#asterisk security

[voipfaqs-blog]

Proper Device Naming In Asterisk

In Asterisk, the concept of an extension number being tied to a specific device does not exist. Asterisk is aware of devices it can call or receive calls from, and how you define in your dialplan how to reach those devices is up to you. Because it has become common practice to think of a specific device as having an extension number associated with it, it only becomes natural to think about naming your devices the same as the extension number you're providing it. But by doing this, you're limiting the powerful concept of separating user from extensions, and extensions from devices. It can also be a security hazard to name your devices (for example a VoIP Phone) with a number, as this can open you up to brute force attacks. Many of the current exploits deal with device configurations which utilize a number, and even worse, a password that matches the devices name in your VoIP software configuration. For example, take a look at this poorly created device in sip.conf: [1000] type=friend context=international_dialing secret=1000 As implied by the context, we've permitted a device named 1000 with a password of 1000 to place calls internationally. If your PBX phone system is accessible via the internet, then your system will be vulnerable to expensive international calls. Even if your system is not accessible via the internet, people within your organization could get access to dialing rules you'd prefer to reserve only for certain people. A more secure example for the device would be to use something like the MAC address of the device, along with a strong password (see the section Secure Passwords). The following example would be more secure: [0004f2040001] type=friend context=international_dialing secret=aE3%B8*$jk^G Then in your dialplan, you would reference the device via the MAC address of the device (or if using the softphone, a MAC address of a network interface on the computer). Also note that you should NOT use this password, as it will likely be one of the first ones added to the dictionary for brute force attacks.

[voipfaqs-blog]

Asterisk Filter Application

The FILTER() dialplan function is used to filter strings by only allowing characters that you have specified. This is a perfect candidate for controlling which characters you want to pass to the Dial() application, or any other application which will contain dynamic information passed to Asterisk from an external source. Lets take a look at how we can use FILTER() to control what data we allow. Using our previous example to accept any string length of 2 or more characters, starting with a number of zero through nine, we can use FILTER() to limit what we will accept to just numbers. Our example would then change to something like: [incoming] exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN}) exten => _X.,n,Dial(SIP/${FILTER(0-9,${EXTEN})}) exten => _X.,n,Hangup() Note how we've wrapped the ${EXTEN} channel variable with the FILTER() function which will then only pass back characters that fit into the numerical range that we've defined. Alternatively, if we didn't want to utilize the FILTER() function within the Dial() application directly, we could save the value to a channel variable, which has a side effect of being usable in other locations of your dialplan if necessary, and to handle error checking in a separate location. [incoming] exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN}) exten => _X.,n,Set(SAFE_EXTEN=${FILTER(0-9,${EXTEN})}) exten => _X.,n,Dial(SIP/${SAFE_EXTEN}) exten => _X.,n,Hangup() Now we can use the ${SAFE_EXTEN} channel variable anywhere throughout the rest of our dialplan, knowing we've already filtered it. We could also perform an error check to verify that what we've received in ${EXTEN} also matches the data passed back by FILTER(), and to fail the call if things do not match. [incoming] exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN}) exten => _X.,n,Set(SAFE_EXTEN=${FILTER(0-9,${EXTEN})}) exten => _X.,n,GotoIf($[${EXTEN} != ${SAFE_EXTEN}]?error,1) exten => _X.,n,Dial(SIP/${SAFE_EXTEN}) exten => _X.,n,Hangup() exten => error,1,Verbose(2,Values of EXTEN and SAFE_EXTEN did not match.) exten => error,n,Verbose(2,EXTEN: "${EXTEN}" -- SAFE_EXTEN: "${SAFE_EXTEN}") exten => error,n,Playback(silence/1&invalid) exten => error,n,Hangup() Another example would be using FILTER() to control the characters we accept when we're expecting to get a SIP URI for dialing. [incoming] exten => _[0-9a-zA-Z].,1,Verbose(2,Incoming call to extension ${EXTEN}) exten => _[0-9a-zA-Z].,n,Dial(SIP/${FILTER(.@0-9a-zA-Z,${EXTEN}) exten => _[0-9a-zA-Z].,n,Hangup() Of course the FILTER() function doesn't check the formatting of the incoming request. There is also the REGEX() dialplan function which can be used to determine if the string passed to it matches the regular expression you've created, and to take proper action on whether it matches or not. The creation of regular expressions is left as an exercise for the reader. More information about the FILTER() and REGEX() dialplan functions can be found by typing "core show function FILTER" and "core show function REGEX" from your Asterisk console.

[voipfaqs-blog]

Filtering Data In Asterisk DialPlan

In the Asterisk dialplan, several channel variables contain data potentially supplied by outside sources. This could lead to a potential security concern where those outside sources may send cleverly crafted strings of data which could be utilized, e.g. to place calls to unexpected locations. An example of this can be found in the use of pattern matching and the ${EXTEN} channel variable. Note that ${EXTEN} is not the only system created channel variable, so it is important to be aware of where the data you're using is coming from. For example, this common dialplan takes 2 or more characters of data, starting with a number 0-9, and then accepts any additional information supplied by the request. [NOTE: We use SIP in this example, but is not limited to SIP only; protocols such as Jabber/XMPP or IAX2 are also susceptible to the same sort of injection problem.] [incoming] exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN}) exten => _X.,n,Dial(SIP/${EXTEN}) exten => _X.,n,Hangup() This dialplan may be utilized to accept calls to extensions, which then dial a numbered device name configured in one of the channel configuration files (such as sip.conf, iax.conf, etc...) (see the section Proper Device Naming for more information on why this approach is flawed). The example we've given above looks harmless enough until you take into consideration that several channel technologies accept characters that could be utilized in a clever attack. For example, instead of just sending a request to dial extension 500 (which in our example above would create the string SIP/500 and is then used by the Dial() application to place a call), someone could potentially send a string like "500&SIP/itsp/14165551212". The string "500&SIP/itsp/14165551212" would then be contained within the ${EXTEN} channel variable, which is then utilized by the Dial() application in our example, thereby giving you the dialplan line of: exten => _X.,n,Dial(SIP/500&SIP/itsp/14165551212)

Our example above has now provided someone with a method to place calls out of your ITSP in a place where you didn't expect to allow it. There are a couple of ways in which you can mitigate this impact:

using strict pattern matching, or

using the FILTER() dialplan function.

[voipfaqs-blog]

Asterisk Security Best Practices

The purpose of this document is to define best practices when working with Asterisk in order to minimize possible security breaches and to provide tried examples in field deployments. This is a living document and is subject to change over time as best practices are defined.

Filtering Data: How to protect yourself from redial attacks

Proper Device Naming: Why to not use numbered extensions for devices

Secure Passwords: Secure passwords limit your risk to brute force attacks

Reducing Pattern Match Typos: Using the 'same' prefix, or using Goto()