[asterisk-users] How to detect fake CallerID? (8xx?)

Adam Goldberg adam at agp-llc.com
Thu May 11 07:59:49 CDT 2017

Seems like this is the best idea (challenge-response), a callback.  No matter the callerid, you don't know where the caller is.  But if you place a call BACK to the callerid, it's going to go to the destination.  Then you either need the phone to be answered, or the phone to be answered and and the challenge entered.

Adam Goldberg

-----Original Message-----
From: asterisk-users-bounces at lists.digium.com [mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of J Montoya or A J Stiles
Sent: Thursday, May 11, 2017 7:48 AM
To: Asterisk Users Mailing List - Non-Commercial Discussion <asterisk-users at lists.digium.com>
Subject: Re: [asterisk-users] How to detect fake CallerID? (8xx?)

On Wednesday 10 May 2017, Steve Edwards wrote:
> On Wed, 10 May 2017, J Montoya or A J Stiles wrote:
> > Presumably your staff carry mobile phones.  What about an app that 
> > gets the ID of the cell tower to which it is connected, and passes 
> > it and the SIM number in a HTTP request to a server you control?
> The problem is that they are supposed to use the 'site landline' to 
> confirm presence -- not their cell phone with the spoofed CID.

Yes; but the whole point is that the caller ID from the site landline is no longer reliable enough as evidence, by itself, that somebody is actually there.

A custom app could read the ID of the cell tower to which it was connected -- or even the phone's GPS co-ordinates -- and transmit that back to base over the Internet.  Preferrably with some sort of precautions to make the request harder to forge  (i.e., *not* just a plain HTTP GET with the MCC, MNC, LAC and CID in the query string).  If your app makes its connection via the site's wi- fi  (which will require the co-operation of the client)  as opposed to the mobile network, so much the better, as there will be an IP address against which you can match.

If you insist to use the site landline for your authentication, you could extend the protocol to a full challenge-and-response as follows:  Play a series of digits down the line to the caller, return the call as soon as they hang up, and ask them to dial the same digits they just heard.  All this can be done in the dialplan  (you might need to record some announcements of your own, such as "Please memorise the following digits" and "Please dial the digits you heard in the last call").  

Intercepting incoming calls *to* a number is much harder  (usually requiring the co-operation of telcos, unless the interloper has access to some equipment through which they know that the call will be routed; that potentially includes your Asterisk, but any tampering there would be evident)  than falsifying outgoing calls *from* a number.  

It would be much more fun to mount a "sting" operation to catch the 
perpetrators red-handed   (say, falsely set off a fire alarm while you know they 
are slacking off down the pub instead of looking after the site like they are paid for)  .....  but maybe I have just been watching too many detective dramas on TV!


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