Sudden Decrease in Reported Events From A Host

Rule ID

PH_Rule_Stat_18

Default Status

Enabled

Description

Detects that a reporting device is suddenly reporting less events. The current average over the one hour time window is less than 3 times the standard deviation and also 50% less than the statistical mean

Severity

MITRE ATT&CK® Tactics

FortiSIEM

MITRE ATT&CK® Techniques

No Technique Specified

Impacts

Network

Data Source

Any Log Ingested by FortiSIEM

Detection

Profiling

Remediation Guidance

Identify the root cause and solve the issue

Time Window

If the following pattern or patterns match an ingested event within the given time window in seconds, trigger an incident.

600 seconds

Trigger Conditions

If the following defined pattern/s occur within a 600 second time window.

Events

SubPattern Definitions

SubPattern Name: Events

This is the named definition of the event query, this is important if multiple subpatterns are defined to distinguish them.

SubPattern Query

This is the query logic that matches incoming events

phEventCategory = 3 AND
	  eventType = "PH_SYSTEM_IP_EVENTS_PER_SEC"

Group by Attributes

This defines how matching events are aggregated, only events with the same matching attribute values are grouped into one unique incident ID

reptDevName,reptDevIpAddr

Aggregate Constraint

This is most typically a numerical constraint that defines when the rule should trigger an incident

(AVG(eventsPerSec)-STAT_AVG(AVG(eventsPerSec):116))/STAT_STDDEV(AVG(eventsPerSec):116) <= -3 AND
	  AVG(eventsPerSec) <= 0.5 * STAT_AVG(AVG(eventsPerSec):116) AND
	  STAT_STDDEV(AVG(eventsPerSec):116) > 0 AND
	  STAT_AVG(AVG(eventsPerSec):116) > 10

Incident Attribute Mapping

This section defines which fields in matching raw events should be mapped to the incident attributes in the resulting incident.

The available raw event attributes to map are limited to the group by attributes and the aggregate event constraint fields for each subpattern

 hostName = Events.reptDevName,
 hostIpAddr = Events.reptDevIpAddr,
 eventsPerSec = Events.AVG(eventsPerSec),
 avgEventsPerSec = Events.STAT_AVG(AVG(eventsPerSec):116),
 sdevEventsPerSec = Events.STAT_STDDEV(AVG(eventsPerSec):116)

FortiSIEM Rules