Detect Polymorphic Code in CrowdStrike LogScale
Adversaries use polymorphic (also called metamorphic or mutating) code to evade signature-based defenses by altering the malware's runtime footprint on each execution. The code mutates into a different version while preserving its original functionality — defeating hash-based and pattern-based detection. Mutation engines perform operations like instruction substitution, code transposition, dead code insertion, register reassignment, and encryption key rotation. BendyBear (attributed to APT41/Winnti) is a documented example. Polymorphic code is often combined with other techniques: software packing, command obfuscation, and encrypted/encoded payloads to create layered evasion. Detection must rely on behavioral indicators rather than static signatures.
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/
LogScale Detection Query
// T1027.014 - Polymorphic Code: CrowdStrike LogScale (Falcon) Detection
// Branch 1: PE files written to staging directories by script interpreters
#event_simpleName=WriteFile
| FileName = /(?i)\.(exe|dll|scr|bin)$/
| FilePath = /(?i)\\(Temp|AppData|ProgramData|Public)\\/
| ImageFileName = /(?i)(powershell|cmd|wscript|cscript|mshta)\.exe$/
| "pe_staged_by_interpreter" as DetectionType
| 65 as RiskScore
// Branch 2: PowerShell using memory manipulation APIs
| union [
#event_simpleName=ProcessRollup2
| ImageFileName = /(?i)powershell\.exe$/
| CommandLine = /(?i)(WriteAllBytes|WriteProcessMemory|VirtualAlloc|VirtualProtect|NtWriteVirtualMemory)/
| "memory_manipulation_api" as DetectionType
| 85 as RiskScore
]
// Branch 3: cmd.exe copy-and-execute binary chain
| union [
#event_simpleName=ProcessRollup2
| ImageFileName = /(?i)cmd\.exe$/
| CommandLine = /(?i)copy.*\.exe.*&&.*start/
| "binary_copy_execute" as DetectionType
| 70 as RiskScore
]
// Branch 4: Module loads from staging directories (non-system path DLL loads)
| union [
#event_simpleName=ClassifiedModuleLoad
| NOT ImageFileName = /(?i)(\\Windows\\|\\Program Files|System32|SysWOW64)/
| ImageFileName = /(?i)\\(Temp|AppData|ProgramData|Public)\\/
| "unsigned_dll_load_staging" as DetectionType
| 75 as RiskScore
]
| table([timestamp, ComputerName, DetectionType, RiskScore, ImageFileName, CommandLine, FileName, FilePath, UserName])
| sort(RiskScore, order=desc)CrowdStrike LogScale (Falcon) CQL query detecting T1027.014 polymorphic code TTPs via Falcon telemetry: PE file writes to staging paths by interpreter processes, PowerShell memory manipulation API chains, cmd.exe copy-execute patterns, and DLL loads from non-standard directories.
Data Sources
Required Tables
False Positives & Tuning
- Software provisioning tools deployed via Falcon Fusion or RMM solutions that legitimately stage executables in AppData during managed deployments
- PowerShell-based application frameworks (.NET, WinForms apps) that use reflection and WriteAllBytes to extract embedded resources at runtime
- Security awareness or red team simulation tools (e.g., Atomic Red Team) running authorized T1027 simulation tests
Other platforms for T1027.014
Testing Methodology
Validate this detection against 4 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 1Self-Modifying Binary Simulation (PowerShell WriteAllBytes)
Expected signal: Sysmon EventCode 11 (FileCreate) for svchost32.exe written to %TEMP%\polytest. EventCode 1 (ProcessCreate) for powershell.exe with 'WriteAllBytes' in CommandLine. PowerShell Script Block Log EventCode 4104 will capture the full script.
- Test 2High-Entropy Binary Drop to AppData (Simulated Mutated Payload)
Expected signal: Sysmon EventCode 11 for explorer32.exe creation in %APPDATA%\Microsoft\Windows\Themes by powershell.exe. The file will have high entropy due to GZip compression.
- Test 3Binary Copy-Then-Execute Pattern (Mutation Simulation)
Expected signal: Sysmon EventCode 11 (FileCreate) for variant_001.exe and variant_002.exe in %TEMP%. EventCode 1 for cmd.exe with 'copy' and '&&' pattern in CommandLine.
- Test 4Unsigned DLL Load from AppData (Polymorphic DLL Variant)
Expected signal: Sysmon EventCode 11 for theme32.dll written to %APPDATA% by powershell.exe. EventCode 7 (ImageLoad) for rundll32.exe loading the DLL from a non-Windows/Program Files path. EventCode 1 for rundll32.exe process.
References (5)
- https://attack.mitre.org/techniques/T1027/014
- https://unit42.paloaltonetworks.com/bendybear/
- https://www.blackberry.com/us/en/solutions/endpoint-security/ransomware-protection/polymorphic-malware
- https://www.sentinelone.com/cybersecurity-101/threat-intelligence/what-is-polymorphic-malware
- https://medium.com/@shellseekerscyber/explainer-packed-malware-16f09cc75035
Unlock Pro Content
Get the full detection package for T1027.014 including response playbook, investigation guide, and atomic red team tests.