{
  "family": "sysn",
  "sample_count": 99,
  "category": "trojan",
  "description": "Trojan:Win32/Sysn is a broad heuristic classification for trojans that aggressively modify critical system files, registry configurations, and system policies to establish deep persistence and compromise the underlying operating system.<br><br><b>Understanding Sysn</b><br>To the end-user, a Sysn infection may manifest as strange system behavior, the inability to open certain applications, or sudden reboots. For incident responders, a Sysn detection indicates a severe compromise of the OS integrity. Sysn is not a single malware family, but rather a behavioral signature used by Microsoft Defender to flag executables that are making unauthorized, high-risk changes to the core of Windows.<br><br><b>Execution and Evasion Strategies</b><br>Sysn variants are distributed through various vectors, including spearphishing, exploit kits, and software vulnerabilities. Upon execution, the primary objective is to alter the system to facilitate further malicious activity. This includes modifying the `HOSTS` file to block access to security vendors, disabling Windows Defender or third-party AV via registry modifications (`DisableAntiSpyware`), altering system proxy settings to intercept traffic, or replacing legitimate system binaries (like `svchost.exe`) with trojanized versions.<br><br><b>Indicators of Compromise & Impact</b><br>The impact is a fundamentally untrustworthy endpoint. EDR platforms will trigger heavily on a Sysn infection due to the aggressive registry modifications (`T1112`) and attempts to impair defenses (`T1562.001`). Threat hunters should utilize file integrity monitoring (FIM) tools to identify modified system files in the `%SystemRoot%\\System32` directory. Anomalous outbound network traffic bypassing the standard corporate proxy (due to modified proxy settings) is also a strong indicator.",
  "cta": "Published by the SystemHelpdesk team.",
  "aliases": [],
  "enrichment_level": "expert-seo",
  "faq": [],
  "faq_count": 0,
  "mitre_attack": [
    "T1543.003",
    "T1562.001",
    "T1112",
    "T1562.004",
    "T1547.001"
  ],
  "cisa_advisory": null,
  "last_updated": "2026-06-09",
  "mitre_attack_detail": [
    {
      "id": "T1112",
      "name": "Modify Registry",
      "tactic": "Defense Evasion"
    },
    {
      "id": "T1562.001",
      "name": "Impair Defenses: Disable or Modify Tools",
      "tactic": "Defense Evasion"
    },
    {
      "id": "T1562.004",
      "name": "Impair Defenses: Disable or Modify System Firewall",
      "tactic": "Defense Evasion"
    },
    {
      "id": "T1547.001",
      "name": "Boot or Logon Autostart Execution: Registry Run Keys / Startup Folder",
      "tactic": "Persistence"
    },
    {
      "id": "T1543.003",
      "name": "Create or Modify System Process: Windows Service",
      "tactic": "Persistence"
    }
  ],
  "containment_steps": [
    "Immediately isolate the endpoint; the aggressive system modifications make the machine fundamentally untrustworthy and likely open to secondary payloads.",
    "Utilize EDR or offline registry analysis tools to identify and reverse the malicious registry modifications (especially those disabling security tools).",
    "Audit the `HOSTS` file (`C:\\Windows\\System32\\drivers\\etc\\hosts`) and remove any entries redirecting security vendor domains to `127.0.0.1`.",
    "Due to the depth of the system modifications, the most secure remediation is a complete, clean rebuild of the operating system."
  ],
  "what_to_avoid": [
    "Do not trust the endpoint's built-in security tools or logs; a Sysn infection is explicitly designed to disable or tamper with these mechanisms.",
    "Avoid relying solely on automated AV removal, as the malware may have replaced legitimate system files that AV cannot safely delete."
  ],
  "target_industries": [
    "Global / Opportunistic"
  ],
  "motivation": "Opportunistic",
  "threat_actors": [
    "Unknown / Cybercriminal"
  ],
  "target_geographies": [
    "Global"
  ]
}