Detects WannaCry Ransomware

Hunt Hypothesis

This hypothesis posits that adversaries are attempting to replicate WannaCry ransomware behavior by exploiting unpatched SMB vulnerabilities within the Azure environment. Proactive hunting is essential because early identification of these specific lateral movement patterns allows the SOC team to isolate affected workloads before encryption spreads across critical cloud resources.

YARA Rule

rule WannaCry_Ransomware_Gen {
   meta:
      description = "Detects WannaCry Ransomware"
      author = "Florian Roth (based on rule by US CERT)"
      reference = "https://www.us-cert.gov/ncas/alerts/TA17-132A"
      date = "2017-05-12"
      hash1 = "9fe91d542952e145f2244572f314632d93eb1e8657621087b2ca7f7df2b0cb05"
      hash2 = "8e5b5841a3fe81cade259ce2a678ccb4451725bba71f6662d0cc1f08148da8df"
      hash3 = "4384bf4530fb2e35449a8e01c7e0ad94e3a25811ba94f7847c1e6612bbb45359"
   strings:
      $s1 = "__TREEID__PLACEHOLDER__" fullword ascii
      $s2 = "__USERID__PLACEHOLDER__" fullword ascii
      $s3 = "Windows for Workgroups 3.1a" fullword ascii
      $s4 = "PC NETWORK PROGRAM 1.0" fullword ascii
      $s5 = "LANMAN1.0" fullword ascii
   condition:
      uint16(0) == 0x5a4d and filesize < 5000KB and all of them
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 5 string patterns in its detection logic.

References

• https://www.us-cert.gov/ncas/alerts/TA17-132A
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the Detects WannaCry Ransomware rule in an enterprise environment, including suggested filters and exclusions:

• Windows Server Update Services (WSUS) Deployment of MS17-010 Patch

  • Scenario: During a scheduled maintenance window, the WSUS server pushes the critical security update KB4013389 (the patch for the EternalBlue exploit used by WannaCry) to thousands of endpoints. The installation process triggers the SMB service and attempts to write files to network shares, mimicking the file encryption behavior and SMB port scanning associated with the initial ransomware infection.
  • Filter/Exclusion: Create an exclusion for processes originating from the UpdateOrchestrator.exe or wuapp.exe services running under the SYSTEM account. Additionally, filter out alerts where the destination IP is the known WSUS server address and the event timestamp falls within the defined maintenance window (e.g., 02:00–04:00 UTC).

• Enterprise Backup Agent Performing Full Image Backups

  • Scenario: A scheduled job using Veeam Backup & Replication or Commvault initiates a full image backup. The agent locks and reads large volumes of data across the network via SMB, often generating high-volume file I/O and temporary file creation that resembles the rapid encryption activity of WannaCry spreading through a domain.
  • Filter/Exclusion: Exclude alerts where the parent process is VeeamAgent.exe, cvp2agent.exe, or similar known backup executables. Implement a logic filter to ignore events where the file extension being accessed matches common backup temporary formats (e.g., .vbk, .vib) rather than user document types like .docx or .xlsx.

• **Group Policy