The Domain Generation Algorithm of Orchard v3

Edit 2022-08-08: Two weeks after this blog post, 360 Netlab published a detailed report on the malware, whose DGA version 3 I have described below. Please read Netlab’s post for general informations about the malware and two older DGA’s that is uses. I have adopted the name “Orchard” that they have given to the malware.

XMRig is an open-source software for mining cryptocurrencies like Monero or Bitcoin. It is also frequently used by cryptojacking malware to mine cryptocurrencies on victims’ computers. These malwares are a dime a dozen and are mostly unremarkable — to the point that they remain unnamed by AV vendors. Orchard is no exception. What sets the sample apart, however, is its Domain Generation Algorithm (DGA) which uses two different sources for seeding:

1. the current date — which is deterministic of course
2. balance of the Bitcoin genesis block (the first block on the Bitcoin blockchain) — which is not deterministic

For both seeds the same algorithm is used to generate the domains. The first 16 domains are derived from the current date, while the next 16 domains are based on the Bitcoin block:

I analyzed the following sample:

File type

PE32 executable (GUI) Intel 80386, for MS Windows

MD5

077b6b101cccb3d77a98e2cc02003526

SHA1

9e4f04d513ef119d7872c7ce0af6ffbdf4f42a7c

SHA256

2e63bbbbbb11c21445885f85fc8ef398737184c603f365c8e77a8cbf7523cac9

Size

507 KB (519680 Bytes)

Compile Timestamp

2022-06-19 16:47:27 UTC

Links

MalwareBazaar, Cape, Dropping_sha256, Dropping_md5, VirusTotal

Filenames

uAAAACCCGGGJ.exe (VirusTotal )

Detections[

Virustotal: 53/73 as of 2022-07-22, nothing specific

The sample unpacks to this binary, which — like the original — can be downloaded from MalwareBazaar:

File type

PE32 executable (GUI) Intel 80386, for MS Windows

MD5

5c13ee5dbe45d02ed74ef101b2e82ae6

SHA1

bdc36bc233675e7a96faa2c4917e9b756cc2a2a0

SHA256

ad1e39076212d8d58ff45d1e24d681fe0c600304bd20388cddcf9182b1d28c2f

Size

400 KB (409600 Bytes)

Compile Timestamp

2022-06-19 19:59:36 UTC

Links

MalwareBazaar, Dropped_by_md5, Dropping_sha256, Cape, Dropped_by_sha256, VirusTotal

Detections

Virustotal: 53/74 as of 2022-07-22, nothing specific.

Many more samples that feature the DGA can be found on VirusTotal since June 22, 2022, with a clear cluster on July 6 and 7. The hashes are summarized in this VT Collection.

Seeding

The first seed is calculated by taking the current date and formatting it as YYYY-mm-dd, e.g., 2022-07-23. Then a hardcoded domain name is appended. In the examined sample, the domain is “ojena.duckdns.org” so the resulting seed string would be “2022-07-23ojena.duckdns.org”.

The second seed is obtained by making a GET request to the following URL:

https://blockchain.info/balance?active=1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa

This url returns the current balance of the Bitcoin genesis block. The response is the seed for the DGA, for example:

{"1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa":{"final_balance":6854870116,"n_tx":3389,"total_received":6854870116}}

Due to the fame of the genesis account, it receives many transactions, which all result in a changed seed string and therefore different domains. The time between transactions varies greatly, but there was only about a 20% chance last year that a domain would have been valid for more than 2 days.

DGAs that use undeterministic sources for seeding are rare: Bedep used the foreign exchange reference rates published by the European Central Bank and Torpig Twitter trends. Unlike for these two examples, the seed in our case can be changed by anyone with a predictable outcome. This allows domains to be registered before they are used by Orchard, something which is not possible for Bedep and Torpig whose seeds can’t be influenced.

Domain Generation

The DGA itself is very simple: The seed string is MD5-hashed, then the hex representation of the hash is split into 4 strings of 8 characters to form the second level domains (sld). These 4 slds are then combined with 4 hardcoded top level domains to form 16 domains:

Reimplementation

The following script is a reimplementation of the DGA in Python. It can generate the first set of domains for arbitrary dates, but if you like to generate the second set of domains seeded by the genesis block, you need to request it with the -b command line argument. This will check if the date is covered by a local database of transactions of the Bitcoin genesis block. If necessary, this database will be updated by downloading transactions from blockchain.info. You find the DGA on my Github ¹, along with a database of transactions until September 2022.

import argparse
import hashlib
import itertools
import json
import os
import time
from datetime import date, datetime
from typing import Iterator, Union

import requests

LIMIT = 100
DB_PATH = "db.json"
BLOCK = "1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa"
PATTERN = '{"1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa":{"final_balance":FB,"n_tx":NTX,"total_received":FB}}'


def refresh_blockchain_db():
    offset = 0
    if os.path.exists(DB_PATH):
        with open(DB_PATH) as r:
            db = json.load(r)
    else:
        db = {}

    while True:
        url = f"https://blockchain.info/multiaddr?active={BLOCK}&limit={LIMIT}&offset={offset}"
        r = requests.get(url)

        data = r.json()
        error = data.get('error')
        if error:
            print(f"error updating blockchain balance: {data}")
            quit()
        txs = data["txs"]
        for tx in txs:
            h = tx['hash']
            if h in db:
                break
            db[h] = tx
        else:
            time.sleep(1)
            offset += LIMIT
            continue
        break
    with open(DB_PATH, "w") as w:
        json.dump(db, w, indent=2)


def get_blockchain_seed(when, updated: bool = False) -> str:

    if when > datetime.now():
        raise ValueError(
            "you can't generate the blockchain domains for the future!")

    with open(DB_PATH) as r:
        transactions = json.load(r)

    transactions = sorted(
        transactions.values(),
        key=lambda x: x['balance'],
        reverse=True
    )
    ntx = len(transactions) + 1
    for i, transaction in enumerate(transactions):
        tt = transaction["time"]
        time = datetime.fromtimestamp(tt)

        if when < time:
            continue

        if i == 0 and not updated:
            """ if the desired date is later than the latest transaction,
                then update the transaction database to make sure it is
                the current transaction you like to access """
            refresh_blockchain_db()
            return get_blockchain_seed(when, updated=True)

        balance = transaction['balance']
        return PATTERN.replace("FB", str(balance)).replace("NTX", str(ntx-i-1))
    raise ValueError("the provided date is before the first transaction")


def dga(when: Union[date, datetime], blockchain: bool = False) -> Iterator[str]:
    for i in range(2):
        if i and not blockchain:
            return

        if i == 0:
            magic = when.strftime("%Y-%m-%d")
            seed = f"{magic}ojena.duckdns.org"
        else:
            magic = get_blockchain_seed(when)
            seed = f"{magic}"

        md5 = hashlib.md5(seed.encode("ascii")).hexdigest()
        slds = [md5[i:i+8] for i in range(0, len(md5), 8)]
        tlds = [".com", ".net", ".org", ".duckdns.org"]

        for sld, tld in itertools.product(slds, tlds):
            yield f"{sld}{tld}"


def date_parser(s):
    return datetime.strptime(s, "%Y-%m-%d")


if __name__ == "__main__":
    now = datetime.now().strftime("%Y-%m-%d")
    parser = argparse.ArgumentParser(
        description="Orchard malware with DGA based on Bitcoin Genesis Block"
    )
    parser.add_argument(
        "-d", "--date",
        help="date for which to generate domains, e.g., 2022-05-09",
        default=now,
        type=date_parser
    )
    parser.add_argument(
        "-b", "--blockchain",
        help="also generate blockchain domains, requires blockchain db",
        action='store_true'
    )
    parser.add_argument(
        "-u", "--update",
        help="just update of blockchain db",
        action='store_true'
    )
    args = parser.parse_args()

    if args.update:
        refresh_blockchain_db()
        exit()

    for domain in dga(args.date, args.blockchain):
        print(domain)

Characteristics of the DGA

The following table summarizes the properties of the DGA

C2 Communication

The DGA domains — along with the hardcoded domain (ojena.duckdns.org) — are used to control XMRig. First, the client sends some fingerprinting information like the following to the C2. The data is sent unencrypted to a hardcoded port, in my sample to port 25654:

{
  "Active_Window": "*internet",
  "Antivirus": ["Windows Defender"],
  "Authenticate_Type": 0,
  "CPU_Model": "12th Gen Intel(R) Core(TM) i9-12900K",
  "Camera": false,
  "Elevated": false,
  "GPU_Models": [
    {
      "Name": "-",
      "Type": 2
    }
  ],
  "Identity": "F12C8A2E\Username\DESKTOP-XYZ",
  "Operating_System": "{\"BuildNumber\":19044,\"MajorVersion\":10,\"MinorVersion\":0,\"ProductType\":false}",
  "Ram_Size": 17179332608,
  "System_Architecture": 1,
  "Threads": 4,
  "Version": 2
}

After that, the XMRig JSON RPC communication is also sent — again unencrypted — to the C2. For example the login:

{
  "id": 1, 
  "jsonrpc": "2.0", 
  "method": "login", 
  "params": {
    "login": "CPU", 
    "pass": "x",
    "agent": "XMRig/6.15.2 (Windows NT 10.0; Win64; x64) libuv/1.38.0 msvc/2019",
    "algo": ["cn/0", "cn/1", "cn/2", "cn/r", "cn/fast", "cn/half", "cn/xao", 
      "cn/rto", "cn/rwz", "cn/zls", "cn/double", "cn/ccx", "cn-lite/0", 
      "cn-lite/1", "cn-heavy/0", "cn-heavy/tube", "cn-heavy/xhv", "cn-pico", 
      "cn-pico/tlo", "cn/upx2", "rx/0", "rx/wow", "rx/arq", "rx/graft", 
      "rx/sfx", "rx/keva", "argon2/chukwa", "argon2/chukwav2", "argon2/ninja", 
      "astrobwt"]
  }
}

These JSON RPC connections of XMRig will be detected by many Suricata rules.

Detection

Speaking of detections: there are many YARA rule hits you will see both on VirusTotal and MalwareBazaar:

Unfortunately, all these rules by The DFIR Report are non functional ². I wrote the following YARA rule, but it only matches on the unpacked sample, e.g., on memory dumps:

rule win_bitcoin_genesis_b9 {

    meta:
        author      = "Johannes Bader @viql"
        date        = "2022-07-22"
        description = "detects a downloader with a DGA based on the Bitcoin Genesis Block"
        tlp         = "TLP:WHITE"
        version     = "v1.0"
        hash_md5    = "5c13ee5dbe45d02ed74ef101b2e82ae6"
        hash_sha1   = "bdc36bc233675e7a96faa2c4917e9b756cc2a2a0"
        hash_sha256 = "ad1e39076212d8d58ff45d1e24d681fe0c600304bd20388cddcf9182b1d28c2f"

    strings:
        $str_json_1 = "\"bytes\": ["
        $str_json_2 = "\"subtype\": "
        $str_json_3 = "{\"bytes\":["
        $str_json_4 = "],\"subtype\":"
        $str_json_5 = "null}"
        $str_json_6 = "<discarded>"
        $str_json_7 = "[json.exception."

        /*
            mov     dl, [ebp+var_14]
            mov     [eax+ecx], dl
            mov     byte ptr [eax+ecx+1], 0
            jmp     short loc_3CBF9F
        */
        $split_hash_1 = {8A 55 ?? 88 14 08 C6 44 08 01 00 EB}
        /*
            inc     ebx
            cmp     ebx, 10h
            jl      loc_3CBF10
        */
        $split_hash_2 = {43 83 FB 10 0F 8C}

        /*
            push    0
            push    0
            mov     [ebp-14h], edx
            mov     [ebp-18h], eax
        */
        $format_the_date = {6A 00 6A 00 89 55 EC 89 45 E8}

    condition:
        uint16(0) == 0x5A4D and 
        all of ($str_json_*) and
        all of ($split_hash_*) and
        $format_the_date
}