Cisco Umbrella - Rare User Agent Detected

Severity: medium
Time window: 14d
Source: github.com/Azure/Azure-Sentinel

'Rule helps to detect a rare user-agents indicating web browsing activity by an unusual process other than a web browser.'

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	No specific technique

Rule body

id: 8c8de3fa-6425-4623-9cd9-45de1dd0569a
name: Cisco Umbrella - Rare User Agent Detected
description: |
  'Rule helps to detect a rare user-agents indicating web browsing activity by an unusual process other than a web browser.'
severity: Medium
requiredDataConnectors:
  - connectorId: CiscoUmbrellaDataConnector
    dataTypes:
      - Cisco_Umbrella_proxy_CL
queryFrequency: 1d
queryPeriod: 14d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - CommandAndControl
query: |
  let lookBack = 14d;
  let timeframe = 1d;
  let user_agents_list = Cisco_Umbrella
  | where EventType == "proxylogs"
  | where TimeGenerated > ago(lookBack) and TimeGenerated < ago(timeframe)
  | summarize count() by HttpUserAgentOriginal
  | summarize make_list(HttpUserAgentOriginal);
  Cisco_Umbrella
  | where EventType == "proxylogs"
  | where TimeGenerated > ago(timeframe)
  | where HttpUserAgentOriginal !in (user_agents_list)
  | extend Message = "Rare User Agent"
  | project Message, SrcIpAddr, DstIpAddr, UrlOriginal, TimeGenerated, HttpUserAgentOriginal
entityMappings:
  - entityType: URL
    fieldMappings:
      - identifier: Url
        columnName: UrlOriginal
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SrcIpAddr
version: 1.1.2
kind: Scheduled

Stages and Predicates

Stage 0: `let`

let lookBack = 14d;
let timeframe = 1d;
let user_agents_list = Cisco_Umbrella
| where EventType == "proxylogs"
| where TimeGenerated > ago(lookBack) and TimeGenerated < ago(timeframe)
| summarize count() by HttpUserAgentOriginal
| summarize make_list(HttpUserAgentOriginal);

Stage 1: `source`

Cisco_Umbrella

Stage 2: `where`

| where EventType == "proxylogs"

Stage 3: `where`

| where TimeGenerated > ago(timeframe)

Stage 4: `where`

| where HttpUserAgentOriginal !in (user_agents_list)

Stage 5: `extend`

| extend Message = "Rare User Agent"

Stage 6: `project`

| project Message, SrcIpAddr, DstIpAddr, UrlOriginal, TimeGenerated, HttpUserAgentOriginal

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`EventType`	eq	`proxylogs`	`field:"EventType" kind:eq value:"proxylogs"`

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`HttpUserAgentOriginal`	eq	`user_agents_list`	`excludes:HttpUserAgentOriginal` `field:"HttpUserAgentOriginal" value:"user_agents_list"`

Output fields

These fields are emitted when the rule matches.

Field	Source
`DstIpAddr`	`project`
`HttpUserAgentOriginal`	`project`
`Message`	`project`
`SrcIpAddr`	`project`
`TimeGenerated`	`project`
`UrlOriginal`	`project`

Cisco Umbrella - Rare User Agent Detected

MITRE ATT&CK coverage

Rule body

Stages and Predicates

Stage 0: let

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: where

Stage 5: extend

Stage 6: project