What is an IMSI Catcher?

A IMSI Catcher is a device that pretends to be a cell tower in order to trick your phone into thinking you are connected to your provider. In truth, your phone is using the IMSI Catcher as an additional cell tower.

Why does my phone connect to a IMSI Catcher?

Cell phones are designed to look for all compatible towers. The IMSI Catcher can be configured to represent a tower from your cell provider. The IMSI Catcher operator must adjust settings to replicate a cell tower in your area. Using higher power output than other legitimate towers will make the IMSI Catcher look like a cell tower with strong signal.

How does a catch-all IMSI Catcher work?

A catch-all IMSI Catcher is configured to tell all cell phones within range that it is the only available cell tower. Tricking your phone into thinking its the only available connection. A catch-all IMSI Catcher can be used for collecting IMSI’s from a particular area or to deny service to cell phone users. By not configuring the IMSI Catcher to pass calls to the networks the users phone can’t call out.

How can a IMSI Catcher track me?

Most IMSI Catchers used by local law enforcement are used for tracking. By knowing the targets IMSI, the operator can program the IMSI Catcher to connect the targets phone if within range. Once connected the operator use a process of RF Mapping to direction find the target.

Can they listen to my calls?

This depends on the IMSI Catcher used. A basic IMSI catcher just captures the cell phone. To listen to calls a IMSI catcher requires a number of additional features charged separately by manufacturers. 2G calls are easy to listen to. IMSI Catchers have been available for well over a decade for this. The price of a IMSI Catcher is based on the number of channels, effective range, type of encryption, time to decrypt desired and the cellular bands to be covered.

But I have a 3G or 4G phone so my calls are encrypted right?

Yes, calls on 3G and 4G have better encryption than calls on 2G. But.. IMSI Catchers can feature add ons that trick a 3G or 4G phone into thinking those connections are unavailable. With a correctly configured IMSI Catcher your phone will still show your normal cellular 3G or 4G connection. Meanwhile your phone has been forced back to the weaker 2G encryption for monitoring.

Can they install software on my phone?

An IMSI Catcher collects the IMSI and the IMEI from your device. So the operator then knows what model phone you are using and potentially where you bought it. By knowing the model number it becomes easier to push a firmware update specifically developed for that phone.

What if I get a new phone and Sim?

A new phone and sim won't be in the IMSI Catchers target database. But the people you call would also need to get new phones. Or your new number will show up again in the target area. Additionally a IMSI Catcher can use direction finding to determine the cell devices used at a location.

Is there an app I can get to detect IMSI Catchers?

There are many apps on the market. Sadly many of them fall short on quality. To be effective an app would need to have access to the phones baseband and radio stack and possess the right heuristics to determine a true IMSI Catcher versus a poorly configured cell tower. Some apps on the market provide many false positives because of poor technical expertise of developers.

What are the legitimate uses for IMSI Catchers?

Well believe it or not there are a few. IMSI Catchers can of course be used by law enforcement to track or monitor suspects in criminal investigations. You could also block someone using a cell phone activated bomb. Locate a child during a amber alert. Find missing hikers in a national park. Or create a small cellular network in a remote area.

What would the bad guys use IMSI Catchers for?

In the wrong hands a IMSI Catcher could be used to cause terror through threatening text messages. Monitor law enforcement investigations. Spy on US businesses and government. Steal personal information given over the phone. And deny cell users access to emergency services.

How many IMSI Catchers are in the USA?

Thats one thing we don’t know for sure. However we do know they are extensively used by law enforcement. And many of the IMSI Catchers detected may not be used by our government.