Detects WannaCryptor spreaded during 2017-May-12th campaign and variants in memory

Hunt Hypothesis

This hunt targets adversary behavior where ransomware variants, specifically those resembling the 2017 WannaCryptor campaign, execute malicious processes directly within system memory to evade traditional disk-based detection. Proactively hunting for these in-memory signatures within Azure Sentinel is critical because it allows the SOC team to identify stealthy lateral movement and early-stage infections that standard file-integrity rules might miss, thereby reducing dwell time before encryption occurs.

YARA Rule

rule wannacry_memory_ransom : wannacry_memory_ransom {

meta:

description = "Detects WannaCryptor spreaded during 2017-May-12th campaign and variants in memory"

author = "Blueliv"

reference = "https://blueliv.com/research/wannacrypt-malware-analysis/"

date = "2017-05-15"

strings:

$s01 = "%08X.eky"

$s02 = "%08X.pky"

$s03 = "%08X.res"

$s04 = "%08X.dky"

$s05 = "@WanaDecryptor@.exe"

condition:

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://blueliv.com/research/wannacrypt-malware-analysis/
• Source Rule

False Positive Guidance

Here are 4 specific false positive scenarios for the WannaCryptor detection rule, tailored to a legitimate enterprise environment:

• Enterprise Patch Deployment via SCCM/Intune

  • Scenario: During the monthly patch Tuesday cycle, System Center Configuration Manager (SCCM) or Microsoft Intune pushes Windows updates containing the MS17-010 security bulletin. The deployment client (ccmexec.exe or Microsoft.Agent.Core) executes a memory scan and file integrity check on the svchost.exe process to verify patch installation status, mimicking the behavior of WannaCryptor scanning for vulnerable systems.
  • Filter/Exclusion: Exclude processes with parent image paths containing \ccm\executive\ or \Microsoft Intune Management Extension\. Additionally, filter out events where the process hash matches known Microsoft-signed binaries (e.g., Publisher: Microsoft Windows).

• Scheduled Antivirus Deep Scan

  • Scenario: A nightly scheduled task triggers a “Deep Scan” using enterprise-grade EDR solutions like CrowdStrike Falcon or Symantec Endpoint Protection. The scanner injects its own memory modules into running processes to detect in-memory threats, generating high-volume file access and registry modification events that overlap with the WannaCryptor signature logic (specifically looking for lsass.exe or explorer.exe interactions).
  • Filter/Exclusion: Create an exclusion rule for process names associated with known EDR agents (e.g., csfalconagent.exe, Symantec Endpoint Protection.exe, McAfeeVirusScanService). Filter events occurring strictly between 02:00 and 04:00 UTC, which aligns with the standard maintenance window for these scans.

• Group Policy Object (GPO) Enforcement

  • Scenario: An administrator forces a Group Policy update (`gpupdate