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Node.js Malware created by pkg project

1. Overview

JavaScript is one of the most used programming languages, and Node.js is also used widely as execution environment. However, Node.js applications cannot be run without Node.js runime, so using them as malware requires a little effort. For this reason, pkg project that convert NodeJS applications to EXE file or other executable formats can be the attractive tool for attackers. This article describes tips for analyzing Node.js malware created by pkg project.

2. Sample

Node Stealer
SHA256: d6aee63ffe429ddb9340090bff2127efad340240954364f1c996a8da6b711374

3. pkg project

pkg project packs js files and runtime into a single file. This executable file created is structured as follows. The custom runtime has the capability to execute js files stored in the virtual file system and downloaded from following repositories.

Repositories URL : https://github.com/vercel/pkg-fetch

The pkg project adds virtual files and index to the back of the runtime. Furthermore, the runtime has the placeholder that stores the offset of the virtual file as follows. Patching this placeholder(PAYLOAD_POSITION) makes it possible for the custom runtime to find out the location of the virtual file system.

The virtual file system index is json format as follows. A virtual file and folder has multiple objects, with offsets and data lengths stored for each object. For example, the data of object type 0 in app.js starts from PAYLOAD_POSITION+0 and its data length is 120648.

{
  "C:\\snapshot\\t2\\app.js": {
    "0": [       //Object type
      0,          // Offset
      120648  //File size
    ],
    "1": [
      120648,
      59506
    ],
    "3": [
      180154,
      121
    ]
  },
...
  "C:\\snapshot\\t2\\node_modules\\minipass-fetch\\lib": {
    "2": [
      29042524,
      106
    ],
    "3": [
      29042630,
      117
    ]
  },
...
Details of each type are shown in the table. Type 0 is only js file's object. This is data compiled from js file into format that can be executed by v8 JavaScript engine. Depending on the options, js file may only have type 0 objects and not type 1 objects.
Type 1 object is raw file data and type 2 object is list of stored files for directories. Type 3 is metadata for pkg, with no interesting data.

Object TypeDescription
0V8 code cache
1File data
2Directory
3Meta data

4. Malware analysis

The executable file created by pkg project has the following surface information. This is information that the custom runtime has, and it is rare for attacker to modify or delete it.

Now that I know this sample was created by pkg project, I dump the virtual file system using script.py. This script saves the virtual file system summary as vfs.txt, the v8 cache as filepath.cache, and the raw data as filepath.dump.

[script.py]
import sys
import json
import os

if(len(sys.argv) != 2):
    print("python script.py [filepath]")
    exit(1)

with open(sys.argv[1], "rb") as f:
    data = f.read()

target = b"\x2f\x2f\x23\x20\x73\x6f\x75\x72\x63\x65\x4d\x61\x70\x70\x69\x6e\x67\x55\x52\x4c\x3d\x63\x6f\x6d\x6d\x6f\x6e\x2e\x6a\x73\x2e\x6d\x61\x70\x0A\x7D\x2C\x0A"
p_st = data.find(target)
if(p_st == -1):
    print("[-] This file doesn't seem nodejs pkg file.")
    exit(1)

p_st += len(target)
p_ed = data[p_st:].index(b"\x0a") + p_st

json_data = json.loads(data[p_st:p_ed].decode())

os.mkdir("dump")
print("[+] Dump virtual file system as vfs.txt.")
with open("./dump/vfs.txt", "w") as f:
  for key in json_data.keys():
    f.write(key+"\n")

target = b"\x50\x41\x59\x4c\x4f\x41\x44\x5f\x50\x4f\x53\x49\x54\x49\x4f\x4e\x20\x3d\x20\x27"
p_st = data.find(target)
if(p_st == -1):
    print("[-] This file doesn't seem nodejs pkg file.")
    exit(1)

p_st += len(target)
p_ed = data[p_st:].index(b"\x20") + p_st
payload_position = int(data[p_st:p_ed])
print(payload_position)

for key in json_data.keys():
  object_info = json_data[key]
  for object_class in object_info.keys():
    if(object_class == "0"):
      print("[+] Dump virtual v8 code cache: %s" % (key))
      data_st = payload_position + object_info[object_class][0]
      data_ed = payload_position + object_info[object_class][0] + object_info[object_class][1]
      object_data = data[data_st:data_ed]
      with open("./dump/" +key.replace(":","").replace("\\","_")+ ".cache", "wb") as f:
        f.write(object_data)

    elif(object_class == "1"):
      print("[+] Dump virtual file: %s" % (key))
      data_st = payload_position + object_info[object_class][0]
      data_ed = payload_position + object_info[object_class][0] + object_info[object_class][1]
      object_data = data[data_st:data_ed]
      with open("./dump/" +key.replace(":","").replace("\\","_")+ ".dump", "wb") as f:
        f.write(object_data)
I first look at summary of the virtual file system. This shows that this file is composed of many js files.

[vfs.txt]
C:\snapshot\t2\app.js
C:\snapshot\t2\package.json
C:\snapshot\t2\node_modules\sqlite3\lib\binding\napi-v6-win32-unknown-x64\node_sqlite3.node
...
A good way to find interesting js files is to look at the entry points. Entry points are found in package.json as follows.

[C:\snapshot\t2\package.json]
...
  "bin": "./app.js",
  "pkg": {
    "scripts": "app.js",
    "targets": [
      "node14-win-x64"
    ],
    "compress": true,
    "outputPath": "bot-0",
    "assets": [
      "./node_modules/sqlite3/lib/binding/napi-v6-win32-unknown-x64/node_sqlite3.node",
      "./node_modules/win32crypt/**/**"
    ]
  },
...
Since I know that the entry point is app.js, I look at the contents of this file. This is highly obfuscated, but it is easy to recognize that it is an interesting file. Analyzing obfuscated js files is not much fun, so I won't describe it in this article.

[C_snapshot_t2_app.js.dump]
function _0xe997(){const _0x1a88d1=['bcbcvbs','log','yeucaudulieumoi','chat_id','sotienNguong44444','2696gGWKSZ','hahahahah','request\x20server\x20error','userFullName\x22:\x22','Thường','1036512kMjefP','agencies','fb_dtsg','18284tWOGPC','BUSINESSES','Chrome','hasOwnProperty','\x20thang\x20','x86','\x20Edg/','adAccountLimit','isDirectory','stdout:\x20','__spin_r\x22:','user','data','console','/accounts?fields=id,name,verification_status,fan_count,access_token&access_token=','indexOf','trunk','__spin_b\x22:\x22','1474VyTKBA','random','cos\x20nha','Win64','all_payment_methods','length','is_httponly','birthday','message','subscription($ip:\x20String){\x0a\x20\x20\x20\x20yeucaudulieumoi(ip:\x20$ip)\x20{\x0a\x20\x20\x20\x20\x20\x20duLieu\x20{\x0a\x20\x20\x20\x20\x20\x20_id\x20\x20ip\x0a\x20\x20\x20\x20\x20\x20}\x0a\x20\x20\x20\x20}\x0a\x20\x20}','facebook','entry\x20comment\x20goes\x20here','.png',
...
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