The Simplocker ransom malware that infected thousands of Android devices last summer has dramatically boosted the power of its encryption design in a new version, security firm Avast as discovered.
First detected by ESET in June 2014 circulating on Russian sites, the original incarnation generated a crude symmetric master key (the same key used for encryption as decryption) to encrypt the files on victim devices, which made providing decryption straightforward once that was found.
Updated weeks later to an English-speaking version, the malware was notable as the first Android attack using the ransom technique to extort money from victims.
At the time considered a proof-of-concept attack to test infectivity, experts predicted that the encryption would be improved in a second and more dangerous version, which has now proved to be the case - Simplocker can now generate a unique key for each device.
On the basis of Avast's rather superficial description, this still sounds like a symmetric key but anyone infected will still be in much deeper water than with the old version because those keys will be harder to trace. If that's correct, the next step up would be to deploy asymmetric public key encryption of the sort used by PC ransom Trojans such as CryptoLocker.
The malware appears to be spreading through the usual shady markets, masquerading as a Flash update or even a full-blown Flash Player app. The attack uses social engineering to bypass Android's restrictions about installing apps from third parties, asking users to activate it in admin mode.
"Once installed, a Flash Player app icon appears on the device and when it is opened the 'Flash Player' requests the user grant it administrator rights, which is when the trouble really begins," said Avast researchers, Nikolaos Chrysaidos.
From that point onwards, the app is simply a variant on the years-old FBI police Trojan attack that insists the user has copyright files on the device and must pay a $200 fee to decrypt the files.
For once the attackers don't ask for Bitcoins which some users might not understand how to get hold of and resorts to old-fashioned money transfer services available to US-based users. That alone underlines that this attack is aimed at English-speaking users although early detections have been in the Middle East.
"If you have been infected by this new strain of Simplocker, back up the encrypted files by connecting your smartphone to your computer. This will not harm your computer, but you may have to wait until a solution to decrypt these files has been found," suggests Avast, holding out little hope that this will happen.
"Then boot your phone into safe mode, go into the administrator settings and remove the malicious app and uninstall the app from the application manager."
Simplocker-AA, to use its full name, appears to have been detected by Slovakian firm ESET in very small numbers as far back as October but the number of victims has stepped up since then.
Probably the most notable mobile ransom attack to date in the US was the Koler Trojan which used a wormlike design to spread quite rapidly to a few thousand subscribers last October.