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SKILL.md
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
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.
Collect Network Telemetry: Aggregate proxy logs, DNS queries, firewall connection logs, and Zeek metadata into the analysis platform.
Calculate Connection Intervals: For each source-destination pair, compute the time delta between consecutive connections and derive mean interval, standard deviation, and CV.
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.
Filter Legitimate Periodic Traffic: Exclude known-good beaconing sources including Windows Update, antivirus definition updates, NTP synchronization, SaaS heartbeat services, and CDN health checks.
: C2 heartbeat packets typically have consistent payload sizes. Calculate the CV of bytes transferred per connection -- low variance suggests automated communication.
Analyze Data Size Consistency
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.
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.
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
Concept
Description
T1071.001
Application Layer Protocol: Web Protocols -- HTTP/HTTPS beaconing
T1071.004
Application Layer Protocol: DNS -- DNS-based C2 tunneling
T1573
Encrypted Channel -- TLS/SSL encrypted C2 communication
T1568.002
Dynamic Resolution: Domain Generation Algorithms
Coefficient of Variation
Standard deviation divided by mean; values below 0.20 indicate periodicity
Jitter
Random variation added to beacon interval to evade detection
RITA Beacon Score
Composite score from connection regularity, data size consistency, and connection count
JA3/JA4 Fingerprinting
TLS client fingerprinting to identify C2 framework signatures
Fast-Flux DNS
Rapidly changing DNS resolution used to protect C2 infrastructure
Tools & Systems
Tool
Purpose
RITA (Real Intelligence Threat Analytics)
Automated beacon scoring from Zeek logs
AC-Hunter
Commercial threat hunting platform with beacon detection
Splunk
SPL-based statistical beacon analysis with streamstats
Elastic Security
ML anomaly detection for periodic network behavior
Network IDS with JA3/JA4 TLS fingerprint extraction
FLARE
C2 profile and beacon pattern detection
VirusTotal
Domain and IP reputation enrichment
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
| stats count avg(interval) as avg_interval stdev(interval) as stdev_interval
min(interval) as min_interval max(interval) as max_interval by src_ip dest
| where count > 50
| eval cv=stdev_interval/avg_interval
| where cv < 0.20 AND avg_interval > 30 AND avg_interval < 86400
| sort cv
| table src_ip dest count avg_interval stdev_interval cv
KQL -- Microsoft Sentinel Beacon Detection
DeviceNetworkEvents
| where Timestamp > ago(24h)
| where RemoteIPType == "Public"
| summarize ConnectionTimes=make_list(Timestamp), Count=count() by DeviceName, RemoteIP, RemoteUrl
| where Count > 50
| extend Intervals = array_sort_asc(ConnectionTimes)
| mv-apply Intervals on (
extend NextTime = next(Intervals)
| where isnotempty(NextTime)
| extend IntervalSec = datetime_diff('second', NextTime, Intervals)
| summarize AvgInterval=avg(IntervalSec), StdDev=stdev(IntervalSec)
)
| extend CV = StdDev / AvgInterval
| where CV < 0.2 and AvgInterval > 30
| sort by CV asc
Cobalt Strike Beacon: Default 60-second interval with configurable 0-50% jitter over HTTPS. Malleable C2 profiles can mimic legitimate traffic patterns.
Sunburst/SUNSPOT: 12-14 day dormancy period, then beaconing every 12-14 minutes with randomized jitter, designed to evade frequency analysis.
DNS Tunneling C2: Encoded data exfiltration via DNS TXT/CNAME queries to attacker-controlled domains, detectable via high subdomain entropy and query volume.
Sliver C2: Modern C2 framework with HTTPS, mTLS, and WireGuard protocols, configurable beacon intervals with built-in jitter support.
Legitimate Service Abuse: C2 communication over Slack, Discord, Telegram, or cloud storage APIs, making destination-based filtering ineffective.
Output Format
Hunt ID: TH-BEACON-[DATE]-[SEQ]
Source IP: [Internal IP]
Source Host: [Hostname from DHCP/DNS]
Destination: [Domain/IP]
Protocol: [HTTP/HTTPS/DNS]
Beacon Interval: [Average seconds]
Jitter Estimate: [Percentage]
Coefficient of Variation: [CV value]
Connection Count: [Total connections in window]
Data Size CV: [Payload consistency metric]
Domain Age: [Days since registration]
TI Match: [Yes/No -- source]
Risk Score: [0-100]
Risk Level: [Critical/High/Medium/Low]
Indicators: [List of triggered risk factors]
