From ef25628b10bb4d60b3e1a5e1a41745ed50b590ec Mon Sep 17 00:00:00 2001 From: Corey Farrell Date: Fri, 1 Dec 2017 17:18:31 -0500 Subject: README-SERIOUSLY.bestpractices.txt: Convert to markdown Follow-up to conversion of README.md. Change-Id: I17ee7cf25bc027ece844efa2c1dfe613aff1e35b --- README-SERIOUSLY.bestpractices.md | 388 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 388 insertions(+) create mode 100644 README-SERIOUSLY.bestpractices.md (limited to 'README-SERIOUSLY.bestpractices.md') diff --git a/README-SERIOUSLY.bestpractices.md b/README-SERIOUSLY.bestpractices.md new file mode 100644 index 000000000..7e18c4aa8 --- /dev/null +++ b/README-SERIOUSLY.bestpractices.md @@ -0,0 +1,388 @@ +# 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() +* [Manager Class Authorizations]: + Recognizing potential issues with certain classes of authorization +* [Avoid Privilege Escalations]: + Disable the ability to execute functions that may escalate privileges + + +## Additional Links + +Additional links that contain useful information about best practices or +security are listed below. + +* [Seven Steps to Better SIP Security][blog-sip-security] +* [Asterisk VoIP Security (webinar)][voip-security-webinar] + + +## Filtering Data + +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. + +```INI +[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 [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 +${EXTEN} channel variable, which is then utilized by the Dial() application in +our example, thereby giving you the dialplan line of: + +```INI +exten => _X.,n,Dial(SIP/500&SIP/itsp/14165551212) +``` + +Our example above has now provided someone with a method to place calls out of +your ITSP in a place where you didn't expect to allow it. There are a couple of +ways in which you can mitigate this impact: stricter pattern matching, or using +the FILTER() dialplan function. + +The CALLERID(num) and CALLERID(name) values are other commonly used values that +are sources of data potentially supplied by outside sources. If you use these +values as parameters to the System(), MixMonitor(), or Monitor() applications +or the SHELL() dialplan function, you can allow injection of arbitrary operating +system command execution. The FILTER() dialplan function is available to remove +dangerous characters from untrusted strings to block the command injection. + + +### Strict Pattern Matching + +The simple way to mitigate this problem is with a strict pattern match that does +not utilize the period (.) or bang (!) characters to match on one-or-more +characters or zero-or-more characters (respectively). To fine tune our example +to only accept three digit extensions, we could change our pattern match to +be: + +```INI +exten => _XXX,n,Dial(SIP/${EXTEN}) +``` + +In this way, we have minimized our impact because we're not allowing anything +other than the numbers zero through nine. But in some cases we really do need to +handle variable pattern matches, such as when dialing international numbers +or when we want to handle something like a SIP URI. In this case, we'll need to +utilize the FILTER() dialplan function. + + +### Using FILTER() + +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: + +```INI +[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. + +```INI +[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. + +```INI +[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. + +```INI +[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. + + +## Proper Device Naming + +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 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. For example, take a look at this poorly created device +in sip.conf: + +```INI +[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 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: + +```INI +[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. + + +## Secure Passwords + +Secure passwords are necessary in many (if not all) environments, and Asterisk +is certainly no exception, especially when it comes to expensive long distance +calls that could potentially cost your company hundreds or thousands of dollars +on an expensive monthly phone bill, with little to no recourse to fight the +charges. + +Whenever you are positioned to add a password to your system, whether that is +for a device configuration, a database connection, or any other secure +connection, be sure to use a secure password. A good example of a secure +password would be something like: + +``` +aE3%B8*$jk^G +``` + +Our password also contains 12 characters with a mixture of upper and +lower case characters, numbers, and symbols. Because these passwords are likely +to only be entered once, or loaded via a configuration file, there is +no need to create simple passwords, even in testing. Some of the holes found in +production systems used for exploitations involve finding the one test extension +that contains a weak password that was forgotten prior to putting a system into +production. + +Using a web search you can find several online password generators such as +[Strong Password Generator] or there are several scripts that can be +used to generate a strong password. + + +## Reducing Pattern Match Typos + +As of Asterisk 1.6.2, a new method for reducing the number of complex pattern +matches you need to enter, which can reduce typos in your dialplan, has been +implemented. Traditionally, a dialplan with a complex pattern match would look +something like: + +```INI +exten => _[3-5]XXX,1,Verbose(Incoming call to ${EXTEN}) +exten => _[3-5]XXX,n,Set(DEVICE=${DB(device/mac_address/${EXTEN})}) +exten => _[3-5]XXX,n,Set(TECHNOLOGY=${DB(device/technology/${EXTEN})}) +exten => _[3-5]XXX,n,GotoIf($[${ISNULL(${TECHNOLOGY})} | ${ISNULL(${DEVICE})}]?error,1) +exten => _[3-5]XXX,n,Dial(${TECHNOLOGY}/${DEVICE},${GLOBAL(TIMEOUT)}) +exten => _[3-5]XXX,n,Set(vmFlag=${IF($[${DIALSTATUS} = BUSY]?b:u)}) +exten => _[3-5]XXX,n,Voicemail(${EXTEN}@${GLOBAL(VOICEMAIL_CONTEXT)},${vmFlag}) +exten => _[3-5]XXX,n,Hangup() + +exten => error,1,Verbose(2,Unable to lookup technology or device for extension) +exten => error,n,Playback(silence/1&num-not-in-db) +exten => error,n,Hangup() +``` + +Of course there exists the possibility for a typo when retyping the pattern +match _\[3-5\]XXX which will match on extensions 3000 through 5999. We can +minimize this error by utilizing the same => prefix on all lines beyond the +first one. Our same dialplan with using same => would look like the following: + +```INI +exten => _[3-5]XXX,1,Verbose(Incoming call to ${EXTEN}) +same => n,Set(DEVICE=${DB(device/mac_address/${EXTEN})}) +same => n,Set(TECHNOLOGY=${DB(device/technology/${EXTEN})}) +same => n,GotoIf($[${ISNULL(${TECHNOLOGY})} | ${ISNULL(${DEVICE})}]?error,1) +same => n,Dial(${TECHNOLOGY}/${DEVICE},${GLOBAL(TIMEOUT)}) +same => n,Set(vmFlag=${IF($[${DIALSTATUS} = BUSY]?b:u)}) +same => n,Voicemail(${EXTEN}@${GLOBAL(VOICEMAIL_CONTEXT)},${vmFlag}) +same => n,Hangup() + +exten => error,1,Verbose(2,Unable to lookup technology or device for extension) +same => n,Playback(silence/1&num-not-in-db) +same => n,Hangup() +``` + + +## Manager Class Authorizations + +Manager accounts have associated class authorizations that define what actions +and events that account can execute/receive. In order to run Asterisk commands +or dialplan applications that affect the system Asterisk executes on, the +"system" class authorization should be set on the account. + +However, Manager commands that originate new calls into the Asterisk dialplan +have the potential to alter or affect the system as well, even though the +class authorization for origination commands is "originate". Take, for example, +the Originate manager command: + +``` +Action: Originate +Channel: SIP/foo +Exten: s +Context: default +Priority: 1 +Application: System +Data: echo hello world! +``` + +This manager command will attempt to execute an Asterisk application, System, +which is normally associated with the "system" class authorication. While some +checks have been put into Asterisk to take this into account, certain dialplan +configurations and/or clever manipulation of the Originate manager action can +circumvent these checks. For example, take the following dialplan: + +```INI +exten => s,1,Verbose(Incoming call) +same => n,MixMonitor(foo.wav,,${EXEC_COMMAND}) +same => n,Dial(SIP/bar) +same => n,Hangup() +``` + +Whatever has been defined in the variable EXEC_COMMAND will be executed after +MixMonitor has finished recording the call. The dialplan writer may have +intended that this variable to be set by some other location in the dialplan; +however, the Manager action Originate allows for channel variables to be set by +the account initiating the new call. This could allow the Originate action to +execute some command on the system by setting the EXEC_COMMAND dialplan variable +in the Variable: header. + +In general, you should treat the Manager class authorization "originate" the +same as the class authorization "system". Good system configuration, such as +not running Asterisk as root, can prevent serious problems from arising when +allowing external connections to originate calls into Asterisk. + + +## Avoid Privilege Escalations + +External control protocols, such as Manager, often have the ability to get and +set channel variables; which allows the execution of dialplan functions. + +Dialplan functions within Asterisk are incredibly powerful, which is wonderful +for building applications using Asterisk. But during the read or write +execution, certain diaplan functions do much more. For example, reading the +SHELL() function can execute arbitrary commands on the system Asterisk is +running on. Writing to the FILE() function can change any file that Asterisk has +write access to. + +When these functions are executed from an external protocol, that execution +could result in a privilege escalation. Asterisk can inhibit the execution of +these functions, if live_dangerously in the \[options\] section of asterisk.conf +is set to no. + +In Asterisk 12 and later, live_dangerously defaults to no. + + +[voip-security-webinar]: https://www.asterisk.org/security/webinar/ +[blog-sip-security]: http://blogs.digium.com/2009/03/28/sip-security/ +[Strong Password Generator]: https://www.strongpasswordgenerator.com +[Filtering Data]: #filtering-data +[Proper Device Naming]: #proper-device-naming +[Secure Passwords]: #secure-passwords +[Reducing Pattern Match Typos]: #reducing-pattern-match-typos +[Manager Class Authorizations]: #manager-class-authorizations +[Avoid Privilege Escalations]: #avoid-privilege-escalations -- cgit v1.2.3