Identify instances where a single source is observed using multiple user agents (ASIM Web Session)

Status: available
Severity: medium
Time window: 5m
Group by: SrcHostname, SrcIpAddr, SrcUsername
Source: github.com/Azure/Azure-Sentinel

'This detection mechanism identifies requests originating from a single source within a brief time period that exhibit multiple user agents. Such behavior could indicate unusual web browsing activities performed by unconventional processes'

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1133` External Remote Services, `T1190` Exploit Public-Facing Application
Credential Access	`T1528` Steal Application Access Token

Rule body kusto

id: 813ccf3b-0321-4622-b0bc-63518fd14454
name: Identify instances where a single source is observed using multiple user agents (ASIM Web Session)
description: |
  'This detection mechanism identifies requests originating from a single source within a brief time period that exhibit multiple user agents. Such behavior could indicate unusual web browsing activities performed by unconventional processes'
severity: Medium
status: Available 
tags:
  - Schema: WebSession
    SchemaVersion: 0.2.6
requiredDataConnectors: []
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
  - CredentialAccess
relevantTechniques:
  - T1190
  - T1133
  - T1528
query: |
  let threshold = 5; // Please update threshold limit as per your environment
  _Im_WebSession(starttime=ago(1h), eventresult="Success")
  | where isnotempty(HttpUserAgent)
  | summarize
      EventCount=count(),
      UserAgentList=make_set(HttpUserAgent, 100),
      URL_List = make_set(Url, 100),
      DestinationIPList = make_set(DstIpAddr, 100)
      by SrcIpAddr, SrcUsername, SrcHostname, bin(TimeGenerated, 5min)
  | extend UserAgentCount = array_length(UserAgentList)
  | where UserAgentCount > threshold
  | extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),""), Threshold=threshold
entityMappings:
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SrcIpAddr
  - entityType: Host
    fieldMappings:
      - identifier: HostName
        columnName: SrcHostname
  - entityType: Account
    fieldMappings:
      - identifier: Name
        columnName: Name
      - identifier: UPNSuffix
        columnName: UPNSuffix
eventGroupingSettings:
  aggregationKind: AlertPerResult
customDetails:
  UserAgentCount: UserAgentCount
  UserAgentArray: UserAgentList
  UserAgentThreshold: Threshold
  URLs: URL_List
  DestinationIPList: DestinationIPList
alertDetailsOverride:
  alertDisplayNameFormat: "User '{{SrcUsername}}' with IP '{{SrcIpAddr}}' has been observed using high number of User Agents within short timeframe"
  alertDescriptionFormat: "The system has detected high User Agent count of '{{UserAgentCount}}' originating from '{{SrcUsername}}'. Further investigation is necessary to determine the reason behind the detection of multiple user agents associated with the SrcIpAddr in this incident. User Agent list include: '{{UserAgentList}}'"
version: 1.0.1
kind: Scheduled

Stages and Predicates

Parameters

let threshold = 5;

Stage 1: `source`

_Im_WebSession(starttime=ago(1h), eventresult="Success")

Stage 2: `where`

| where isnotempty(HttpUserAgent)

Stage 3: `summarize`

| summarize
    EventCount=count(),
    UserAgentList=make_set(HttpUserAgent, 100),
    URL_List = make_set(Url, 100),
    DestinationIPList = make_set(DstIpAddr, 100)
    by SrcIpAddr, SrcUsername, SrcHostname, bin(TimeGenerated, 5min)

Stage 4: `extend`

| extend UserAgentCount = array_length(UserAgentList)

Stage 5: `where`

| where UserAgentCount > threshold

Stage 6: `extend`

| extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),""), Threshold=threshold

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
`HttpUserAgent`	is_not_null	(no value, null check)
`UserAgentCount`	gt	`5` 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
`DestinationIPList`	`summarize`
`EventCount`	`summarize`
`SrcHostname`	`summarize`
`SrcIpAddr`	`summarize`
`SrcUsername`	`summarize`
`URL_List`	`summarize`
`UserAgentList`	`summarize`
`UserAgentCount`	`extend`
`Name`	`extend`
`Threshold`	`extend`
`UPNSuffix`	`extend`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`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)

Identify instances where a single source is observed using multiple user agents (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: `source`

Stage 2: `where`

Stage 3: `summarize`

Stage 4: `extend`

Stage 5: `where`

Stage 6: `extend`

Indicators

Output fields

Keyboard shortcuts

Search operators

Identify instances where a single source is observed using multiple user agents (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: source

Stage 2: where

Stage 3: summarize

Stage 4: extend

Stage 5: where

Stage 6: extend

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `summarize`

Stage 4: `extend`

Stage 5: `where`

Stage 6: `extend`