Detect known risky user agents (ASIM Web Session)

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

'This rule is designed to flag web requests that contain a user agent header that is recognized as malicious. It relies on a predefined list of known user agents, which is referenced from a specific CSV file'

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1133` External Remote Services, `T1190` Exploit Public-Facing Application
Command & Control	`T1071` Application Layer Protocol

Rule body kusto

id: 6a4dbcf8-f5e2-4b33-b34f-2db6487613f0
name: Detect known risky user agents (ASIM Web Session)
description: |
  'This rule is designed to flag web requests that contain a user agent header that is recognized as malicious. It relies on a predefined list of known user agents, which is referenced from a specific CSV file'
severity: Medium
status: Available 
tags:
  - Schema: WebSession
    SchemaVersion: 0.2.6
requiredDataConnectors: []
queryFrequency: 5m
queryPeriod: 5m
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
  - CommandAndControl
relevantTechniques:
  - T1190
  - T1133
  - T1071
query: |
  let lookback = 5m;
  let knownUserAgentsIndicators = materialize(externaldata(UserAgent: string, UserAgentCategory: string)
    [@"https://raw.githubusercontent.com/Azure/Azure-Sentinel/master/Sample%20Data/Feeds/UnusualUserAgents.csv"] 
    with(format="csv", ignoreFirstRecord=True)
    | extend joiner = 1);
  let CustomUserAgentIndicators = (_ASIM_GetWatchlistRaw("UnusualUserAgents") // Create new Watchlist and add your custom indicators(Optional)
    | extend
        UserAgent = tostring(WatchlistItem["UserAgent"]),
        UserAgentCategory = tostring(WatchlistItem["UserAgentCategory"])
    | project UserAgent, UserAgentCategory, joiner = 1
    | where isnotempty(UserAgent) and isnotempty(UserAgentCategory));
  let CombinedUAList = union knownUserAgentsIndicators, CustomUserAgentIndicators;
  let knownUserAgents=toscalar(CombinedUAList
    | where isnotempty(UserAgent)
    | summarize make_list(UserAgent, 1000));
  _Im_WebSession (starttime=ago(lookback), httpuseragent_has_any=knownUserAgents)
  | project SrcIpAddr, SrcUsername, SrcHostname, HttpUserAgent, Url, DstIpAddr, DstPortNumber, TimeGenerated
  | extend joiner = 1
  | join kind=inner CombinedUAList on joiner
  | where HttpUserAgent has UserAgent
  | summarize
    EventCount=count(),
    EventStartTime = min(TimeGenerated),
    EventEndTime=max(TimeGenerated)
    by SrcIpAddr, SrcUsername, SrcHostname, Url, HttpUserAgent, DstIpAddr, DstPortNumber, UserAgentCategory
  | extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")
entityMappings:
  - entityType: URL
    fieldMappings:
      - identifier: Url
        columnName: Url
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: SrcIpAddr
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: DstIpAddr
  - entityType: Account
    fieldMappings:
      - identifier: Name
        columnName: Name
      - identifier: UPNSuffix
        columnName: UPNSuffix
  - entityType: Host
    fieldMappings:
      - identifier: HostName
        columnName: SrcHostname
eventGroupingSettings:
  aggregationKind: AlertPerResult
customDetails:
  EventCount: EventCount
  EventStartTime: EventStartTime
  EventEndTime: EventEndTime
  HttpUserAgent: HttpUserAgent
  UserAgentCategory: UserAgentCategory
alertDetailsOverride:
  alertDisplayNameFormat: "User '{{SrcUsername}}' with IP '{{SrcIpAddr}}' has been observed using User Agent categorized as '{{UserAgentCategory}}'"
  alertDescriptionFormat: "User accessed this URL '{{Url}}' using User Agent '{{HttpUserAgent}}'. Perform a thorough analysis of the requests associated with this user agent header"
version: 1.0.0
kind: Scheduled

Stages and Predicates

Parameters

let lookback = 5m;
let CombinedUAList = union knownUserAgentsIndicators, CustomUserAgentIndicators;

Let binding: `knownUserAgentsIndicators`

let knownUserAgentsIndicators = materialize(externaldata(UserAgent: string, UserAgentCategory: string)
  [@"https://raw.githubusercontent.com/Azure/Azure-Sentinel/master/Sample%20Data/Feeds/UnusualUserAgents.csv"] 
  with(format="csv", ignoreFirstRecord=True)
  | extend joiner = 1);

Let binding: `CustomUserAgentIndicators`

let CustomUserAgentIndicators = (_ASIM_GetWatchlistRaw("UnusualUserAgents")
  | extend
      UserAgent = tostring(WatchlistItem["UserAgent"]),
      UserAgentCategory = tostring(WatchlistItem["UserAgentCategory"])
  | project UserAgent, UserAgentCategory, joiner = 1
  | where isnotempty(UserAgent) and isnotempty(UserAgentCategory));

Let binding: `knownUserAgents`

let knownUserAgents = toscalar(CombinedUAList
  | where isnotempty(UserAgent)
  | summarize make_list(UserAgent, 1000));

Derived from CombinedUAList.

Stage 1: `source`

_Im_WebSession (starttime=ago(lookback), httpuseragent_has_any=knownUserAgents)

Stage 2: `project`

| project SrcIpAddr, SrcUsername, SrcHostname, HttpUserAgent, Url, DstIpAddr, DstPortNumber, TimeGenerated

Stage 3: `extend`

| extend joiner = 1

Stage 4: `join`

| join kind=inner CombinedUAList on joiner

Stage 5: `where`

| where HttpUserAgent has UserAgent

Stage 6: `summarize`

| summarize
  EventCount=count(),
  EventStartTime = min(TimeGenerated),
  EventEndTime=max(TimeGenerated)
  by SrcIpAddr, SrcUsername, SrcHostname, Url, HttpUserAgent, DstIpAddr, DstPortNumber, UserAgentCategory

Stage 7: `extend`

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

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`	match	`UserAgent` transforms: `term`
`UserAgent`	is_not_null	(no value, null check)
`UserAgentCategory`	is_not_null	(no value, null check)

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`	`summarize`
`DstPortNumber`	`summarize`
`EventCount`	`summarize`
`EventEndTime`	`summarize`
`EventStartTime`	`summarize`
`HttpUserAgent`	`summarize`
`SrcHostname`	`summarize`
`SrcIpAddr`	`summarize`
`SrcUsername`	`summarize`
`Url`	`summarize`
`UserAgentCategory`	`summarize`
`Name`	`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)

Detect known risky user agents (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: `knownUserAgentsIndicators`

Let binding: `CustomUserAgentIndicators`

Let binding: `knownUserAgents`

Stage 1: `source`

Stage 2: `project`

Stage 3: `extend`

Stage 4: `join`

Stage 5: `where`

Stage 6: `summarize`

Stage 7: `extend`

Indicators

Output fields

Keyboard shortcuts

Search operators

Detect known risky user agents (ASIM Web Session)

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Let binding: knownUserAgentsIndicators

Let binding: CustomUserAgentIndicators

Let binding: knownUserAgents

Stage 1: source

Stage 2: project

Stage 3: extend

Stage 4: join

Stage 5: where

Stage 6: summarize

Stage 7: extend

Indicators

Output fields

Let binding: `knownUserAgentsIndicators`

Let binding: `CustomUserAgentIndicators`

Let binding: `knownUserAgents`

Stage 1: `source`

Stage 2: `project`

Stage 3: `extend`

Stage 4: `join`

Stage 5: `where`

Stage 6: `summarize`

Stage 7: `extend`