This is the second security flaw that has been detected in Android. (For the first one, read this post). A new technique allows attackers to hide encrypted malicious Android applications inside images. This can be used to evade detection by antivirus products and possibly Google Play’s malware scanner.
The attack was developed by a researcher at Fortinet, Axelle Apvrille, and reverse engineer Ange Albertini. It’s based on a technique devised by Albertini dubbed AngeCryption that allows controlling both the input and the output of a file encryption operation using the Advanced Encryption Standard (AES) by taking advantage of the properties of some file formats that allow files to remain valid despite having junk data appended to them.
AngeCryption allows the user to choose an input and an output file and makes the necessary modifications so that when the input file is encrypted with a specified key using AES in cipher-block chaining (CBC) mode, it produces the desired output file.
Apvrille and Albertini took the idea further and applied it to APK (Android application package) files. They created a proof-of-concept wrapping application that simply displays a PNG image of Star Wars character Anakin Skywalker. However, the app can also decrypt the image with a particular key in order to produce a second APK file that it can then install.
In their demonstration, the APK hidden inside the image was designed to display a picture of Darth Vader, but an actual attacker can use a malicious application to steal text messages, photos, contacts, or other data.
During the demonstration, Android displayed a permission request when the wrapper application tried to install the decrypted APK file, but this can be bypassed using a method called DexClassLoader so that the user doesn’t see anything, Apvrille said. The image wouldn’t even have to be included in the wrapper application and could be downloaded from a remote server after installation, she said.
In order for the attack to work, some data needs to be appended at the end of the original application, but the APK format, which is derived from ZIP, does not allow appended data after a marker called the End of Central Directory (EOCD), which signals the end of the file.
However, the researchers found that adding a second EOCD after the appended data tricks Android into accepting the file as valid. According to Apvrille, this is the result of an error in Android’s APK parser.
The attack works on the latest version of Android, 4.4.2, but the Android security team has been notified and is developing a fix, Apvrille said.
Because of fragmentation in the Android ecosystem, especially when it comes to firmware updates, many devices will likely remain vulnerable to this attack for a long time, giving Android malware authors ample time to take advantage of it.
Source: pcworld
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