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EFF – SecUpwN/Android-IMSI-Catcher-Detector



Both law enforcement agencies and criminals use IMSI-Catchers, which are false mobile towers acting between the target mobile phone(s) and the service providers real towers. As such it is considered a Man In the Middle (MITM) attack. It was patented and first commercialized by Rohde & Schwarz in 2003, although it would be hard to maintain such a patent, since in reality it is just a modified cell tower with a malicious operator.


On 24 January 2012, the Court of Appeal of England and Wales held that the patent is invalid for obviousness. But ever since it was first invented, the technology has been used and “improved” by many different companies around the world. Other manufacturers (like Anite) prefer to refer to this spying and tracking equipment in cozy marketing words as “Subscriber Trackers“. In the USA this technology is known under the name “StingRay“, which is even capable to track the people who are traveling together with the owner of a targeted phone across the country. Here you can see alleged StingRay tracking devices mounted to the roof of three SUVs. The FBI or local police might deploy the device at a protest to obtain a record of everyone who attended with a cell phone. IMSI-Catchers also allow adversaries to intercept your conversations, text messages, and data. Police can use them to determine your location, or to find out who is in a given geographic area at what time. Identity thieves might operate an IMSI-Catcher in a parked car in a residential neighborhood, stealing passwords or credit card information from people nearby who make purchases on their phones.


There is more: Powerful, expensive IMSI-Catchers are in use at federal agencies and some police departments. And if you think that IMSI-Catchers are not used in your own town, think twice! If you ever happen to be near a riot or demonstration (hint: leave you phone at home if participating), pay close attention to cars standing along the path of the demonstration – those might be IMSI-Catchers. It is common practice for police to position IMSI-Catchers at the beginning as well as the end of roads where the demonstrating crowd moves to capture and compare data in order to find out who participated. But most of the time IMSI-Catchers are well hidden and can be even body-worn – therefore you won’t even discover these creepy devices. Current technology shrinks them to be as tiny as your phone! So again, if you really have to participate in a riot or demonstration, leave your phones at home or build yourself a signal blocking phone pouch!


In addition, all IMSI-Catchers can crack A5/1 encryption, which is most commonly used for GSM traffic, on the fly (passively)! A5/3 encryption which is used for securing 3G and is offered as new security standard for GSM encryption remains secure in practice while susceptible to theoretical attacks. Although 3G and 4G offer sufficient protection from eavesdropping, the security measures can be bypassed by IMSI-Catchers forcing a mobile device into 2G mode and downgrade encryption to A5/1 or disable it. For further reading on the algorithms, check out the Cryptome GSM Files.

There are almost no phones on the market which offer an option to check what kind of encryption is used to secure GSM traffic. And although the Issue of not having a convenient display of the Ciphering Indicator has been assigned to Google since 2009, it seems they’re getting paid (or are forced to) blatantly ignoring it. Just recently, a new open source project called the “Android-CipheringIndicator-API” opened its doors to finally craft an API which fixes this Issue and merge the resulting API into the Android AOSP branch. But currently, the only way to protect a mobile device from downgrade attacks is to disable 2G if this option is available. In this case, the phone will not be able to receive or make calls in areas without 3G coverage. This is why the original author named “E:V:A” started this project. Let’s detect and protect against these threats! Never think you’ve got “nothing to hide“.

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