Hunting for Beaconing with Frequency Analysis

When to Use

• When proactively searching for compromised endpoints calling back to C2 infrastructure
• After threat intelligence reports indicate active C2 frameworks targeting your sector
• When network logs show periodic outbound connections to unfamiliar destinations
• During purple team exercises validating C2 detection capabilities
• When investigating a potential breach and need to identify active C2 channels

Prerequisites

• Network proxy/firewall logs with timestamps and destination data (minimum 24 hours)
• Zeek conn.log, dns.log, and ssl.log or equivalent NetFlow/IPFIX data
• SIEM platform with statistical analysis capability (Splunk, Elastic, Microsoft Sentinel)
• RITA (Real Intelligence Threat Analytics) or AC-Hunter for automated beacon analysis
• Threat intelligence feeds for domain/IP reputation enrichment

Workflow

1. Define Beacon Parameters: Establish detection thresholds -- coefficient of variation (CV) below 0.20 indicates strong periodicity, minimum 50 connections over 24 hours, average interval between 30 seconds and 24 hours.
2. Collect Network Telemetry: Aggregate proxy logs, DNS queries, firewall connection logs, and Zeek metadata into the analysis platform.
3. Calculate Connection Intervals: For each source-destination pair, compute the time delta between consecutive connections and derive mean interval, standard deviation, and CV.
4. Apply Jitter Analysis: Sophisticated C2 frameworks like Cobalt Strike add jitter (randomness) to beacon intervals. The Sunburst backdoor beaconed every 15 minutes plus/minus 90 seconds. Analyze jitter patterns to detect even randomized beaconing.
5. Filter Legitimate Periodic Traffic: Exclude known-good beaconing sources including Windows Update, antivirus definition updates, NTP synchronization, SaaS heartbeat services, and CDN health checks.
6. Analyze Data Size Consistency: C2 heartbeat packets typically have consistent payload sizes. Calculate the CV of bytes transferred per connection -- low variance suggests automated communication.
7. Enrich with Threat Intelligence: Check identified beaconing destinations against VirusTotal, WHOIS registration data (flag domains under 30 days old), certificate transparency logs, and passive DNS history.
8. Correlate with Endpoint Telemetry: Map beaconing source IPs to endpoint hostnames via DHCP logs, then correlate with process creation events (Sysmon Event ID 1, 3) to identify the responsible process.
9. Score and Prioritize: Assign risk scores based on CV value, domain age, TI matches, data size consistency, and suspicious port usage. Escalate high-confidence findings.

Key Concepts

Tools & Systems

Detection Queries

Splunk -- HTTP/S Beacon Frequency Analysis

index=proxy OR index=firewall
| where NOT match(dest, "(?i)(microsoft|google|amazonaws|cloudflare|akamai)")
| bin _time span=1s
| stats count by src_ip dest _time
| streamstats current=f last(_time) as prev_time by src_ip dest
| eval interval=_time-prev_time

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