26zl Detecting Process Hollowing Technique
When to Use
- When investigating suspected fileless malware or in-memory threats
- After EDR alerts on process injection or suspicious memory operations
- When hunting for defense evasion techniques in a compromised environment
- When threat intel reports indicate process hollowing in active campaigns
- During purple team exercises validating T1055.012 detection coverage
Prerequisites
- EDR with memory protection monitoring (CrowdStrike, MDE, SentinelOne)
- Sysmon with Event IDs 1 (Process Create), 8 (CreateRemoteThread), 25 (ProcessTampering)
- Windows ETW providers for process hollowing (Microsoft-Windows-Kernel-Process)
- Memory forensics capabilities (Volatility, WinDbg)
- Process integrity monitoring tools
Workflow
- Understand Hollowing Mechanics: Process hollowing involves creating a legitimate process in suspended state, unmapping its memory, writing malicious code, then resuming execution.
- Monitor Suspended Process Creation: Hunt for processes created with CREATE_SUSPENDED flag followed by memory writes and thread resumption.
- Detect Memory Section Anomalies: Identify processes where the in-memory image differs from the on-disk binary (image mismatch).
- Analyze Parent-Child Process Trees: Flag processes whose behavior does not match their binary name (e.g., svchost.exe making unusual network connections).
- Check Process Integrity: Compare process memory sections against the legitimate binary on disk.
- Correlate with Network Activity: Hollowed processes often establish C2 connections - correlate suspicious process behavior with network logs.
- Document and Contain: Report findings, isolate affected endpoints, and update detection rules.
Key Concepts
| Concept | Description |
|---|---|
| T1055.012 | Process Injection: Process Hollowing |
| T1055 | Process Injection (parent technique) |
| T1055.001 | DLL Injection |
| T1055.003 | Thread Execution Hijacking |
| T1055.004 | Asynchronous Procedure Call |
| CREATE_SUSPENDED | Windows flag to create a process in suspended state |
| NtUnmapViewOfSection | API to unmap process memory sections |
| WriteProcessMemory | API to write into another process's memory |
| ResumeThread | API to resume a suspended thread |
| Image Mismatch | Process memory content differs from on-disk binary |
| Process Doppelganging | Related technique using NTFS transactions (T1055.013) |
Tools & Systems
| Tool | Purpose |
|---|---|
| CrowdStrike Falcon | Memory protection and hollowing detection |
| Microsoft Defender for Endpoint | ProcessTampering alerts |
| Sysmon v13+ | Event ID 25 ProcessTampering detection |
| Volatility | Memory forensics - malfind plugin |
| pe-sieve | Process memory scanner for hollowed processes |
| Hollows Hunter | Automated hollowed process detection |
| Process Hacker | Live process memory inspection |
| API Monitor | Monitor NtUnmapViewOfSection calls |
Common Scenarios
- Svchost.exe Hollowing: Malware creates svchost.exe suspended, hollows it, injects backdoor code - process appears legitimate but behaves maliciously.
- Explorer.exe Hollowing: Attacker hollows explorer.exe to inherit its network permissions and trusted process context.
- Rundll32 Hollowing: Malicious loader creates rundll32.exe, replaces its memory with implant code for C2 beaconing.
- Multi-Stage Hollowing: Loader uses process hollowing as first stage, then performs additional injection into services.
Output Format
Hunt ID: TH-HOLLOW-[DATE]-[SEQ]
Technique: T1055.012
Hollowed Process: [Process name and PID]
Original Binary: [Expected on-disk path]
Parent Process: [Parent name and PID]
Memory Mismatch: [Yes/No]
Suspicious APIs: [NtUnmapViewOfSection, WriteProcessMemory, etc.]
Network Activity: [C2 connections if any]
Host: [Hostname]
User: [Account context]
Risk Level: [Critical/High/Medium/Low]