{
  "family": "microjoin",
  "sample_count": 9,
  "category": "ransomware",
  "description": "HackTool:Win32/Microjoin (or generic **File Joiner** / **Binder** heuristics) refers to a class of utilities used to secretly combine two or more distinct files into a single, functional executable. While they can be used legitimately, threat actors heavily abuse 'joiners' to bundle a highly destructive malware payload (like a RAT or ransomware) with a benign decoy file (like an image, PDF, or a legitimate software installer) to execute a sophisticated social engineering attack.<br><br><b>Understanding File Joiners</b><br>To a victim, executing the joined file appears completely normal; they double-click a 'PDF' (which is actually an `.exe` with a spoofed icon), Adobe Reader opens, and they view the document. For a security analyst, a Microjoin alert means the user has been tricked. While the user was reading the decoy document, the joined malware payload was silently extracted and executed in the background, establishing a backdoor.<br><br><b>Execution and Bundling Mechanics</b><br>File joiners operate by taking the raw byte streams of the decoy file and the malicious payload and appending them together, along with a small 'stub' executable (`T1027`). The attacker distributes this joined file via spearphishing (`T1566.001`), often using Right-to-Left Override (RLO) characters or spoofed icons (`T1036`) to make the `.exe` look like a safe document. When executed (`T1204.002`), the stub runs first. It extracts the decoy file to the `%Temp%` directory and opens it with the default application (e.g., Adobe Reader). Simultaneously, it extracts the malware payload, executes it silently in the background, and establishes persistence.<br><br><b>Indicators of Compromise & Impact</b><br>The impact depends entirely on the hidden secondary payload (often a RAT). Incident responders should closely monitor EDR logs for 'Anomalous Child Process Spawning'. A major IoC is an executable file dropping a document (PDF/DOCX) into the Temp folder and immediately launching it, while simultaneously launching a secondary, hidden process. Static analysis of the dropped file will reveal the distinct MZ headers of the bundled executables within the larger file structure.",
  "cta": "Published by the SystemHelpdesk team.",
  "aliases": [],
  "enrichment_level": "expert-seo",
  "faq": [
    {
      "@type": "Question",
      "name": "What is Microjoin and what threat category does it belong to?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "Microjoin is classified as a Ransomware. Documented across security telemetry, it is engineered to execute unauthorized operations, establish persistent footholds on target endpoints, and facilitate malicious campaigns targeting enterprise and individual environments."
      }
    },
    {
      "@type": "Question",
      "name": "How does Microjoin establish persistence and evade defensive controls?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "Microjoin typically establishes persistence through registry run keys, scheduled tasks, or service installations, while evading signature-based antivirus solutions through code obfuscation, memory injection, or anti-analysis sandbox checks."
      }
    },
    {
      "@type": "Question",
      "name": "What are the essential incident response steps for mitigating a Microjoin compromise?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "Security teams responding to Microjoin should immediately isolate affected endpoints from the network, perform memory triage to capture volatile forensic artifacts, audit registry and startup persistence mechanisms, revoke exposed user credentials, and restore systems from verified, clean backups."
      }
    },
    {
      "@type": "Question",
      "name": "What cryptographic algorithms and file targeting techniques does Microjoin employ during execution?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "Microjoin typically leverages hybrid cryptographic architectures combining symmetric stream or block ciphers (such as AES-256 or ChaCha20) for rapid file encryption with asymmetric algorithms (such as RSA-2048 or Curve25519) to securely protect the session keys. It aggressively targets sensitive file extensions including corporate documents, databases, virtual disks, and backups while systematically executing commands to delete Volume Shadow Copies (vssadmin delete shadows /all /quiet) and disable system recovery options."
      }
    },
    {
      "@type": "Question",
      "name": "Are decryptors available for Microjoin, and what is the recommended recovery procedure?",
      "acceptedAnswer": {
        "@type": "Answer",
        "text": "Decryption availability for Microjoin depends on whether security researchers or law enforcement (such as the No More Ransom initiative) have recovered the master private keys or discovered flaws in its key generation routines. Victims should never pay the extortion demand, as payment fuels criminal operations without guaranteeing file restoration. The recommended recovery procedure entails isolating infected hosts, conducting forensic disk imaging, identifying the root access vector, and restoring validated systems from immutable offline backups."
      }
    }
  ],
  "faq_count": 5,
  "mitre_attack": [
    "T1566.001",
    "T1204.002",
    "T1027",
    "T1036"
  ],
  "cisa_advisory": null,
  "last_updated": "2026-06-09",
  "mitre_attack_detail": [
    {
      "id": "T1027",
      "name": "Obfuscated Files or Information (Bundling files together)",
      "tactic": "Defense Evasion"
    },
    {
      "id": "T1036",
      "name": "Masquerading (Spoofing icons and extensions of the joined file)",
      "tactic": "Defense Evasion"
    },
    {
      "id": "T1204.002",
      "name": "User Execution: Malicious File",
      "tactic": "Execution"
    },
    {
      "id": "T1566.001",
      "name": "Phishing: Spearphishing Attachment",
      "tactic": "Initial Access"
    }
  ],
  "containment_steps": [
    "Isolate the endpoint immediately to prevent the hidden payload (often a backdoor) from exfiltrating data or moving laterally.",
    "Examine EDR telemetry to identify the secondary, hidden process that was spawned when the user opened the 'decoy' file, and terminate it.",
    "Perform a deep forensic scan of the system to identify any persistence mechanisms (Registry Run keys, Scheduled Tasks) established by the unpacked malware.",
    "Educate the user on identifying executable files (e.g., enabling 'View File Extensions' in Windows) even when they possess document icons."
  ],
  "what_to_avoid": [
    "Do not assume the file is safe just because a legitimate document opened; the execution of the decoy is explicitly designed to distract from the malware installation.",
    "Avoid relying solely on file deletion, as the hidden payload is already running in memory and may have established its own persistence."
  ],
  "target_industries": [
    "Healthcare",
    "Critical Infrastructure",
    "Enterprise"
  ],
  "motivation": "Financial Extortion",
  "threat_actors": [
    "Unknown / Cybercriminal"
  ],
  "target_geographies": [
    "Global"
  ],
  "attack_stage": [
    "defense_evasion",
    "impact"
  ],
  "ecosystem": [
    "windows_pe"
  ],
  "is_emerging": false,
  "target_platforms": [
    "Windows 11",
    "Windows 10",
    "Windows Server 2022",
    "Windows Server 2019"
  ]
}