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Simplocker Authors Try to Prevent File Decryption Without Payment

The developers of Simplocker, a piece of ransomware that locks Android smartphones and encrypts files on their external storage, have improved the malware to make it more difficult to recover files without paying the ransom.

The developers of Simplocker, a piece of ransomware that locks Android smartphones and encrypts files on their external storage, have improved the malware to make it more difficult to recover files without paying the ransom.

The Android ransomware Simplocker has evolved a great deal over the past months. The first variants of the threat only claimed to encrypt files on infected devices in hopes that they would scare victims into paying the ransom without actually checking the claims.

In June 2014, Avast spotted a version of the malware that actually encrypted files. However, it wasn’t difficult to recover lost files because the cybercriminals had used a single encryption key that was hardcoded in the malware. The security firm even released an app to help users get their files back.

Now, Simplocker developers have stepped up their game and started using unique encryption keys for each of the infected devices. This should make it more difficult to recover the encrypted files without paying the $200 ransom.

However, Avast recommends against paying up. The security firm advises users whose devices have been infected to back up the encrypted files on a computer and wait until researchers figure out how to recover the files. In the meantime, the threat can be removed from the phone by booting it in safe mode.

According to the security firm, at least 5,000 devices had been infected with the new Simplocker variant within days after the threat was discovered. SecurityWeek has asked Avast about the chances of finding a way to help users recover their lost files.

“When doing this job, you must have confidence in finding and supplying solutions to the affected user. For the new Simplocker version, we’ll need to do more research to find a solution,” responded Nikolaos Chrysaidos, malware analyst at Avast. “For future variants, it is difficult to predict what the bad guys’ next move will be.”

Simplocker is distributed via ads that instruct users to install Flash Player in order to view videos. The served application is not Flash Player, but the Android ransomware. Once it’s installed, the app requests administrator privileges. If these privileges are granted, the malware locks the device, displays what appears to be a warning from the FBI, and encrypts the user’s files.

The malware uses the the XMPP protocol and Jabber to communicate with its command and control (C&C) server. Simplocker connects to the C&C server every 60 minutes. First, the threat collects information on the infected device (IMEI, OS, phone number, operator name, country) and sends it back to the server.

“[The malware] checks whether the files have been encrypted or not. Also if a voucher has been entered, it sends back the type and the code. All the data that gets sent back to the server is formatted as: Base64 ( CRC(data) + MalwareEncryption(data) ),” Chrysaidos explained in a blog post.

Written By

Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.

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