[asterisk-users] Important security alert: update your dialplans now!

Tue Feb 16 16:38:15 CST 2010

I have read the posts about the security issue and from what I  
understand there should be a check to make sure that the characters  
used are actually allowed. I wrote a very straightforward and not so  
rocket science kind of macro that will do the job I guess. Just two  
parameters, one with the allowed characters and one with the string to  
check. The allowed characters can be stored in a global variable if  
this set has a global character. It is just one extra line in the  
dialplan and a little bit extra cpu load.

It is used like this:

exten => _XXXXXXXXX.,1,Macro(alfa_num_check,1234567890,${EXTEN})
exten => _XXXXXXXXX.,n,Dial(SIP/315927xxxxxx/${EXTEN},40,t)
exten => _XXXXXXXXX.,n,Hangup()

If not allowed characters are used the call setup will end.

And the actual macro does just have 20 lines of Asterisk code. I'm  
sure there will be better solutions in the future but for the short  
time this might be useful. A clever scripter might be able to write a  
script that scans the dialplan line by line and adds the macro line  
above the _XXXX.,n,DIAL() lines and adjust the numbering of the line  
if needed.   I have done some testing and the code in this mail  
ishould work, at least it did on my Asterisk box.

Friendly regards,

Erik

[macro-alfa_num_check]
;;;;;;;;;;;;;;;;;;;;
; written by  : erik de wild
; company  : Tripple-o    Your Asterisk migration partner
; e-mail        : info   at   tripple-o.nl
; date           : 16 februari 2010
;
; everybody is free to use, improve and modify this code
; as long as credits will be addressed, no costs are charged
; and this header stays with the code.
;;;;;;;;;;;;;;;;;;;;;
exten => s,1,NoOp(This is a macro to check the content of the ${EXTEN}  
variable to cope with the vulnerabilily of number injection)
exten => s,n,Set(ALLOWED_CHARACTERS=${ARG1})
exten => s,n,Set(STRING_TO_CHECK=${ARG2})
exten => s,n,Set(LEN_ALLOWED_CHARACTERS=${LEN(${ALLOWED_CHARACTERS})})
exten => s,n,Set(LEN_STRING_TO_CHECK=${LEN(${STRING_TO_CHECK})})
exten => s,n,Set(COUNTER=0)

exten => s,n(start_loop),SET(CHAR_TO_CHECK=${STRING_TO_CHECK:$ 
{COUNTER}:1})
exten => s,n,NoOp(Character to check is ${CHAR_TO_CHECK}. This  
Character should be in this list of characters -> ${ALLOWED_CHARACTERS})
exten => s,n,Set(COUNTER2=0)
exten => s,n,NoOp(character to check: ${CHAR_TO_CHECK}    -> $ 
{ALLOWED_CHARACTERS:${COUNTER2}:1})

exten => s,n(start_loop2),GotoIf($["${CHAR_TO_CHECK}" = "$ 
{ALLOWED_CHARACTERS:${COUNTER2}:1}"]?exit_loop2)     ;
exten => s,n,Set(COUNTER2=$[${COUNTER2} + 1])
exten => s,n,GotoIf($["${COUNTER2}" = "${LEN_ALLOWED_CHARACTERS}"]? 
hangup)
; if the character to control is not in the list of allowed   
characters then the setup of the call has to stop

exten => s,n,Goto(start_loop2)
exten => s,n(exit_loop2),NoOp(character checked is allowed, go for the  
next charachter!!)

exten => s,n,Set(COUNTER=$[${COUNTER} + 1])
exten => s,n,GotoIf($["${COUNTER}" = "${LEN_STRING_TO_CHECK}"]? 
macroexit)

exten => s,n,Goto(start_loop)

exten => s,n(hangup),NoOp(the checked character is not in the allowed  
charachter string. Call setup has to stop)
exten => s,n,Hangup()

exten => s,n(macroexit),MacroExit()

On 14 feb 2010, at 00:04, Olle E. Johansson wrote:

> Friends,
>
> Last week, Hans Petter Selansky alerted us of a potential security  
> issue in all releases of Asterisk. In fact, it doesn't involve the  
> code, but the most common way to construct dialplans. If you have  
> something like this in your Asterisk, you need to update your  
> dialplans:
>
> [incoming-from-voip]
> exten => _X., 1, dial(SIP/${EXTEN})
>
> Many VoIP protocols support a large character set, that may cause  
> harm in your dialplan
> ====================================================================
>
> I've written an article about this on my blog, where my summary says:
>
> "Because of a conflict between allowed characters in the called  
> number or name in many VoIP protocols and the way Asterisk handles  
> channel variables, there is a security risk hidden in many dialplans  
> based on examples provided over the years by the Asterisk  
> developers, trainers and community. The primary risk is that by  
> using an ampersand in the dialstring, a user can access protected  
> resources or misuse the pbx services. However, this character is not  
> the only problem, as other characters may cause unexpected or  
> problematic behaviour."
>
> There will be an Asterisk Security Advisory document coming out from  
> Digium soon, as well as updated documentation and examples within  
> the Asterisk source code tree. I strongly advise everyone to follow  
> these and stay updated. (I have no access to the ASA system myself  
> and can't generate an official security alert).
>
> For more information about this issue and some code examples of what  
> I personally currently think are good ways to prevent misuse of your  
> services, please read my blog article at
>
> http://www.voip-forum.com/?p=241&preview=true
>
> Please help us to distribute this message!
> =================================
> We need help from all involved in the Asterisk eco-system. This is  
> not something that  the development team can solve by itself. We can  
> add documents, READMEs and fix our own examples. But that won’t fix  
> it. We need everyone involved to pump this information out in all  
> the veins that runs through the Asterisk eco-system. In all  
> languages needed, we shall say: "Audit your dialplans, fix this  
> issue. And do it now."
>
> Everyone that runs a web site with dialplan examples - audit your  
> examples, fix them. Everyone that has published books on Asterisk -  
> publish errata on your web site. Please help us - and do it now.
>
> If you add web links, please add links both to http:// 
> www.asterisk.org where the official documents will soon be  
> published, as well as to my blog (if you like, of course). But don't  
> just refer to my blog entry alone.
>
> I have updated my own servers and will now start auditing my  
> customers' servers. After that I will have to update all my training  
> materials so I don't repeat the bad examples. There's no magic  
> bullet, no wonderful code patch, that can fix this, just hard work  
> with all dialplans that accept calls over VoIP channels.
>
> Let us all work together to fix this!
>
> With Asterisk greetings!
>
> /Olle
>
> PS. If someone can update the entries on Queue() and Dial() in voip- 
> info.org, that would be considered a good thing (TM).
> -- 
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