Zitmo trojan for Android defeats two-factor authentication

Malware intercepts TANs sent via SMS.

A new variant of the Zitmo trojan has been discovered that infects mobile devices running the Android platform and which intercepts SMS messages from banks sending mobile TAN numbers, thus potentially defeating two-factor authentication.

Two-factor authentication is used by many banks to prevent a customer's online banking account being compromised by password theft. One common way for it to work is for the customer to be required to enter both their password and a 'Transaction Authentication Number' (TAN) - which is sent to their mobile device via SMS - in order to complete a transaction. This is considered to be more secure as it is deemed unlikely that criminals would be able both to steal passwords and have access to the user's mobile device.

However, it is certainly not impossible - as the Zitmo trojan (first discovered in September 2010 for Symbian devices) shows. The trojan co-operates with the ZeuS crime kit (Zitmo stands for 'Zeus In The MObile'): when a user who is infected with ZeuS visits one of a number of particular websites, code is injected into the session, prompting the user to enter their mobile number as well as the model of the device. An SMS is then sent to that number with a link to the malicious application, which is a Zitmo variant targeting that particular operating system.

The combination of ZeuS, which steals the user's login credentials for the online banking system, and Zitmo, which intercepts mobile TANs, gives the criminals effective control of the user's bank account.

Two-factor authentication should still be a minimum requirement for online banking, but neither banks nor their customers should assume that this makes the systems undefeatable.

More at Fortinet's blog here and at CSIS;'s blog here. Fortinet's Axelle Apvrille and Kyle Yang wrote a two-part analysis of Zitmo for the March and April editions of Virus Bulletin (subscription required).

Axelle Apvrille will give a presentation on analysing mobile malware at VB2011 later this year. The conference takes place 5-7 October in Barcelona. Registration for the event is now open.

Tags: android, authentication, banking, mobile, trojan, zeus, zitmo. Posted on 11 July 2011 by Virus Bulletin. 2 comments.

2 comments

2 comments

Is there a reason they would continually risk interception with a TAN rather than just use a unique auth app (mock key-fob), like Blizzard / Google do with the iPhone?

Obviously those are not perfect, and could be the target of local memory hacks, but I'm under the impression that attacking data transfer is always the weakest link in security.

by ThisIsSaei, 12 July 2011, 20:37

Is there a reason they would continually risk interception with a TAN rather than just use a unique auth app (mock key-fob), like Blizzard / Google do with the iPhone?

Obviously those are not perfect, and could be the target of local memory hacks, but I'm under the impression that attacking data transfer is always the weakest link in security.

by ThisIsSaei, 12 July 2011, 20:37

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