malware-analyst – Security skill for AI agents

Defensive malware triage

Use only an authorized sample in a disposable isolated analysis environment. Disable shared folders, clipboard, host credentials and live outbound networking before dynamic execution; use simulated services. Do not upload private samples to public sandboxes without explicit disclosure approval. Static-tool parsers are also attack surfaces.

# File identification
file sample.exe
sha256sum sample.exe

# String extraction
strings -a sample.exe | head -100
FLOSS sample.exe  # Obfuscated strings

# Packer detection
diec sample.exe   # Detect It Easy
exeinfope sample.exe

# Import analysis
rabin2 -i sample.exe
dumpbin /imports sample.exe

Phase 3: Static Analysis

1. Load in disassembler: IDA Pro, Ghidra, or Binary Ninja

2. Identify main functionality: Entry point, WinMain, DllMain

3. Map execution flow: Key decision points, loops

4. Identify capabilities: Network, file, registry, process operations

5. Extract IOCs: C2 addresses, file paths, mutex names

Phase 4: Dynamic Analysis

1. Environment Setup:
   - Windows VM with common software installed
   - Process Monitor, Wireshark, Regshot
   - API Monitor or x64dbg with logging
   - INetSim or FakeNet for network simulation

2. Execution:
   - Start monitoring tools
   - Execute sample
   - Observe behavior for 5-10 minutes
   - Trigger only approved behavior against simulated or contained network services

3. Documentation:
   - Network connections attempted
   - Files created/modified
   - Registry changes
   - Processes spawned
   - Persistence mechanisms

Use this skill when

• Working on file identification tasks or workflows
• Needing guidance, best practices, or checklists for file identification

Do not use this skill when

• The task is unrelated to file identification
• You need a different domain or tool outside this scope

Instructions

• Clarify goals, constraints, and required inputs.
• Apply relevant best practices and validate outcomes.
• Provide actionable steps and verification.
• The referenced implementation playbook is not bundled; use the inline methodology and inspected tool documentation.

Common Malware Techniques

Persistence Mechanisms

Registry Run keys       - HKCU/HKLM\Software\Microsoft\Windows\CurrentVersion\Run
Scheduled tasks         - schtasks, Task Scheduler
Services               - CreateService, sc.exe
WMI subscriptions      - Event subscriptions for execution
DLL hijacking          - Plant DLLs in search path
COM hijacking          - Registry CLSID modifications
Startup folder         - %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup
Boot records           - MBR/VBR modification

Evasion Techniques

Anti-VM                - CPUID, registry checks, timing
Anti-debugging         - IsDebuggerPresent, NtQueryInformationProcess
Anti-sandbox           - Sleep acceleration detection, mouse movement
Packing                - UPX, Themida, VMProtect, custom packers
Obfuscation           - String encryption, control flow flattening
Process hollowing      - Inject into legitimate process
Living-off-the-land    - Use built-in tools (PowerShell, certutil)

C2 Communication

HTTP/HTTPS            - Web traffic to blend in
DNS tunneling         - Data exfil via DNS queries
Domain generation     - DGA for resilient C2
Fast flux             - Rapidly changing DNS
Tor/I2P               - Anonymity networks
Social media          - Twitter, Pastebin as C2 channels
Cloud services        - Legitimate services as C2

Tool Proficiency

Analysis Platforms

Cuckoo Sandbox       - Open-source automated analysis
ANY.RUN              - Interactive cloud sandbox
Hybrid Analysis      - VirusTotal alternative
Joe Sandbox          - Enterprise sandbox solution
CAPE                 - Cuckoo fork with enhancements

Monitoring Tools

Process Monitor      - File, registry, process activity
Process Hacker       - Advanced process management
Wireshark            - Network packet capture
API Monitor          - Win32 API call logging
Regshot              - Registry change comparison

Unpacking Tools

Unipacker            - Automated unpacking framework
x64dbg + plugins     - Scylla for IAT reconstruction
OllyDumpEx           - Memory dump and rebuild
PE-sieve             - Detect hollowed processes
UPX                  - For UPX-packed samples

IOC Extraction

Indicators to Extract

Network:
  - IP addresses (C2 servers)
  - Domain names
  - URLs
  - User-Agent strings
  - JA3/JA3S fingerprints

File System:
  - File paths created
  - File hashes (MD5, SHA1, SHA256)
  - File names
  - Mutex names

Registry:
  - Registry keys modified
  - Persistence locations

Process:
  - Process names
  - Command line arguments
  - Injected processes

YARA Rules

rule Malware_Generic_Packer
{
    meta:
        description = "Detects common packer characteristics"
        author = "Security Analyst"

    strings:
        $mz = { 4D 5A }
        $upx = "UPX!" ascii
        $section = ".packed" ascii

    condition:
        $mz at 0 and ($upx or $section)
}

Reporting Framework

Analysis Report Structure

# Malware Analysis Report

## Executive Summary
- Sample identification
- Key findings
- Threat level assessment

## Sample Information
- Hashes (MD5, SHA1, SHA256)
- File type and size
- Compilation timestamp
- Packer information

## Static Analysis
- Imports and exports
- Strings of interest
- Code analysis findings

## Dynamic Analysis
- Execution behavior
- Network activity
- Persistence mechanisms
- Evasion techniques

## Indicators of Compromise
- Network IOCs
- File system IOCs
- Registry IOCs

## Recommendations
- Detection rules
- Mitigation steps
- Remediation guidance

Ethical Guidelines

Appropriate Use

• Incident response and forensics
• Threat intelligence research
• Security product development
• Academic research
• CTF competitions

Never Assist With

• Creating or distributing malware
• Attacking systems without authorization
• Evading security products maliciously
• Building botnets or C2 infrastructure
• Any offensive operations without proper authorization

Response Approach

1. Verify context: Ensure defensive/authorized purpose

2. Assess sample: Quick triage to understand what we're dealing with

3. Recommend approach: Appropriate analysis methodology

4. Guide analysis: Step-by-step instructions with safety considerations

5. Extract value: IOCs, detection rules, understanding

6. Document findings: Clear reporting for stakeholders

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.