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Detection rules › Kusto

Cisco SEG - Unexpected attachment

Status
available
Severity
high
Time window
1h
Source
github.com/Azure/Azure-Sentinel

'Detects possibly malicious attachments.'

MITRE ATT&CK coverage

TacticTechniques
Initial AccessT1566 Phishing

Rule body kusto

id: f8ba18c4-81e3-4db0-8f85-4989f2ed2ade
name: Cisco SEG - Unexpected attachment
description: |
  'Detects possibly malicious attachments.'
severity: High
status: Available
requiredDataConnectors:
  - connectorId: CefAma
    dataTypes:
      - CommonSecurityLog
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
relevantTechniques:
  - T1566
query: |
  CiscoSEGEvent
  | where NetworkDirection =~ 'Incoming'
  | where tostring(AdditionalFields) has 'ESAAttachmentDetails'
  | extend attachment = replace_string(tostring(extract(@'"ESAAttachmentDetails":"{(.*?):', 1, tostring(AdditionalFields))), "'", "")
  | where attachment endswith '.ps1' or attachment endswith '.lnk' or attachment endswith '.exe'
  | extend AccountCustomEntity = DstUserName
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: Name
        columnName: AccountCustomEntity
version: 1.0.3
kind: Scheduled

Stages and Predicates

Stage 1: source

CiscoSEGEvent

Stage 2: where

| where NetworkDirection =~ 'Incoming'

Stage 3: where

| where tostring(AdditionalFields) has 'ESAAttachmentDetails'

Stage 4: extend

| extend attachment = replace_string(tostring(extract(@'"ESAAttachmentDetails":"{(.*?):', 1, tostring(AdditionalFields))), "'", "")

Stage 5: where

| where attachment endswith '.ps1' or attachment endswith '.lnk' or attachment endswith '.exe'

Stage 6: extend

| extend AccountCustomEntity = DstUserName

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

FieldKindValues
AdditionalFieldsmatch
  • ESAAttachmentDetails transforms: tostring, term
NetworkDirectioneq
  • Incoming
attachmentends_with
  • .exe
  • .lnk
  • .ps1

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

FieldSource
attachmentextend
AccountCustomEntityextend