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Firewalls 101: Why Every Asterisk Server Should Have a Functioning Firewall

Part of our fundamental disagreement with the FreePBX® design can be summed up in one word: FIREWALL or the lack of a functioning firewall in the FreePBX Distro and in the functionally identical Digium product, AsteriskNOW®.1 Most of the other design choices including the controversial, non-GPL compliant Module Signature Checking mechanism are touted as failsafe ways to detect altered systems even though changes in FreePBX MySQL tables and Asterisk config files can be modified easily without triggering alerts. In short, the Band-Aid® approach to module tampering does nothing to address the fundamental problem, prevention of unauthorized intrusions in the first place.

Some would contend that the included Fail2Ban product is specifically designed to prevent unauthorized intrusions by locking out the bad guys after a certain number of failed login attempts. Assuming Fail2Ban were functioning properly, which does not appear to be the case, putting all your eggs in the Fail2Ban basket also ignores several critical shortcomings in Fail2Ban. First, it has been documented that powerful servers such as Amazon EC2 and Twitter botnets give hackers almost unlimited intrusion attempts before Fail2Ban ever gets a time slice sufficient to scan logs for intrusion attempts. Second, Fail2Ban provides no protection against stealthy distributed bruteforcing activity. For example, if a botnet with 770,000 PCs attacked your server and each PC executed only two login attempts, Fail2Ban never gets triggered even assuming your server could handle the load and Fail2Ban got sufficient server resources to actually scan your logs. Finally, Fail2Ban provides no protection against Zero Day vulnerabilities where an intruder basically walks right into your server because of an unidentified vulnerability lurking in the existing code. Unfortunately, these are not hypothetical situations but regular occurrences over the past 10 years of Asterisk and FreePBX development. In a nutshell, that’s why you need a real firewall. It completely blocks all access to your server by unauthorized users all of the time.

Numerous companies have intentionally exposed Asterisk® servers to the public Internet in a continuing effort to identify problems before they affect "real servers." We know of no similar efforts with a platform that includes FreePBX as an integral component of the server. Why? Because the potential for Zero Day Vulnerabilities in a platform of modular design is enormous. One vulnerable component in FreePBX and the entire house of cards collapses because of the blank check server access that a compromised FreePBX asterisk user account gives to an intruder. It’s the fundamental reason that services such as Apache were engineered to run with different user credentials than a root user in the real world. In essence, the current FreePBX design with Asterisk has elevated asterisk user credentials to allow root-like access to almost every server file and function with the exception of SSH access. And SSH access becomes all but unnecessary given the scope of the GUI functionality provided within FreePBX and the escalated privileges it enjoys.

On FreePBX-based Asterisk servers, the absence of any user account separation means Asterisk, Apache, and FreePBX services all operate under the single asterisk user account. If any piece collapses due to a vulnerability, the intruder gets the keys to the castle including read/write access to Asterisk and FreePBX manager credentials and config files as well as broad MySQL access. This, in turn, exposes your VoIP account credentials in addition to facilitating SQL injection into any and all FreePBX database tables. Because FreePBX "hides" numerous settings in over a hundred MySQL tables, the Asterisk DB, and dozens of Asterisk config files, once the asterisk user account access is compromised, many of the major components on your server could be cleverly reconfigured without leaving much of a hint that your server had been compromised. In fact, VoIP account credentials could be extracted and used elsewhere with no traceable footprint back to your server. For all you would know, your provider compromised your credentials rather than the other way around. Just another reminder that keeping a credit card on file for automatic replenishment with VoIP providers is a very bad idea!

Providing the asterisk user with these broad permissions was a (poor) design choice. Why was it done? To make it easy for the developers to alter virtually everything on your Asterisk server using FreePBX’s integrated Module Admin component. Root user permissions are never required to do much of anything other than server platform upgrades once the FreePBX Distro or AsteriskNOW product is installed. That’s exactly the design one would expect to find in a commercial, closed source software platform. But it’s unusual in the open source community to put it charitably. We trust we’ve made the case why a rock-solid firewall with any product that uses FreePBX modules is absolutely essential. FreePBX is a wonderful GUI, but use of the platform without a properly configured, fully functional firewall could be financially catastrophic not to mention the serious damage it could cause to others including the good reputation of Asterisk in the Internet community.

Our objective next week will be to help you implement a functioning Linux-based software firewall on the FreePBX Distro and AsteriskNOW platforms. It’s FREE! Not only will this improve the security of your server, but it will deny the bad guys a platform from which to launch mischievous acts against the rest of us. Unless you’re running Asterisk on a Cloud-based platform, do all of us a favor NOW! Run, don’t walk, to your nearest electronics store (including WalMart and BestBuy) and purchase one of the dozens of inexpensive NAT-based routers. Install it between the Internet and your server TODAY! This is the one we use, but there are plenty from which to choose including our refurbished one.2

Download the new FUD-Free Firewall for FreePBX Distro and AsteriskNOW.

Originally published: Monday, August 3, 2015

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Some Recent Nerd Vittles Articles of Interest…

  1. Technically, IPtables is running on the FreePBX Distro and AsteriskNOW platforms; however, it’s sole function is to act as the shutdown mechanism for Fail2Ban-detected breaches. It does not independently examine packets. There is no functioning iptables config file. From our vantage point, serving as the Fail2Ban traffic cop doesn’t qualify as a functioning firewall since it lacks any of the traditional IPtables rules that manage PREROUTING, INPUT, FORWARD, OUTPUT, and POSTROUTING of packets. []
  2. Where prices are competitive or availability is a factor, we often recommend Amazon because Amazon provides financial support to Nerd Vittles through our referral links. We encourage everyone to shop independently and purchase products from suppliers that best meet your own requirements. []

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