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Unlock with Pro - from £29/user/moDetect Distributed Low-and-Slow Port/Service Scanning Sweep in CrowdStrike LogScale
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
LogScale Detection Query
#event_simpleName=FirewallInboundConnDenied OR #event_simpleName=NetworkConnectIP4
| RemoteAddressIP4=/^(203\.0\.113\.|198\.51\.10[0-1]\.)/
| ConnectionDirection="Inbound"
| groupBy([RemoteAddressIP4], function=([count(as=TouchCount), distinct_count(LocalAddressIP4, as=DistinctDestIPs)]))
| test(DistinctDestIPs<=3)
| Signal := "LowTouchSource"
// Companion aggregate search (run after collecting the low-touch source set above):
// #event_simpleName=FirewallInboundConnDenied OR #event_simpleName=NetworkConnectIP4
// | RemoteAddressIP4=in(subset=$low_touch_sources)
// | groupBy([], function=([distinct_count(RemoteAddressIP4, as=UniqueSources), distinct_count(LocalAddressIP4, as=UniqueDestIPsSwept)]))
// | test(UniqueSources>=25 and UniqueDestIPsSwept>=50)CrowdStrike LogScale query over firewall/host network telemetry ingested into Falcon LogScale (either via FirewallInboundConnDenied on hosts running Falcon Firewall Management, or via a perimeter firewall forwarder repository). First stage groups by remote source IP within the org's public CIDR prefix and flags low-touch sources (3 or fewer distinct local destination IPs contacted). The commented companion search aggregates across the resulting low-touch source set to test for the distributed-sweep thresholds (25+ unique sources, 50+ unique destinations swept), matching the two-stage KQL/SPL detection logic; CIDR prefix matching is approximated via regex since LogScale lacks a native CIDR-range function.
Data Sources
Required Tables
False Positives & Tuning
- Known internet research scanners and vulnerability-scanning vendors with published IP ranges
- Distributed synthetic-monitoring or health-check services probing multiple hosts from many 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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