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Yesterday β€” 27 October 2021Reverse Engineering

Avast releases decryptor for AtomSilo and LockFile ransomware

27 October 2021 at 15:59

On Oct 17, 2021, JiΕ™Γ­ Vinopal published information about a weakness in the AtomSilo ransomware and that it is possible to decrypt files without paying the ransom. Slightly later, he also analyzed another ransomware strain, LockFile. We prepared our very own free Avast decryptor for both the AtomSilo and LockFile strains.

Limitation of the decryptor

During the decryption process, the Avast AtomSilo decryptor relies on a known file format in order to verify that the file was successfully decrypted. For that reason, some files may not be decrypted. This canΒ  include files with proprietary or unknown format, or with no format at all, such as text files.

How AtomSilo and LockFile Work

Both the AtomSilo and LockFile ransomware strains are very similar to each other and except for minor differences, this description covers both of them.Β 

AtomSilo ransomware searches local drives using a fixed drive list, whilst LockFile calls GetLogicalDriveStringsA() and processes all drives that are fixed drives.

A separate thread is created for each drive in the list. This thread recursively searches the given logical drive and encrypts files found on it. To prevent paralyzing the compromised PC entirely, AtomSilo has a list of folders, file names and file types that are left unencrypted which are listed here:

Excluded folders
Boot Windows Windows.old Tor Browser
Internet Explorer Google Opera Opera Software
Mozilla Mozilla Firefox $Recycle.Bin ProgramData
All Users
Excluded files
autorun.inf index.htmlΒ  boot.ini bootfont.bin
bootsect.bak bootmgr bootmgr.efi bootmgfw.efi
desktop.ini iconcache.db ntldr ntuser.dat
ntuser.dat.log ntuser.ini thumbs.db #recycle
Excluded extensions
.hta .html .exe .dll .cpl .ini
.cab .cur .cpl .drv .hlp .icl
.icns .ico .idx .sys .spl .ocx

LockFile avoids files and folders, containing those sub-strings:

Excluded sub-strings
Windows NTUSER LOCKFILE .lockfile

In addition to that, there is a list of 788 file types (extensions), which won’t be encrypted. Those include .exe, but also .jpg, .bmp and .gif. You may noticed that some of them are included repeatedly.

The ransomware generates RSA-4096 session keypair for each victim. Its private part is then stored in the ransom note file, encrypted by the master RSA key (hardcoded in the binary). A new AES-256 file key is generated for each file. This key is then encrypted by the session RSA key and stored at the end of the encrypted file, together with original file size.Β 

Each encrypted file contains a ransom note file with one of the names:

  • README-FILE-%ComputerName%-%TimeStamp%.hta
  • LOCKFILE-FILE-%ComputerName%-%TimeStamp%.hta

Encrypted files can be recognized by the .ATOMSILO or .lockfile extension:Β 

When the encryption process is complete, the ransom note is shown to the user. Each strain’s ransom note has its own look:

AtomSilo Ransom Message
LockFile Ransom Message

How to use the Decryptor

To decrypt your files, please, follow these steps:

  1. Download the free decryptor. The single EXE file covers both ransomware strains.
  2. Simply run the EXE. It starts in form of wizard, which leads you through configuration of the decryption process.
  3. On the initial page, you can see a list of credits. Simply click β€œNext”
  1. On the next page, select the list of locations which you want to be decrypted. By default, it contains a list of all local drives.
  1. On the third page, you can select whether you want to backup encrypted files. These backups may help if anything goes wrong during the decryption process. This option is turned on by default, which we recommend. After clicking β€œDecrypt”, the decryption process begins.
  1. Let the decryptor work and wait until it finishes.

Credits

We would like to thank JiΕ™Γ­ Vinopal for sharing analysis of both ransomware strains.

IOCs

SHA filename
d9f7bb98ad01c4775ec71ec66f5546de131735e6dba8122474cc6eb62320e47b .ATOMSILO
bf315c9c064b887ee3276e1342d43637d8c0e067260946db45942f39b970d7ce .lockfile

The post Avast releases decryptor for AtomSilo and LockFile ransomware appeared first on Avast Threat Labs.

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