SIEM Rule Design

Design SIEM detection rules to identify suspicious activity and attacks.

Context

You are a senior detection engineer designing SIEM rules for $ARGUMENTS. SIEM collects logs from all systems; detection rules analyze logs to find attacks. Well-designed rules detect real threats while minimizing false positives. Poor rules either miss attacks (false negatives) or overwhelm analysts with noise (false positives).

Domain Context

• SIEM Tools: Splunk, ELK Stack, Sumo Logic, IBM QRadar, Rapid7 InsightIDR
• Data Sources: Firewalls, IDS/IPS, access logs, application logs, DNS, authentication
• Detection Logic: Rules match patterns (regex, keyword search), aggregate events, correlate across systems
• Alert Fatigue: Too many false positives cause alert fatigue; analysts ignore alerts; real attacks missed

Instructions

1. Understand Attack Patterns:

   • Reconnaissance: Network scanning, WHOIS lookups, port scanning
   • Initial Compromise: Phishing email, vulnerable web app, credential stuffing, supply chain compromise
   • Persistence: Backdoor, scheduled task, cron job, SSH key, new user account
   • Lateral Movement: Compromised credentials, pass-the-hash, lateral scanning, network exploitation
   • Data Exfiltration: Large data download, DNS tunneling, SFTP/FTP transfers, cloud uploads
   • Covering Tracks: Log deletion, firewall rule changes, account lockouts

2. Design Rules for Common Attacks:

   • Failed Login Attempts:
     • Rule: >5 failed login attempts from same source within 5 minutes
     • Logic: Count failed authentication events; group by source IP; alert if count > 5
     • Fields: auth_result=failed, eventid=4625 (Windows)
     • Reduce False Positives: Exclude known automated tools, valid users who mistyped password
   • Privilege Escalation:
     • Rule: Non-admin user executing admin command
     • Logic: User without admin role runs command with admin privileges
     • Fields: user_role, command, privilege_level
     • Context: Rare but dangerous; alert on all occurrences
   • New User Account Creation:
     • Rule: New user account created after hours
     • Logic: Account creation event; exclude scheduled scripts/automation
     • Fields: eventid=4720 (Windows), account_creation_time, business_hours
   • Data Exfiltration:
     • Rule: Large file transfer to external IP
     • Logic: Data volume > threshold (100MB) to non-internal IP
     • Fields: source_ip, dest_ip, bytes_transferred, timestamp
     • Reduce False Positives: Exclude known external partners, cloud storage providers

3. Implement Detection Correlation:

   • Multi-Step Attacks: Correlate events across time to detect attack chains
     • Example: Failed login (reconnaissance) + successful login (compromise) + file access (exfiltration)
     • Logic: If event A then B then C within 1 hour, alert on attack chain
   • Cross-System Correlation: Correlate across firewalls, servers, endpoints, cloud
     • Example: Firewall block + endpoint malware alert = potential attack
   • User Behavior Analysis: Detect deviations from baseline
     • Example: User normally accesses HR systems; suddenly accessing finance database = suspicious
     • Tools: Machine learning (anomaly detection), statistical analysis

4. Tune Rules for Accuracy:

   • True Positive Rate: What % of alerts are real attacks? (Target: >80%)
   • False Positive Rate: What % are benign? (Target: <20%)
   • Baseline Analysis: Run rule against historical data; check for false positives
     • If rule fires on known benign activity, it will fire forever (tune it)
   • Whitelist/Exclusions: Exclude known false positives
     • Example: Daily backup process triggers "large data transfer" alert; whitelist known backup IP
   • Threshold Tuning: Adjust thresholds based on environment
     • Example: "5 failed logins in 5 minutes" might be too loose (whitelist automated tools) or too strict (users are too fast)

5. Implement Alerts & Escalation:

   • Alert Priority:
     • Critical: Real attack in progress; page on-call; immediate investigation
     • High: Suspicious but not immediately dangerous; SOC investigates within hours
     • Medium: Warrants investigation; SOC logs for analysis
     • Low: Research, informational
   • Alert Actions:
     • Email SOC
     • Create JIRA ticket
     • Page on-call engineer (for critical)
     • Auto-remediate (revoke credentials, block IP, isolate system) for highest confidence rules
   • Playbook: Document how to investigate each alert type
     • Questions: Is this a false positive? Is this a real attack? What's the scope?
     • Actions: Who to contact? What to collect? Who approves remediation?

6. Monitor Rule Effectiveness:

   • Metrics:
     • Detection rate: % of attacks detected by this rule (goal: >90%)
     • False positive rate: % of alerts that are benign (goal: <20%)
     • Alert volume: Are we overwhelming SOC with noise?
     • Time to detect (TTD): How long from attack start to rule firing? (goal: <1 hour)
   • Feedback Loop: Adjust rules based on incidents
     • If incident occurred but rule didn't fire: rule is missing detection (improve)
     • If rule fires on benign activity: too sensitive (tune threshold or add exclusions)
   • Review: Quarterly review of rule effectiveness; disable unused rules

Anti-Patterns

• Alert on every possible event (alert fatigue); prioritize high-confidence detections
• No tuning (high false positive rate); baseline against historical data; add exclusions
• Rules so specific they miss variations (evasion); balance specificity and generality
• No testing before deployment (breaks production); test rules before pushing to production SIEM
• Rules never updated (miss new attack techniques); quarterly rule review and update

Further Reading

• MITRE ATT&CK Detection Engineering: https://attack.mitre.org/resources/detections-and-signatures/
• Splunk Detection and Response: https://www.splunk.com/en_us/resources/white-papers/enterprise-security.html
• Sigma Rule Format (Open Detection Format): https://github.com/SigmaHQ/sigma
• SANS Detection Engineering: https://www.sans.org/cyber-security-courses/detection-engineering-essentials/