{
  "family": "pynamer",
  "sample_count": 67,
  "category": "ransomware",
  "description": "VirTool:Win32/Pynamer is a heuristic detection used by antivirus engines to flag suspicious executables that have been compiled or packaged using Python-based compilation tools (such as PyInstaller, py2exe, or cx_Freeze). While not exclusively malicious, it strongly indicates a payload designed to evade traditional signature-based detection.<br><br><b>Understanding Pynamer</b><br>To an end-user, this detection simply indicates a blocked threat. For a malware analyst, the 'Pynamer' (Python-namer) tag provides immediate structural context. Python is a highly versatile scripting language heavily favored by modern threat actors to quickly write infostealers, ransomware, and custom RATs. To execute on a victim machine that doesn't have Python installed, the attacker 'freezes' the script into a standalone `.exe` using tools like PyInstaller. This bundles the Python interpreter, the required libraries, and the malicious script (often obfuscated or encrypted) into a single, massive executable.<br><br><b>Execution and Threat Hunting</b><br>Pynamer-flagged executables are large (often 5MB to 20MB+) due to the bundled interpreter. When executed, the PyInstaller stub unpacks the Python environment and the malicious bytecode (`.pyc` files) into the `%Temp%` directory, often creating a folder named `_MEIxxxxxx`. The stub then executes the script. Threat hunters should investigate EDR alerts for massive executables suddenly dropping hundreds of `.pyd` or `.dll` files into the Temp directory and executing them. Because the core logic is Python bytecode, static analysis of the original `.exe` is ineffective; analysts must unpack the binary to retrieve the `.pyc` files.<br><br><b>Forensic Analysis & Impact</b><br>The impact depends entirely on the hidden Python script. Incident responders must locate the `_MEIxxxxxx` folder in the user's Temp directory. Forensic analysts will use tools like `pyinstxtractor` to unpack the executable, followed by decompilers like `uncompyle6` or `decompyle3` to reverse the `.pyc` bytecode back into readable Python source code, revealing the exact capabilities of the malware.",
  "cta": "Published by the SystemHelpdesk team.",
  "aliases": [],
  "enrichment_level": "expert-seo",
  "faq": [],
  "faq_count": 0,
  "mitre_attack": [
    "T1566.001",
    "T1027.002",
    "T1059.006",
    "T1027",
    "T1129"
  ],
  "cisa_advisory": null,
  "last_updated": "2026-06-09",
  "mitre_attack_detail": [
    {
      "id": "T1027.002",
      "name": "Obfuscated Files or Information: Software Packing",
      "tactic": "Defense Evasion"
    },
    {
      "id": "T1059.006",
      "name": "Command and Scripting Interpreter: Python",
      "tactic": "Execution"
    },
    {
      "id": "T1129",
      "name": "Shared Modules",
      "tactic": "Execution"
    },
    {
      "id": "T1027",
      "name": "Obfuscated Files or Information",
      "tactic": "Defense Evasion"
    },
    {
      "id": "T1566.001",
      "name": "Phishing: Spearphishing Attachment",
      "tactic": "Initial Access"
    }
  ],
  "containment_steps": [
    "Quarantine the endpoint to halt the execution of the unpacked Python payload, which could be an infostealer or ransomware.",
    "Clear the Windows `%Temp%` directory, specifically looking for and deleting any `_MEIxxxxxx` folders created by the PyInstaller stub.",
    "Submit the original massive executable to a malware analyst for unpacking and decompilation to understand the specific threat.",
    "Perform a full system sweep to determine the initial delivery vector (e.g., a phishing email or a compromised download)."
  ],
  "what_to_avoid": [
    "Do not waste time trying to statically analyze the original `.exe` file using standard dissassemblers; you must unpack the Python bytecode first.",
    "Avoid assuming the threat is contained just by deleting the `.exe`; the script may have already executed and established persistence elsewhere."
  ],
  "target_industries": [
    "Global / Opportunistic"
  ],
  "motivation": "Opportunistic",
  "threat_actors": [
    "Unknown / Cybercriminal"
  ],
  "target_geographies": [
    "Global"
  ]
}