Writing Custom IDS Rules (Detecting Scanning and Suspicious Activity)

#Tailoring Detection to Your Environment#link

Prebuilt IDS signatures are valuable, but they cannot account for every organization's unique applications, network architecture, and threat landscape. Custom detection rules allow defenders to identify environment-specific threats, policy violations, and unusual behaviors that generic signatures may miss.

Detecting Internal Reconnaissance

Internal reconnaissance is often an early stage of lateral movement. IDS rules can identify repeated connection attempts, unusual access patterns, or traffic that deviates from established baselines. Thresholding helps detect repeated activity while reducing excessive alert generation.

# Example Snort Rule
alert tcp $HOME_NET any -> $HOME_NET any (msg:"Potential Internal Scan"; flags:S; threshold:type threshold, track by_src, count 30, seconds 60; sid:500001; rev:1;)

This example alerts when a source host generates a large number of SYN packets within a short time window. While it may indicate scanning activity, additional analysis is required because legitimate applications can also generate bursts of connection attempts.

Identifying Suspicious Beaconing Behavior

Malware often communicates with command-and-control (C2) infrastructure using periodic network connections. While IDS signatures can help identify known malicious traffic patterns, effective beaconing detection typically combines network telemetry, DNS monitoring, flow analysis, and behavioral analytics.

• ▪Identify visibility gaps in the environment
• ▪Develop detection logic for specific behaviors
• ▪Test rules using packet captures and lab traffic
• ▪Apply thresholding to reduce alert noise
• ▪Continuously tune rules based on operational feedback