for SIP and IAX ports, but it works great for HTTP, FTP, and SSH access. WE STRONGLY DISCOURAGE EVER OPENING HTTP ACCESS TO YOUR SERVER FROM THE INTERNET. But you may need SSH access from remote locations. For example, port 22 typically is the default SSH port on Asterisk aggregations, and this port normally can be used on your internal LAN assuming you know and trust your users. For external (aka Internet) SSH access, simply remap TCP port 22 to some obscure port and change it periodically. For example, you might redirect TCP port 22 to port 2319. Once the setting is saved, you access SSH like this from the Internet: ssh -p 2319 root@pbx.mydomain.com. Then (and just as important!) next month, change the port to 4382, then 6109, and so on. Don’t use these numbers obviously! Make up your own.

The key here is that 2 minutes work every month will keep SSH access to your PBX much more secure than letting every Tom, Dick, and Ivan hammer away at port 22 every night while you’re sleeping. As previously mentioned, most of these routers also will let you block access to certain ports during certain hours of the day. If you’re sleeping, there’s really not much need to provide SSH access to your Asterisk server. At the risk of being labeled xenophobic, keep in mind that many of the world’s best crackers reside in countries where daytime happens to be nighttime in the U.S.

10. Really Secure Passwords Really Do Matter. While we have no hard evidence to back this up, our guess is that 90% of the security breaches in Asterisk systems have been the direct result of folks using passwords that matched the extension numbers on their phone systems. Since most Asterisk PBX systems are configured with extension numbers beginning in the 200, 700, or 800 range of numbers, it really wasn’t Rocket Science to remotely log into these servers and make unlimited SIP telephone calls. It may seem obvious but really secure passwords really do matter. And it’s more than having a secure root password. All of your passwords need to be secure including those on your phone extensions and voicemail accounts unless you are absolutely certain that you have blocked all access to your system from everyone except trusted users. If you use DISA, multiply this advice by 10. Part of having really secure passwords is regularly changing them. And our rule of thumb on Asterisk system passwords goes one step further. Never, ever use passwords on your PBX that you use for other important personal information (such as financial accounts). Remember, it’s your phone bill.

11: Minimize Web Access To Your PBX. Most of the Asterisk aggregations utilize FreePBX as the graphical user interface to configure your Asterisk PBX. Because FreePBX is web-based, it is extremely dangerous to leave it exposed on the Internet. As much as we love FreePBX, keep in mind that it was written by dozens and dozens of contributors of various skill levels over a very long period of time. Spaghetti code doesn’t begin to describe some of what lies under the FreePBX covers. While the FreePBX Dev Team is vigorously rewriting much of this old code, some of it still lingers. Our recommendation is to make absolutely certain that you have .htaccess password protection in place for all web directories in at least these directory trees: admin, maint, meetme, and panel.

Our rule of thumb on Internet web accessibility to any Asterisk PBX goes like this. Don’t! And, for FreePBX web access from the Internet. Never! If the bad guys ever get into FreePBX, the security of your PBX has been compromised… permanently! This means you need to start over with all-new passwords and install a fresh system. You can’t fix every possible hole that has been opened on a FreePBX-compromised system!

12. Choosing VoIP Providers. So long as you use reputable VoIP providers that support registration of your SIP and IAX accounts, NO INTERNET PORT EXPOSURE TO YOUR SERVER IS EVER REQUIRED! If a VoIP provider doesn’t support SIP/IAX account registration, don’t use them! Add your public and private IP addresses in FreePBX’s Asterisk SIP Settings module to eliminate one-way audio issues.

13. Never Activate Auto-Replenishment. If you’re using VoIP providers that you pay by the minute, do your wallet a favor. Never, ever activate auto-replenishment on your accounts. By manually controlling the money flow to your accounts, you automatically insulate yourself from a huge phone bill. If something does come unglued, your financial exposure is limited to the preauthorized amount in each of your VoIP provider accounts.

14. Tighten Up International Calling. Almost every VoIP provider gives you the option of restricting international calls. If you don’t make international calls, use it! If you do make international calls, implement Outbound Routes in your FreePBX® dial plan with designated country codes. If you never call Africa, China, or cruise ships in international waters, make sure your dialplan doesn’t allow these calls.

15. Time of Day Calling Restrictions. Whether your server is for business or home use, time of day restrictions can save you a bundle. If remote telephone extensions are a must have for your server, chances are that those extensions don’t place calls in the middle of the night. Almost every hardware-based router/firewall allows creation of time of day rules for access. Implement these restrictions to minimize exposure to those that are hacking while you’re sleeping.

16. Minimize Simultaneous Calls. Especially with pay-as-you-go VoIP providers, often there is no limit to the number of simultaneous calls that can be placed from a trunk on your server. If someone manages to gain access to your accounts or your server, that can be really bad news. Some providers offer tools to restrict the number of simultaneous calls that can be placed. Take advantage of it to limit your financial exposure. Similarly, FreePBX includes a Maximum Channels option when you configure a Trunk. Don’t leave it blank. Set it to what you need to meet your needs.

17. Outbound Route Passwords. For outbound routes to international numbers and 900 numbers, always take advantage of the FreePBX Outbound Route option to prompt for a password. Just enter a numeric Route Password when you configure these outbound routes, and FreePBX will handle the rest.

18. IP Address Filtering with Asterisk Extensions. With the number of Asterisk SIP vulnerabilities reported over the years, suffice it to say IP address filtering at the Asterisk extension level is not something you should rely upon exclusively to protect your server. But it’s better than nothing. And, when used in conjunction with the other security mechanisms we’ve outlined, it provides another layer of security for your server. The extension setup in FreePBX includes the permit field which can be used to limit connections to a particular extension based upon an IP address or range of IP addresses. In addition, Travelin’ Man 2 deploys additional permit tables using an include list in sip_custom_post.conf in conjunction with include files for specified extensions, e.g. 701.inc, to define additional authorized IP addresses.

To restrict an extension to a private LAN address with a FreePBX extension entry in permit like this: Then you can broaden this restricted access with specified WhiteList addresses using an include file in /etc/asterisk that looks like this:


You, of course, would also have to authorize the specified IP address in your iptables configuration as well. That’s essentially how Travelin’ Man 2 works.

19: Check Your Logs Every Day. We’re still dumbfounded by the following quote from the article we cited above: "115,000 international mobile calls were made using the small business’s VoIP system over a six month period." Six months and they never checked their call logs? FreePBX provides an incredibly simple way to review your call logs. Click the CDR Reports link and look at your call log showing the number of calls each day and the combined length of those calls. Nothing could be easier. Do it every single day!

20: Do Some Reading… Regularly. No security implementation is complete without a little regular effort on your part: reading. If you’re going to manage your own network or PBX, then you need to keep abreast of what’s happening in the business. There are any number of ways to do this, none of which take much time. The simplest approach is just to scan the Open Discussion, Add-Ons, and Bug Reporting topics on the PBX in a Flash Forum, the FreePBX Forum, and Asterisk News. Aside from reviewing your call logs, it’s the best 15 minutes you could spend to safeguard your system.

Originally published: Monday, October 1, 2012

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