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name: containing-active-security-breach description: Rapidly contain an active security breach by isolating compromised systems, blocking attacker communications, and preserving evidence while minimizing business disruption. domain: cybersecurity subdomain: incident-response tags: [incident-response, containment, breach-response, network-isolation, dfir] version: "1.0" author: mahipal license: Apache-2.0

Containing an Active Security Breach

When to Use

• Active unauthorized access detected on network or systems
• IDS/IPS alerts indicate ongoing exploitation or data exfiltration
• SOC analysts confirm a true positive security incident requiring immediate containment
• Lateral movement or privilege escalation observed in real time
• Ransomware encryption activity detected before full deployment

Prerequisites

• Incident Response Plan with defined containment procedures
• Network access to firewalls, switches, and endpoint management consoles
• EDR/XDR platform deployed across endpoints (CrowdStrike, SentinelOne, Microsoft Defender for Endpoint)
• SIEM access with real-time log correlation (Splunk, Elastic, QRadar)
• Pre-approved authority to isolate systems (documented in IR plan)
• Forensic imaging tools ready for evidence preservation

Workflow

Step 1: Validate and Classify the Incident

# Check SIEM for correlated alerts - Splunk example
index=security sourcetype=ids_alerts severity=critical
| stats count by src_ip, dest_ip, signature
| where count > 5
| sort -count

# Verify endpoint alerts via CrowdStrike Falcon API
curl -X GET "https://api.crowdstrike.com/detects/queries/detects/v1?filter=status:'new'+max_severity_displayname:'Critical'" \
  -H "Authorization: Bearer $FALCON_TOKEN"

Step 2: Identify Scope of Compromise

# Identify all systems communicating with attacker C2
# Using Zeek connection logs
cat conn.log | zeek-cut id.orig_h id.resp_h id.resp_p duration orig_bytes resp_bytes \
  | awk '$3 == 443 && $5 > 1000000' | sort -t#x27;\t' -k5 -rn | head -20

# Check for lateral movement in Windows Event Logs
wevtutil qe Security /q:"*[System[(EventID=4624)] and EventData[Data[@Name='LogonType']='3']]" /f:text /c:50

# Query Active Directory for recent authentication anomalies
Get-WinEvent -FilterHashtable @{LogName='Security';ID=4625} -MaxEvents 100 |
  Group-Object -Property {$_.Properties[5].Value} | Sort-Object Count -Descending

Step 3: Execute Network Containment

# Block attacker IP at perimeter firewall (Palo Alto example)
set cli pager off
configure
set rulebase security rules emergency-block from any to any source [attacker_ip] action deny
set rulebase security rules emergency-block from any to any destination [attacker_ip] action deny
commit force

# Isolate compromised VLAN at switch level (Cisco)
configure terminal
interface vlan 100
  shutdown
end
write memory

# Block C2 domains at DNS level
# Add to DNS sinkhole or RPZ
echo "attacker-c2-domain.com CNAME ." >> /etc/bind/rpz.local
rndc reload

Step 4: Isolate Compromised Endpoints

# CrowdStrike - Network contain host via API
curl -X POST "https://api.crowdstrike.com/devices/entities/devices-actions/v2?action_name=contain" \
  -H "Authorization: Bearer $FALCON_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"ids": ["device_id_1", "device_id_2"]}'

# Microsoft Defender for Endpoint - Isolate machine
curl -X POST "https://api.securitycenter.microsoft.com/api/machines/{machineId}/isolate" \
  -H "Authorization: Bearer $MDE_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"Comment": "IR-2024-001: Active breach containment", "IsolationType": "Full"}'

# SentinelOne - Disconnect from network
curl -X POST "https://usea1.sentinelone.net/web/api/v2.1/agents/actions/disconnect" \
  -H "Authorization: ApiToken $S1_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"filter": {"ids": ["agent_id"]}, "data": {}}'

Step 5: Preserve Volatile Evidence Before Full Isolation

# Capture live memory from compromised Windows host
winpmem_mini_x64.exe memdump_hostname_$(date +%Y%m%d).raw

# Capture network connections and running processes
netstat -anob > netstat_capture_$(date +%Y%m%d_%H%M).txt
tasklist /V /FO CSV > process_list_$(date +%Y%m%d_%H%M).csv
wmic process list full > process_detail_$(date +%Y%m%d_%H%M).txt

# Linux volatile evidence collection
dd if=/proc/kcore of=/mnt/forensics/memory_$(hostname)_$(date +%Y%m%d).raw bs=1M
ss -tulnp > /mnt/forensics/network_$(hostname).txt
ps auxwwf > /mnt/forensics/processes_$(hostname).txt

Step 6: Disable Compromised Accounts

# Disable compromised Active Directory accounts
Import-Module ActiveDirectory
Disable-ADAccount -Identity "compromised_user"
Set-ADUser -Identity "compromised_user" -Description "Disabled - IR-2024-001 $(Get-Date)"

# Revoke all active sessions
Revoke-AzureADUserAllRefreshToken -ObjectId "user_object_id"

# Reset service account credentials
Set-ADAccountPassword -Identity "svc_compromised" -Reset -NewPassword (ConvertTo-SecureString "TempP@ss$(Get-Random)" -AsPlainText -Force)

Step 7: Validate Containment Effectiveness

# Verify no active C2 communications
tcpdump -i eth0 host attacker_ip -c 100 -w verification_capture.pcap

# Check for new lateral movement attempts
index=security sourcetype=wineventlog EventCode=4624 LogonType=3
  earliest=-15m
| stats count by src_ip, dest_ip
| where src_ip IN ("compromised_hosts")

# Validate endpoint isolation status
curl -X GET "https://api.crowdstrike.com/devices/entities/devices/v2?ids=device_id" \
  -H "Authorization: Bearer $FALCON_TOKEN" | jq '.resources[].status'

Key Concepts

Concept	Description
Short-term Containment	Immediate actions to stop active damage (network isolation, account disable)
Long-term Containment	Sustainable measures while investigation continues (VLAN segmentation, enhanced monitoring)
Evidence Preservation	Capturing volatile data before containment actions destroy forensic artifacts
Blast Radius	Total scope of systems, accounts, and data affected by the breach
Containment Boundary	Network and logical perimeter established to prevent further spread
Kill Chain Disruption	Breaking the attacker's operational chain at the earliest possible stage
Business Continuity	Maintaining critical operations while containing the threat

Tools & Systems

Tool	Purpose
CrowdStrike Falcon	Endpoint detection, network containment of hosts
Microsoft Defender for Endpoint	Endpoint isolation and automated investigation
Palo Alto NGFW	Perimeter firewall rules for IP/domain blocking
Splunk/Elastic SIEM	Real-time alert correlation and scope analysis
Zeek (Bro)	Network traffic analysis for C2 identification
Velociraptor	Remote forensic collection and endpoint querying
Active Directory	Account management and authentication control

Common Scenarios

1. Ransomware Pre-Encryption: Attacker has deployed ransomware binary but encryption hasn't started. Isolate patient zero, block C2, and prevent lateral deployment.
2. Active Data Exfiltration: Data is being exfiltrated to external server. Block egress to C2, capture network evidence, isolate affected systems.
3. Compromised Domain Controller: Attacker has DC access. Isolate DC from network, reset KRBTGT twice, rotate all privileged credentials.
4. Supply Chain Compromise: Malicious update deployed across environment. Block update server, isolate systems that received the update, assess scope.
5. Insider Threat - Active Exfil: Employee actively copying sensitive data. Disable account, block USB access, preserve evidence chain.

Output Format

• Containment action log with timestamps (who, what, when)
• Network isolation verification report
• List of compromised/isolated systems with justification
• Evidence preservation checksums and chain of custody records
• Containment effectiveness validation results
• Stakeholder notification with current status and next steps