W32/Petya.EOB!tr is a generic detection for a type of ransomware that encrypts files on the victim's computer. Since this is a generic detection, files that are detected as W32/Petya.EOB!tr may have varying behavior.
Below are examples of some of these behaviors:

  • This malware arrives as a DLL. The malicious actions can be found in the DLL's first exported function.

  • It checks if there is a file in the C:\Windows folder that has the same name as itself (with no extension name). For example, if the malware is executing as Petya.dll, it checks if the file C:\Windows\Petya  exists. If the file can be found, it exits. If not, it proceeds with the rest of its actions.

  • It overwrites the Master Boot Record and encrypts the disk.

  • In order to encrypt disk, it will schedule a restart/shutdown using these commands:
    • cmd.exe "undefinedWINDIRundefined\system32\shutdown.exe /r /f"
    • schtasks.exe " /TR "undefinedWINDIRundefined\system32\shutdown.exe /r /f" /ST {HH:MM}

  • Upon reboot, it displays a fake Chkdsk message that it is "Repairing file system on C:". However, it is likely already using this time to encrypt the victim's disk.

    • Figure 1: Boot Chkdsk message.

  • Once this progress prompt is complete, it displays a a ransom message on the screen stating that the victim's files are encrypted and in order to use their decryption, the victim needs to send $300 worth of Bitcoins to this address: 1Mz7153H{Removed}AtNbBWX.

    • Figure 2: Ransom message.

  • It drops a file [Random].tmp  in the Temporary folder, then runs it. This file is the Mimikatz tool which is used to gain access to logged on users and their passwords. The malware uses a named pipe to communicate with it.

  • It drops the file dllhost.dat  to the Windows folder. This is the PsExec  tool.

  • There are indications that the EternalBlue exploit is used to spread this malware onto unpatched machines within the victim's network.

Recommended Action

  • Make sure that your FortiGate/FortiClient system is using the latest AV database.
  • Quarantine/delete files that are detected and replace infected files with clean backup copies.