Eradicating Malware from Infected Systems

When to Use

• Malware infection confirmed and containment is in place
• Forensic investigation has identified all persistence mechanisms
• All compromised systems have been identified and scoped
• Ready to remove attacker artifacts and restore clean state
• Post-containment phase requires systematic cleanup

Prerequisites

• Completed forensic analysis identifying all malware artifacts
• List of all compromised systems and accounts
• EDR/AV with updated signatures deployed
• YARA rules for the specific malware family
• Clean system images or verified backups for restoration
• Network isolation still in effect during eradication

Workflow

Step 1: Map All Persistence Mechanisms

# Windows - Check all known persistence locations
# Autoruns (Sysinternals) - comprehensive autostart enumeration
autorunsc.exe -accepteula -a * -c -h -s -v > autoruns_report.csv

# Registry Run keys
reg query "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /s
reg query "HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /s
reg query "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce" /s
reg query "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer\Run" /s

# Scheduled tasks
schtasks /query /fo CSV /v > schtasks_all.csv

# WMI event subscriptions
Get-WMIObject -Namespace root\Subscription -Class __EventFilter
Get-WMIObject -Namespace root\Subscription -Class CommandLineEventConsumer
Get-WMIObject -Namespace root\Subscription -Class __FilterToConsumerBinding

# Services
Get-Service | Where-Object {$_.Status -eq 'Running'} | Select-Object Name, DisplayName, BinaryPathName

# Linux persistence
cat /etc/crontab
ls -la /etc/cron.*/
ls -la /etc/init.d/
systemctl list-unit-files --type=service | grep enabled
cat /etc/rc.local
ls -la ~/.bashrc ~/.profile ~/.bash_profile

Step 2: Identify All Malware Artifacts

# Scan with YARA rules specific to the malware family
yara -r -s malware_rules/specific_family.yar C:\ 2>/dev/null

# Scan with multiple AV engines
# ClamAV scan
clamscan -r --infected --remove=no /mnt/infected_disk/

# Check for known malicious file hashes
find / -type f -newer /tmp/baseline_timestamp -exec sha256sum {} \; 2>/dev/null | \
  while read hash file; do
    grep -q "$hash" known_malicious_hashes.txt && echo "MALICIOUS: $file ($hash)"
  done

# Check for web shells
find /var/www/ -name "*.php" -newer /tmp/baseline -exec grep -l "eval\|base64_decode\|system\|passthru\|shell_exec" {} \;

# Check for unauthorized SSH keys
find / -name "authorized_keys" -exec cat {} \; 2>/dev/null

Step 3: Remove Malware Files and Artifacts

# Remove identified malicious files (after forensic imaging)
# Windows
Remove-Item -Path "C:\Windows\Temp\malware.exe" -Force
Remove-Item -Path "C:\Users\Public\backdoor.dll" -Force

# Remove malicious scheduled tasks
schtasks /delete /tn "MaliciousTaskName" /f

# Remove WMI persistence
Get-WMIObject -Namespace root\Subscription -Class __EventFilter -Filter "Name='MalFilter'" | Remove-WMIObject
Get-WMIObject -Namespace root\Subscription -Class CommandLineEventConsumer -Filter "Name='MalConsumer'" | Remove-WMIObject

# Remove malicious registry entries
reg delete "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /v "MalEntry" /f

# Remove malicious services
sc stop "MalService" && sc delete "MalService"

# Linux - Remove malicious cron entries, binaries, SSH keys
crontab -r  # Remove entire crontab (or edit specific entries)
rm -f /tmp/.hidden_backdoor
sed -i '/malicious_key/d' ~/.ssh/authorized_keys
systemctl disable malicious-service && rm /etc/systemd/system/malicious-service.service

Step 4: Reset Compromised Credentials

# Reset all compromised user passwords
Import-Module ActiveDirectory
Get-ADUser -Filter * -SearchBase "OU=CompromisedUsers,DC=domain,DC=com" |
  Set-ADAccountPassword -Reset -NewPassword (ConvertTo-SecureString "TempP@ss!$(Get-Random)" -AsPlainText -Force)

# Reset KRBTGT password (twice, 12+ hours apart for Kerberos golden ticket attack)
Reset-KrbtgtPassword -DomainController DC01
# Wait 12+ hours, then reset again
Reset-KrbtgtPassword -DomainController DC01

# Rotate service account passwords
Get-ADServiceAccount -Filter * | ForEach-Object {
  Reset-ADServiceAccountPassword -Identity $_.Name
}

# Revoke all Azure AD tokens
Get-AzureADUser -All $true | ForEach-Object {
  Revoke-AzureADUserAllRefreshToken -ObjectId $_.ObjectId
}

# Rotate API keys and secrets
# Application-specific credential rotation

Step 5: Patch Vulnerability Used for Initial Access

# Identify and patch the entry point vulnerability
# Windows Update
Install-WindowsUpdate -KBArticleID "KB5001234" -AcceptAll -AutoReboot

# Linux patching
apt update && apt upgrade -y  # Debian/Ubuntu
yum update -y                 # RHEL/CentOS

# Application-specific patches
# Update web application frameworks, CMS, etc.

# Verify patch was applied
Get-HotFix -Id "KB5001234"

Step 6: Validate Eradication

# Full system scan with updated signatures
# CrowdStrike Falcon - On-demand scan
curl -X POST "https://api.crowdstrike.com/scanner/entities/scans/v1" \
  -H "Authorization: Bearer $FALCON_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"ids": ["device_id"]}'

# Verify no persistence mechanisms remain
autorunsc.exe -accepteula -a * -c -h -s -v | findstr /i "unknown verified"

# Check for any remaining suspicious processes
Get-Process | Where-Object {$_.Path -notlike "C:\Windows\*" -and $_.Path -notlike "C:\Program Files*"}

# Verify no unauthorized network connections
Get-NetTCPConnection -State Established |
  Where-Object {$_.RemoteAddress -notlike "10.*" -and $_.RemoteAddress -notlike "172.16.*"} |
  Select-Object LocalPort, RemoteAddress, RemotePort, OwningProcess

# Run YARA rules again to confirm no artifacts remain
yara -r malware_rules/specific_family.yar C:\ 2>/dev/null

Key Concepts

Concept	Description
Persistence Mechanism	Method attacker uses to maintain access across reboots
Root Cause Remediation	Fixing the vulnerability that enabled initial compromise
Credential Rotation	Resetting all potentially compromised passwords and tokens
KRBTGT Reset	Invalidating Kerberos tickets after golden ticket attack
Indicator Sweep	Scanning all systems for known malicious artifacts
Validation Scan	Confirming eradication was successful before recovery
Re-imaging	Rebuilding systems from clean images rather than cleaning

Tools & Systems

Tool	Purpose
Sysinternals Autoruns	Enumerate all Windows autostart locations
YARA	Custom rule-based malware scanning
CrowdStrike/SentinelOne	EDR-based scanning and remediation
ClamAV	Open-source antivirus scanning
PowerShell	Scripted cleanup and validation
Velociraptor	Remote artifact collection and remediation

Common Scenarios

1. RAT with Multiple Persistence: Remote access trojan using registry, scheduled task, and WMI subscription. Must remove all three persistence mechanisms.
2. Web Shell on IIS/Apache: PHP/ASPX web shell in web root. Remove shell, audit all web files, patch application vulnerability.
3. Rootkit Infection: Kernel-level rootkit that survives cleanup. Requires full re-image from known-good media.
4. Fileless Malware: PowerShell-based attack living in memory and registry. Remove registry entries, clear WMI subscriptions, restart system.
5. Active Directory Compromise: Attacker created backdoor accounts and golden tickets. Reset KRBTGT, remove rogue accounts, audit group memberships.

Output Format

• Eradication action log with all removed artifacts
• Credential rotation confirmation report
• Vulnerability patching verification
• Post-eradication validation scan results
• Systems cleared for recovery phase