Detect Polymorphic Code in Elastic Security
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/
Elastic Detection Query
sequence by host.id with maxspan=5m
[file where event.action in ("creation", "overwrite") and
file.extension in ("exe", "dll", "scr", "bin") and
file.path : ("*\\Temp\\*", "*\\AppData\\*", "*\\ProgramData\\*", "*\\Users\\Public\\*") and
process.name : ("powershell.exe", "cmd.exe", "wscript.exe", "cscript.exe", "mshta.exe")] as event0
[process where event.action == "start" and
process.executable : ("*\\Temp\\*", "*\\AppData\\*", "*\\ProgramData\\*", "*\\Users\\Public\\*") and
process.executable == event0.file.path]
OR
process where event.action == "start" and
process.name : ("powershell.exe", "cmd.exe") and
process.args : ("*WriteAllBytes*", "*WriteProcessMemory*", "*VirtualAlloc*", "*VirtualProtect*", "*NtWriteVirtualMemory*", "*Set-Content*") and
process.args : ("*.exe*", "*.dll*", "*.scr*")
OR
sequence by host.id, process.parent.entity_id with maxspan=2m
[file where event.action in ("creation", "overwrite") and
file.extension in ("exe", "dll") and
file.directory == process.working_directory]
[process where event.action == "start" and
process.parent.name == process.name]Detects polymorphic code behavioral indicators using Elastic EQL sequences: PE files staged by interpreters then executed, memory manipulation API usage in script hosts, and self-modifying binary patterns where a process drops a new executable in its own working directory and immediately executes it.
Data Sources
Required Tables
False Positives & Tuning
- Software updaters and patch managers (e.g., Chrome, Firefox auto-update) that download and replace their own binaries in AppData
- Development tools like npm, pip, or cargo that compile and stage executables in temp directories during legitimate builds
- Enterprise software deployment tools (SCCM, Chocolatey, Ninite) that use PowerShell to stage and execute installers
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
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