Locky Ransomware Switches to DLLs for Distribution

Locky, one of the most popular ransomware families at the moment, has changed its distribution method once again and is now using DLLs for infection, Cyren researchers warn.

When it emerged in February this year, Locky was using weaponized documents for distribution, but has changed the delivery method a few times since then. In addition to malicious macros, the ransomware was seen employing JavaScript attachments for infection and has recently switched to Windows script (WSF) files, only to revert to macros soon after.

Over the past half a year, Locky has seen other changes as well, including the addition of new sandbox evasion techniques, the switch to a new extension (.zepto) for the encrypted files, the use of offline encryption, and the drop of downloaders in favor of JavaScript files.

Now, Cyren researchers say, Locky has added another layer of obfuscation to its downloader script, which is delivered via spam emails in the form of a ZIP-archived JavaScript. Just as before, however, the ransomware employs finance-related topics as lures to trick users into opening the malicious attachment.

The downloader script features a similar structure to the previously used Locky downloaders, as well as a similar behavior: the downloaded files are decrypted and saved in the Windows Temp directory. However, the malicious payload is not an EXE binary as before, but a DLL file, Cyren reports. The dropped DLL is loaded using rundll32.exe.

Upon closer inspection, researchers discovered that the DLL is using a custom packer to prevent anti-malware scanners from detecting it. When executed, the file does the usual ransomware routine to search the affected system and network shares for files to encrypt.

Encrypted files are appended the .zepto extension, and, as soon as the encryption process has been completed, the malware drops and displays a ransom payment instruction page.

According to researchers, the .onion link displayed in the ransom note directs victims to the same Locky decryptor page that has been used in previous infection campaigns as well.

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