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Unlock with Pro - from £29/user/moDetect Distributed Low-and-Slow Port/Service Scanning Sweep in IBM QRadar
Adversaries increasingly avoid classic single-source port scan detection (which alerts on one IP hitting many ports/hosts in a short window) by distributing the sweep across a large pool of source IPs — botnet nodes, residential proxies, or rented scanning infrastructure — with each individual source only touching a handful of destination hosts or ports. No single source ever crosses a per-IP threshold, but the aggregate campaign methodically walks the organization's entire public IP block (T1595.001 — Active Scanning: Scanning IP Blocks) looking for exposed services ahead of exploitation. This is a persistent detection gap because most SIEM scan-detection rules key off a single source IP's touch count within a window; a coordinated low-and-slow sweep spread across dozens or hundreds of source IPs, each staying under that threshold, sails through undetected. Reliable detection requires aggregating perimeter firewall deny/reset telemetry at the destination CIDR level across ALL source IPs in a rolling window, then flagging when a large number of distinct low-touch sources collectively achieve high coverage of the organization's address space — the fingerprint of a coordinated sweep rather than background internet noise.
MITRE ATT&CK
- Tactic
- Reconnaissance
QRadar Detection Query
SELECT
sourceip AS SourceIP,
destinationip AS DestinationIP,
destinationport AS DestinationPort,
DATEFORMAT(starttime, 'YYYY-MM-dd HH:mm:ss') AS EventTime
FROM events
WHERE
QIDNAME(qid) IN ('Firewall Deny', 'Firewall Drop', 'Firewall Session Denied', 'Connection Reset')
AND INCIDR('203.0.113.0/24', destinationip)
AND starttime > NOW() - 86400000
GROUP BY SourceIP, DestinationIP, DestinationPort, EventTime
HAVING UNIQUECOUNT(destinationip) <= 3
ORDER BY EventTime DESC
LAST 500QRadar AQL detection over firewall deny/drop/reset events using INCIDR() (not string equality) against the org's registered CIDR block, isolating candidate low-touch source IPs (each contacting 3 or fewer distinct destination hosts). Feed the resulting source IP list into a companion QRadar Custom Rule with an aggregate 'unique source count >= 25 AND unique destination count >= 50' test over a rolling 24h window (grouping by destination CIDR) to fully reproduce the two-stage distributed-sweep logic, since AQL's single-pass aggregation cannot natively chain the per-source and cross-source aggregations in one query.
Data Sources
Required Tables
False Positives & Tuning
- Known internet research scanners and vulnerability-scanning vendors with published or contracted IP ranges
- Distributed synthetic-monitoring services probing from many geographically dispersed nodes
Other platforms for THREAT-Recon-DistributedPortScanSweep
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Simulated Low-Touch Multi-Source Port Probe (Single Host, Multiple Ports)
Expected signal: Perimeter firewall deny/reset log entries for the connection attempts to closed/filtered ports on the lab target, recorded with the source IP and each probed destination port.
- Test 2Simulated Coordinated Sweep (Multiple Scripted Sources, Sequential IP Block Walk)
Expected signal: Firewall deny/reset log entries showing the same source IP touching multiple sequential destination IPs in the lab CIDR block, each with a small number of probed ports.
- Test 3Masscan High-Speed IP Block Sweep (Single Source, Lab Range)
Expected signal: Firewall/NetFlow records showing SYN-only or single-packet connection attempts across every host in the lab /28 range from one source IP.
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