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Upgrade to ProDetect Polymorphic Code in Splunk
This detection identifies behaviors associated with polymorphic or metamorphic malware that mutates its code or binary footprint at runtime to evade signature-based defenses. Because the binary hash changes with each execution, detections focus on behavioral indicators rather than static signatures: repeated file writes of executables with changing hashes, use of known mutation/packing engines, self-modifying process memory patterns, abnormal entropy in dropped files, and processes that write then immediately execute new PE files from temporary or user-writable directories. Correlation of these behaviors — especially when combined with Command and Scripting Interpreter activity or injection patterns — provides high-confidence indicators of polymorphic malware activity.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1027 Obfuscated Files or Information
- Sub-technique
- T1027.014 Polymorphic Code
- Canonical reference
- https://attack.mitre.org/techniques/T1027/014/
SPL Detection Query
index=* sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=11
| eval FolderPath=lower(TargetFilename)
| where match(FolderPath, "(?i)(\\temp\\|\\appdata\\|\\programdata\\|\\users\\public\\)")
| where match(TargetFilename, "(?i)\.(exe|dll|bin|scr)$")
| eval FileHash=coalesce(SHA256, MD5)
| stats
dc(FileHash) as UniqueHashes,
count as FilesWritten,
values(TargetFilename) as FileList
by _time span=1h, Computer, Image, ProcessId
| where UniqueHashes >= 3
| eval AlertReason="Process wrote PE files with ".UniqueHashes." distinct hashes to writable path"
| join type=inner Computer
[search index=* sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
| eval ExecPath=lower(Image)
| where match(ExecPath, "(?i)(\\temp\\|\\appdata\\|\\programdata\\|\\users\\public\\)")
| stats values(CommandLine) as ExecCmdLine, values(Image) as ExecImage by Computer, ParentImage]
| table _time, Computer, Image, ProcessId, UniqueHashes, FilesWritten, FileList, ExecImage, ExecCmdLine, AlertReason
| sort -UniqueHashesDetects polymorphic malware using Sysmon Event ID 11 (FileCreate) to find processes writing multiple uniquely-hashed PE files to temp/user-writable directories within an hour window (threshold: 3+ distinct hashes), correlated with Sysmon Event ID 1 (ProcessCreate) for subsequent execution from those directories.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Software deployment tools that stage multiple distinct binaries to AppData or ProgramData directories before installation
- Game launchers (Steam, Epic, Battle.net) patching game files with differentially-patched executable segments
- Antivirus or EDR update mechanisms that write multiple engine or definition binary blobs with unique hashes
Other platforms for T1027.014
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Simulate Polymorphic PE Mutation via XOR Re-encoding
Expected signal: DeviceFileEvents: 5 FileCreated events in %TEMP% with distinct SHA256 hashes, all with InitiatingProcessFileName=powershell.exe. Sysmon EventID 11 entries for each variant_N.bin file.
- Test 2Drop and Execute Multiple Renamed Copies of a Binary (Hash-Change via Append)
Expected signal: DeviceFileEvents: 4 FileCreated events for svc_update_N.exe in %TEMP% with distinct SHA256 hashes. DeviceProcessEvents: 4 ProcessCreate events for svc_update_N.exe running from %TEMP% with parent cmd.exe. Sysmon EventID 11 and 1 for each iteration.
- Test 3BendyBear-Style In-Memory Stub via PowerShell Reflection (Simulation)
Expected signal: DeviceProcessEvents: PowerShell process with unusual memory allocation patterns. If Defender for Endpoint AMSI/behavior monitoring is enabled: alerts on VirtualAlloc+VirtualProtect RW->RX page permission flip from powershell.exe. Process creation event for powershell.exe with -ExecutionPolicy Bypass flag.
References (5)
- https://attack.mitre.org/techniques/T1027/014/
- https://www.sentinelone.com/cybersecurity-101/polymorphic-malware/
- https://unit42.paloaltonetworks.com/bendybear/
- https://www.linkedin.com/pulse/polymorphic-malware-how-it-works-detection-evasion-/
- https://medium.com/@understanding-polymorphic-code-in-malware
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