Server Disk space Warning

Rule ID

PH_Rule_Perf_209

Default Status

Enabled

Description

Detects that server disk space usage has reached a warning level (utilization between 90% and 95% and free disk still greater than 100 MB) based on 2 successive readings in a 10 minute interval

Severity

MITRE ATT&CK® Tactics

Impact

Impact consists of techniques that adversaries use to disrupt availability or compromise integrity by manipulating business and operational processes. Techniques used for impact can include destroying or tampering with data. In some cases, business processes can look fine, but may have been altered to benefit the adversaries’ goals. These techniques might be used by adversaries to follow through on their end goal or to provide cover for a confidentiality breach.

https://attack.mitre.org/tactics/TA0040

MITRE ATT&CK® Techniques

T1499.001

Endpoint Denial of Service: OS Exhaustion Flood

Adversaries may target the operating system (OS) for a DoS attack, since the (OS) is responsible for managing the finite resources on a system. These attacks do not need to exhaust the actual resources on a system since they can simply exhaust the limits that an OS self-imposes to prevent the entire system from being overwhelmed by excessive demands on its capacity.

https://attack.mitre.org/techniques/T1499/001

Impacts

Server

Data Source

Windows Performance Monitoring via OMI or FortiSIEM Agent, Linux via SNMP

Detection

Correlation

Remediation Guidance

Identify the largest files consuming most disk space and delete them. Or, enlarge the disk. If it is an OS disk, then the system may stop working if this problem is not taken care of.

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.

 ServDiskWarn

SubPattern Definitions

SubPattern Name: ServDiskWarn

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

eventType = "PH_DEV_MON_SYS_DISK_UTIL" AND
	   hostIpAddr IN (Group@PH_SYS_DEVICE_Server) AND diskName NOT IN ("/boot")

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

hostIpAddr,hostName,diskName

Aggregate Constraint

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

((AVG(diskUtil) >= DeviceToCMDBAttr(hostIpAddr,diskName,DiskSpaceUtilThreshWarn) AND
	   AVG(diskUtil) <= DeviceToCMDBAttr(hostIpAddr,diskName,DiskSpaceUtilThreshCrit)) OR
	   (AVG(diskUtil) >= DeviceToCMDBAttr(hostIpAddr,diskName,DiskSpaceUtilThreshCrit) AND
	   AVG(freeDiskMB) > DeviceToCMDBAttr(hostIpAddr,diskName,FreeDiskMBThreshCrit)))
	   AND COUNT(*) >= 2

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

 hostIpAddr = ServDiskWarn.hostIpAddr,
 hostName = ServDiskWarn.hostName,
 diskName = ServDiskWarn.diskName,
 diskUtil = ServDiskWarn.AVG(diskUtil),
 freeDiskMB = ServDiskWarn.AVG(freeDiskMB)

FortiSIEM Rules