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https://origsvn.digium.com/svn/asterisk/branches/1.4 ........ r247508 | lmadsen | 2010-02-18 11:53:44 -0500 (Thu, 18 Feb 2010) | 1 line Add additional link to best practices document per jsmith. ........ git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@247509 65c4cc65-6c06-0410-ace0-fbb531ad65f3
296 lines
12 KiB
Plaintext
296 lines
12 KiB
Plaintext
==================
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| Best Practices |
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==================
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The purpose of this document is to define best practices when working with
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Asterisk in order to minimize possible security breaches and to provide tried
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examples in field deployments. This is a living document and is subject to
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change over time as best practices are defined.
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--------
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Sections
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--------
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* Filtering Data:
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How to protect yourself from redial attacks
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* Proper Device Naming:
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Why to not use numbered extensions for devices
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* Secure Passwords:
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Secure passwords limit your risk to brute force attacks
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* Reducing Pattern Match Typos:
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Using the 'same' prefix, or using Goto()
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----------------
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Additional Links
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----------------
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Additional links that contain useful information about best practices or
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security are listed below.
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* Seven Steps to Better SIP Security:
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http://blogs.digium.com/2009/03/28/sip-security/
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* Asterisk VoIP Security (webinar):
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http://www.asterisk.org/security/webinar/
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==============
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Filtering Data
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==============
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In the Asterisk dialplan, several channel variables contain data potentially
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supplied by outside sources. This could lead to a potential security concern
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where those outside sources may send cleverly crafted strings of data which
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could be utilized, e.g. to place calls to unexpected locations.
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An example of this can be found in the use of pattern matching and the ${EXTEN}
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channel variable. Note that ${EXTEN} is not the only system created channel
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variable, so it is important to be aware of where the data you're using is
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coming from.
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For example, this common dialplan takes 2 or more characters of data, starting
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with a number 0-9, and then accepts any additional information supplied by the
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request.
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[NOTE: We use SIP in this example, but is not limited to SIP only; protocols
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such as Jabber/XMPP or IAX2 are also susceptible to the same sort of
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injection problem.]
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[incoming]
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exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN})
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exten => _X.,n,Dial(SIP/${EXTEN})
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exten => _X.,n,Hangup()
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This dialplan may be utilized to accept calls to extensions, which then dial a
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numbered device name configured in one of the channel configuration files (such
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as sip.conf, iax.conf, etc...) (see the section Proper Device Naming for more
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information on why this approach is flawed).
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The example we've given above looks harmless enough until you take into
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consideration that several channel technologies accept characters that could
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be utilized in a clever attack. For example, instead of just sending a request
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to dial extension 500 (which in our example above would create the string
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SIP/500 and is then used by the Dial() application to place a call), someone
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could potentially send a string like "500&SIP/itsp/14165551212".
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The string "500&SIP/itsp/14165551212" would then be contained within the
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${EXTEN} channel variable, which is then utilized by the Dial() application in
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our example, thereby giving you the dialplan line of:
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exten => _X.,n,Dial(SIP/500&SIP/itsp/14165551212)
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Our example above has now provided someone with a method to place calls out of
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your ITSP in a place where you didn't expect to allow it. There are a couple of
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ways in which you can mitigate this impact: stricter pattern matching, or using
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the FILTER() dialplan function.
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Strict Pattern Matching
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-----------------------
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The simple way to mitigate this problem is with a strict pattern match that does
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not utilize the period (.) or bang (!) characters to match on one-or-more
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characters or zero-or-more characters (respectively). To fine tune our example
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to only accept three digit extensions, we could change our pattern match to
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be:
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exten => _XXX,n,Dial(SIP/${EXTEN})
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In this way, we have minimized our impact because we're not allowing anything
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other than the numbers zero through nine. But in some cases we really do need to
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handle variable pattern matches, such as when dialing international numbers
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or when we want to handle something like a SIP URI. In this case, we'll need to
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utilize the FILTER() dialplan function.
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Using FILTER()
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--------------
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The FILTER() dialplan function is used to filter strings by only allowing
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characters that you have specified. This is a perfect candidate for controlling
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which characters you want to pass to the Dial() application, or any other
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application which will contain dynamic information passed to Asterisk from an
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external source. Lets take a look at how we can use FILTER() to control what
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data we allow.
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Using our previous example to accept any string length of 2 or more characters,
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starting with a number of zero through nine, we can use FILTER() to limit what
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we will accept to just numbers. Our example would then change to something like:
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[incoming]
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exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN})
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exten => _X.,n,Dial(SIP/${FILTER(0-9,${EXTEN})})
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exten => _X.,n,Hangup()
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Note how we've wrapped the ${EXTEN} channel variable with the FILTER() function
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which will then only pass back characters that fit into the numerical range that
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we've defined.
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Alternatively, if we didn't want to utilize the FILTER() function within the
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Dial() application directly, we could save the value to a channel variable,
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which has a side effect of being usable in other locations of your dialplan if
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necessary, and to handle error checking in a separate location.
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[incoming]
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exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN})
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exten => _X.,n,Set(SAFE_EXTEN=${FILTER(0-9,${EXTEN})})
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exten => _X.,n,Dial(SIP/${SAFE_EXTEN})
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exten => _X.,n,Hangup()
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Now we can use the ${SAFE_EXTEN} channel variable anywhere throughout the rest
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of our dialplan, knowing we've already filtered it. We could also perform an
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error check to verify that what we've received in ${EXTEN} also matches the data
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passed back by FILTER(), and to fail the call if things do not match.
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[incoming]
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exten => _X.,1,Verbose(2,Incoming call to extension ${EXTEN})
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exten => _X.,n,Set(SAFE_EXTEN=${FILTER(0-9,${EXTEN})})
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exten => _X.,n,GotoIf($[${EXTEN} != ${SAFE_EXTEN}]?error,1)
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exten => _X.,n,Dial(SIP/${SAFE_EXTEN})
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exten => _X.,n,Hangup()
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exten => error,1,Verbose(2,Values of EXTEN and SAFE_EXTEN did not match.)
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exten => error,n,Verbose(2,EXTEN: "${EXTEN}" -- SAFE_EXTEN: "${SAFE_EXTEN}")
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exten => error,n,Playback(silence/1&invalid)
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exten => error,n,Hangup()
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Another example would be using FILTER() to control the characters we accept when
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we're expecting to get a SIP URI for dialing.
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[incoming]
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exten => _[0-9a-zA-Z].,1,Verbose(2,Incoming call to extension ${EXTEN})
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exten => _[0-9a-zA-Z].,n,Dial(SIP/${FILTER(.@0-9a-zA-Z,${EXTEN})
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exten => _[0-9a-zA-Z].,n,Hangup()
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Of course the FILTER() function doesn't check the formatting of the incoming
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request. There is also the REGEX() dialplan function which can be used to
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determine if the string passed to it matches the regular expression you've
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created, and to take proper action on whether it matches or not. The creation of
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regular expressions is left as an exercise for the reader.
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More information about the FILTER() and REGEX() dialplan functions can be found
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by typing "core show function FILTER" and "core show function REGEX" from your
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Asterisk console.
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====================
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Proper Device Naming
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====================
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In Asterisk, the concept of an extension number being tied to a specific device
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does not exist. Asterisk is aware of devices it can call or receive calls from,
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and how you define in your dialplan how to reach those devices is up to you.
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Because it has become common practice to think of a specific device as having an
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extension number associated with it, it only becomes natural to think about
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naming your devices the same as the extension number you're providing it. But
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by doing this, you're limiting the powerful concept of separating user from
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extensions, and extensions from devices.
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It can also be a security hazard to name your devices with a number, as this can
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open you up to brute force attacks. Many of the current exploits deal with
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device configurations which utilize a number, and even worse, a password that
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matches the devices name. For example, take a look at this poorly created device
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in sip.conf:
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[1000]
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type=friend
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context=international_dialing
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secret=1000
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As implied by the context, we've permitted a device named 1000 with a password
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of 1000 to place calls internationally. If your PBX system is accessible via
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the internet, then your system will be vulnerable to expensive international
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calls. Even if your system is not accessible via the internet, people within
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your organization could get access to dialing rules you'd prefer to reserve only
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for certain people.
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A more secure example for the device would be to use something like the MAC
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address of the device, along with a strong password (see the section Secure
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Passwords). The following example would be more secure:
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[0004f2040001]
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type=friend
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context=international_dialing
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secret=aE3%B8*$jk^G
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Then in your dialplan, you would reference the device via the MAC address of the
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device (or if using the softphone, a MAC address of a network interface on the
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computer).
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Also note that you should NOT use this password, as it will likely be one of the
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first ones added to the dictionary for brute force attacks.
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================
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Secure Passwords
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================
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Secure passwords are necessary in many (if not all) environments, and Asterisk
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is certainly no exception, especially when it comes to expensive long distance
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calls that could potentially cost your company hundreds or thousands of dollars
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on an expensive monthly phone bill, with little to no recourse to fight the
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charges.
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Whenever you are positioned to add a password to your system, whether that is
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for a device configuration, a database connection, or any other secure
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connection, be sure to use a secure password. A good example of a secure
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password would be something like:
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aE3%B8*$jk^G
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Our password also contains 12 characters with a mixture of upper and
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lower case characters, numbers, and symbols. Because these passwords are likely
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to only be entered once, or loaded via a configuration file, there is
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no need to create simple passwords, even in testing. Some of the holes found in
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production systems used for exploitations involve finding the one test extension
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that contains a weak password that was forgotten prior to putting a system into
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production.
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Using a web search you can find several online password generators such as
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http://www.strongpasswordgenerator.com or there are several scripts that can be
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used to generate a strong password.
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============================
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Reducing Pattern Match Typos
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============================
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As of Asterisk 1.6.2, a new method for reducing the number of complex pattern
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matches you need to enter, which can reduce typos in your dialplan, has been
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implemented. Traditionally, a dialplan with a complex pattern match would look
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something like:
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exten => _[3-5]XXX,1,Verbose(Incoming call to ${EXTEN})
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exten => _[3-5]XXX,n,Set(DEVICE=${DB(device/mac_address/${EXTEN})})
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exten => _[3-5]XXX,n,Set(TECHNOLOGY=${DB(device/technology/${EXTEN})})
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exten => _[3-5]XXX,n,GotoIf($[${ISNULL(${TECHNOLOGY})} | ${ISNULL(${DEVICE})}]?error,1)
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exten => _[3-5]XXX,n,Dial(${TECHNOLOGY}/${DEVICE},${GLOBAL(TIMEOUT)})
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exten => _[3-5]XXX,n,Set(vmFlag=${IF($[${DIALSTATUS} = BUSY]?b:u)})
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exten => _[3-5]XXX,n,Voicemail(${EXTEN}@${GLOBAL(VOICEMAIL_CONTEXT)},${vmFlag})
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exten => _[3-5]XXX,n,Hangup()
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exten => error,1,Verbose(2,Unable to lookup technology or device for extension)
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exten => error,n,Playback(silence/1&num-not-in-db)
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exten => error,n,Hangup()
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Of course there exists the possibility for a typo when retyping the pattern
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match _[3-5]XXX which will match on extensions 3000 through 5999. We can
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minimize this error by utilizing the same => prefix on all lines beyond the
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first one. Our same dialplan with using same => would look like the following:
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exten => _[3-5]XXX,1,Verbose(Incoming call to ${EXTEN})
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same => n,Set(DEVICE=${DB(device/mac_address/${EXTEN})})
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same => n,Set(TECHNOLOGY=${DB(device/technology/${EXTEN})})
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same => n,GotoIf($[${ISNULL(${TECHNOLOGY})} | ${ISNULL(${DEVICE})}]?error,1)
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same => n,Dial(${TECHNOLOGY}/${DEVICE},${GLOBAL(TIMEOUT)})
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same => n,Set(vmFlag=${IF($[${DIALSTATUS} = BUSY]?b:u)})
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same => n,Voicemail(${EXTEN}@${GLOBAL(VOICEMAIL_CONTEXT)},${vmFlag})
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same => n,Hangup()
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exten => error,1,Verbose(2,Unable to lookup technology or device for extension)
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same => n,Playback(silence/1&num-not-in-db)
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same => n,Hangup()
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