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 kusto

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

Parameters

let lookBack = 14d;
let timeframe = 1d;

Let binding: `user_agents_list`

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);

Derived from lookBack, timeframe.

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)

References user_agents_list (defined above).

Stage 5: `extend`

| extend Message = "Rare User Agent"

Stage 6: `project`

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

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`HttpUserAgentOriginal`	eq	`user_agents_list`

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.

Field	Kind	Values
`EventType`	eq	`proxylogs` transforms: `cased`

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.

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

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`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

Cisco Umbrella - Rare User Agent Detected

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: `user_agents_list`

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `where`

Stage 5: `extend`

Stage 6: `project`

Exclusions

Indicators

Output fields

Keyboard shortcuts

Search operators

Cisco Umbrella - Rare User Agent Detected

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: user_agents_list

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: where

Stage 5: extend

Stage 6: project

Exclusions

Indicators

Output fields

Let binding: `user_agents_list`

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `where`

Stage 5: `extend`

Stage 6: `project`