Excessive Failed Authentication from Invalid Inputs

Status: available
Severity: medium
Time window: 15m
Group by: ClientIP, User
Source: github.com/Azure/Azure-Sentinel

'Creates an incident in the event that a user generates an excessive amount of failed authentications due to invalid inputs, indications of a potential brute force.'

MITRE ATT&CK coverage

Tactic	Techniques
Credential Access	`T1110` Brute Force

Rule body

id: c775a46b-21b1-46d7-afa6-37e3e577a27b
name: Excessive Failed Authentication from Invalid Inputs
description: |
  'Creates an incident in the event that a user generates an excessive amount of failed authentications due to invalid inputs, indications of a potential brute force.'
severity: Medium
requiredDataConnectors:
  - connectorId: SyslogAma
    datatypes:
      - Syslog
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - CredentialAccess
relevantTechniques:
  - T1110
query: |
  let threshold = 15;
  SymantecVIP
  | where isnotempty(RADIUSAuth)
  | where RADIUSAuth =~ "Reject"
  | summarize Total = count() by bin(TimeGenerated, 15m), User, ClientIP
  | where Total > threshold
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: User
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: ClientIP
version: 1.0.4
status: Available
kind: Scheduled

Stages and Predicates

Parameters

let threshold = 15;

Stage 1: `source`

SymantecVIP

Stage 2: `where`

| where isnotempty(RADIUSAuth)

Stage 3: `where`

| where RADIUSAuth =~ "Reject"

Stage 4: `summarize`

| summarize Total = count() by bin(TimeGenerated, 15m), User, ClientIP

Stage 5: `where`

| where Total > threshold

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`RADIUSAuth`	eq	`Reject`	`field:"RADIUSAuth" kind:eq value:"Reject"`
`RADIUSAuth`	is_not_null	(no value, null check)	`field:"RADIUSAuth" kind:is_not_null`
`Total`	gt	`15`	`field:"Total" kind:gt value:"15"`

Output fields

These fields are emitted when the rule matches.

Field	Source
`ClientIP`	`summarize`
`Total`	`summarize`
`User`	`summarize`