Detect Dynamic API Resolution in Splunk
Adversaries may obfuscate then dynamically resolve API functions called by their malware in order to conceal malicious functionalities and impair defensive analysis. API functions called by malware leave static artifacts such as strings in payload files and in the Import Address Table (IAT). To avoid static analysis, adversaries use dynamic API resolution: hashes of function names are stored in malware in lieu of literal strings, and malware uses GetProcAddress() and LoadLibrary() to manually reproduce the linking process. Threat actors including Mustang Panda, Lazarus Group, Latrodectus, Bazar, Brute Ratel C4, TONESHELL, PlugX, Raccoon Stealer, AvosLocker, and CHIMNEYSWEEP use this technique.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1027 Obfuscated Files or Information
- Sub-technique
- T1027.007 Dynamic API Resolution
- Canonical reference
- https://attack.mitre.org/techniques/T1027/007/
SPL Detection Query
index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=7
(ImageLoaded="*\\kernel32.dll" OR ImageLoaded="*\\kernelbase.dll" OR ImageLoaded="*\\ntdll.dll")
NOT (Image="C:\\Windows\\*" OR Image="C:\\Program Files\\*" OR Image="C:\\Program Files (x86)\\*")
| stats
dc(ImageLoaded) as CoreDLLLoads,
values(ImageLoaded) as LoadedLibs,
count as TotalEvents
by host, Image, ProcessGuid
| where CoreDLLLoads >= 1
| join type=left ProcessGuid [
search index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=7
NOT (Image="C:\\Windows\\*" OR Image="C:\\Program Files\\*")
| stats dc(ImageLoaded) as AllModules by ProcessGuid
]
| where AllModules < 5
| table host, Image, CoreDLLLoads, AllModules, LoadedLibs
| sort AllModules ascDetects processes with very few total DLL loads (fewer than 5) that are loading core Windows libraries, from non-standard paths. This minimal import pattern indicates a packed or dynamically-resolving binary that suppresses its static IAT. Malware using API hashing typically loads only LoadLibrary and GetProcAddress at import time, then resolves all other APIs at runtime.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Portable executable tools with genuinely minimal imports for cross-compatibility reasons
- Certain .NET executables that use P/Invoke rather than static imports for cross-version compatibility
- Security research tools designed to minimize their import footprint for analysis purposes
- Packed legitimate software where the packer stub has minimal imports before decompressing the real binary
Other platforms for T1027.007
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 1Demonstrate GetProcAddress Dynamic API Resolution in PowerShell
Expected signal: Sysmon Event ID 1: PowerShell process creation with Add-Type and DllImport. PowerShell ScriptBlock Log Event ID 4104: the P/Invoke declarations and GetProcAddress call. Sysmon Event ID 7: user32.dll loaded by powershell.exe.
- Test 2API Hash Resolution Simulation
Expected signal: PowerShell ScriptBlock Log Event ID 4104: the hash function implementation and the list of API names being hashed. The output shows API-to-hash mappings as adversarial malware would store them.
- Test 3Inspect Binary IAT for Dynamic Resolution Indicators
Expected signal: Sysmon Event ID 1: dumpbin.exe execution with /imports argument on calc.exe. The findstr filter shows LoadLibrary and GetProcAddress imports if present.
- Test 4Create Minimal-Import Executable for Testing
Expected signal: Sysmon Event ID 1: csc.exe compilation (T1027.004 indicator). Sysmon Event ID 1: dynapi.exe execution from Temp. Sysmon Event ID 7: kernel32.dll and user32.dll loaded by dynapi.exe. The dynapi.exe IAT will contain only LoadLibrary and GetProcAddress.
References (5)
- https://attack.mitre.org/techniques/T1027/007/
- https://www.huntress.com/blog/hackers-no-hashing-randomizing-api-hashes-to-evade-cobalt-strike-shellcode-detection
- https://www.blackhat.com/docs/us-15/materials/us-15-Choi-API-Deobfuscator-Resolving-Obfuscated-API-Functions-In-Modern-Packers.pdf
- https://www.ired.team/offensive-security/defense-evasion/windows-api-hashing-in-malware
- https://dr4k0nia.github.io/dotnet/coding/2022/08/10/HInvoke-and-avoiding-PInvoke.html
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