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

1. Understand Hollowing Mechanics: Process hollowing involves creating a legitimate process in suspended state, unmapping its memory, writing malicious code, then resuming execution.
2. Monitor Suspended Process Creation: Hunt for processes created with CREATE_SUSPENDED flag followed by memory writes and thread resumption.
3. Detect Memory Section Anomalies: Identify processes where the in-memory image differs from the on-disk binary (image mismatch).
4. Analyze Parent-Child Process Trees: Flag processes whose behavior does not match their binary name (e.g., svchost.exe making unusual network connections).
5. Check Process Integrity: Compare process memory sections against the legitimate binary on disk.
6. Correlate with Network Activity: Hollowed processes often establish C2 connections - correlate suspicious process behavior with network logs.
7. 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

1. Svchost.exe Hollowing: Malware creates svchost.exe suspended, hollows it, injects backdoor code - process appears legitimate but behaves maliciously.
2. Explorer.exe Hollowing: Attacker hollows explorer.exe to inherit its network permissions and trusted process context.
3. Rundll32 Hollowing: Malicious loader creates rundll32.exe, replaces its memory with implant code for C2 beaconing.
4. 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]