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An international mobile subscriber identity-catcher, or IMSI-catcher, is a telephone eavesdropping device used for intercepting mobile phone traffic and tracking location data of mobile phone users.
Essentially a "fake" mobile tower acting between the target mobile phone and the service provider's real towers, it is considered a man-in-the-middle (MITM) attack. The 3G wireless standard offers some risk mitigation due to mutual authentication required from both the handset and the network.
IMSI-catchers are used in a number of countries by law enforcement and intelligence agencies, but their use has raised significant civil liberty and privacy concerns and is strictly regulated in some countries such as under the German Strafprozessordnung (StPO / Code of Criminal Procedure).
Every mobile phone has the requirement to optimize the reception. If there is more than one base station of the subscribed network operator accessible, it will always choose the one with the strongest signal. An IMSI-catcher masquerades as a base station and causes every mobile phone of the simulated network operator within a defined radius to log in. With the help of a special identity request, it is able to force the transmission of the IMSI.
The IMSI-catcher subjects the phones in its vicinity to a man-in-the-middle attack, appearing to them as a preferred base station in terms of signal strength. With the help of a SIM, it simultaneously logs into the GSM network as a mobile station. Since the encryption mode is chosen by the base station, the IMSI-catcher can induce the mobile station to use no encryption at all. Hence it can encrypt the plain text traffic from the mobile station and pass it to the base station.
A targeted mobile phone is sent signals where the user will not be able to tell apart the device from authentic cell service provider infrastructure.
The assignment of an IMSI catcher has a number of difficulties:
It must be ensured that the mobile phone of the observed person is in standby mode and the correct network operator is found out. Otherwise, for the mobile station, there is no need to log into the simulated base station.
Depending on the signal strength of the IMSI-catcher, numerous IMSIs can be located. The problem is to find out the right one.
All mobile phones in the area covered by the catcher have no access to the network. Incoming and outgoing calls cannot be patched through for these subscribers. Only the observed person has an indirect connection.
There are some disclosing factors. In most cases, the operation cannot be recognized immediately by the subscriber. But there are a few mobile phones that show a small symbol on the display, e.g. an exclamation point, if encryption is not used. This "Ciphering Indication Feature" can be suppressed by the network provider, however, by setting the OFM bit in EFAD on the SIM card. Since the network access is handled with the SIM/USIM of the IMSI-catcher, the receiver cannot see the number of the calling party. Of course, this also implies that the tapped calls are not listed in the itemized bill.
The assignment near the base station can be difficult, due to the high signal level of the original base station.
As most mobile phones prefer the faster modes of communication such as 4G or 3G, downgrading to 2G can require blocking frequency ranges for 4G and 3G.
Some preliminary research has been done in trying to detect and frustrate IMSI-catchers. One such project is through the Osmocom open source mobile station software. This is a special type of mobile phone firmware that can be used to detect and fingerprint certain network characteristics of IMSI-catchers, and warn the user that there is such a device operating in their area. But this firmware/software-based detection is strongly limited to a select few, outdated GSM mobile phones (i.e. Motorola) that are no longer available on the open market. The main problem is the closed-source nature of the major mobile phone producers.
The application Android IMSI-Catcher Detector (AIMSICD) is being developed to detect and circumvent IMSI-catchers by StingRay and silent SMS.
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