Locky, one of the most prolific ransomware families this year, has changed its modus operandi once again by adopting a new extension that is appended to encrypted files.

First observed in February, when it stood out because it could encrypt files on unmapped network shares, Locky was originally renaming the encrypted files to [unique_id][identifier].locky. In early summer, researchers revealed that Locky switched to the .zepto extension, which has been used in multiple campaigns since.

Now, Locky is appending the .ODIN extension to encrypted files, which is bound to create some confusion, as victims might believe they have been infected with a new ransomware variant. However, BleepingComputer’s Lawrence Abrams notes that this is not the Odin ransomware, but the well-known Locky, which is using the .ODIN extension instead of .zepto.

Just as before, the new malware version is distributed via spam emails containing script files as attachments. As soon as the recipient opens the attachment, the malicious code in these script files downloads an encrypted DLL installer, after which it decrypts and executes to infect the system with Locky.

Once executed, the ransomware encrypts user’s files, renames them, and appends the .ODIN extension. Then, the malware drops ransom notes on the system to inform the user on the attack. In this new variant, the names of the ransom notes have been changed to _HOWDO_text.html, _HOWDO_text.bmp, and _[2_digit_number]_HOWDO_text.html.

Recently, Locky also returned to the use of a command and control (C&C) server, after switching to an offline mode back in mid-July. At the time, the change made it more difficult for IT admins and security researchers to stop Locky infections, because blocking its C&C connections no longer had the desired effect.

Now, Avira researchers reveal that the ransomware has switched back to using C&C servers, while also saying that only few affiliates continue to use the offline-only mode. While there’s no info on what exactly determined Locky’s operators to revert the change, Avira explains that the use of an offline mode was a two-edged sword for cybercriminals.

“On one hand, by not giving C&C information – and an IP address – it lets the Locky network keep out of sight from law enforcement and security researchers. But on the other hand, it reduces the feedback the cybercriminals can collect on the effectiveness of individual Locky distribution campaigns run by their affiliates,” Avira notes.

Related: Locky Ransomware Fuels Surge in .RAR, JavaScript Attachments

Related: Locky Ransomware Switches to DLLs for Distribution