Anomalous User Agent connection attempt

Severity: low
Time window: 1d
Group by: Computer, cIP, csMethod, csUriStem, csUserAgent, csUserAgent_size, csUserName, sIP, sPort, sSiteName, scStatus, scSubStatus, scWin32Status
Author: Microsoft Security Research
Source: github.com/Azure/Azure-Sentinel

'Identifies connection attempts (success or fail) from clients with very short or very long User Agent strings and with less than 100 connection attempts.'

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1190` Exploit Public-Facing Application

Rule body kusto

id: f845881e-2500-44dc-8ed7-b372af3e1e25
name: Anomalous User Agent connection attempt
description: |
  'Identifies connection attempts (success or fail) from clients with very short or very long User Agent strings and with less than 100 connection attempts.'
severity: Low
requiredDataConnectors:
  - connectorId: AzureMonitor(IIS)
    dataTypes:
      - W3CIISLog
queryFrequency: 1d
queryPeriod: 1d
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
relevantTechniques:
  - T1190
query: |
  let short_uaLength = 5;
  let long_uaLength = 1000;
  let c_threshold = 100;
  W3CIISLog
  // Exclude local IPs as these create noise
  | where cIP !startswith "192.168." and cIP != "::1"
  | where isnotempty(csUserAgent) and csUserAgent !in~ ("-", "MSRPC") and (string_size(csUserAgent) <= short_uaLength or string_size(csUserAgent) >= long_uaLength)
  | extend csUserAgent_size = string_size(csUserAgent)
  | summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), ConnectionCount = count() by Computer, sSiteName, sPort, csUserAgent, csUserAgent_size, csUserName , csMethod, csUriStem, sIP, cIP, scStatus, scSubStatus, scWin32Status
  | where ConnectionCount < c_threshold
  | extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
  | extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
  | extend AccountName = tostring(split(csUserName, "@")[0]), AccountUPNSuffix = tostring(split(csUserName, "@")[1])
entityMappings:
  - entityType: Account
    fieldMappings:
      - identifier: FullName
        columnName: csUserName
      - identifier: Name
        columnName: AccountName
      - identifier: UPNSuffix
        columnName: AccountUPNSuffix
  - entityType: Host
    fieldMappings:
      - identifier: FullName
        columnName: Computer
      - identifier: HostName
        columnName: HostName
      - identifier: NTDomain
        columnName: HostNameDomain
  - entityType: IP
    fieldMappings:
      - identifier: Address
        columnName: cIP
version: 1.0.3
kind: Scheduled
metadata:
    source:
        kind: Community
    author:
        name: Microsoft Security Research
    support:
        tier: Community
    categories:
        domains: [ "Security - Threat Protection" ]

Stages and Predicates

Parameters

let short_uaLength = 5;
let long_uaLength = 1000;
let c_threshold = 100;

Stage 1: `source`

W3CIISLog

Stage 2: `where`

| where cIP !startswith "192.168." and cIP != "::1"

Stage 3: `where`

| where isnotempty(csUserAgent) and csUserAgent !in~ ("-", "MSRPC") and (string_size(csUserAgent) <= short_uaLength or string_size(csUserAgent) >= long_uaLength)

Stage 4: `extend`

| extend csUserAgent_size = string_size(csUserAgent)

Stage 5: `summarize`

| summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), ConnectionCount = count() by Computer, sSiteName, sPort, csUserAgent, csUserAgent_size, csUserName , csMethod, csUriStem, sIP, cIP, scStatus, scSubStatus, scWin32Status

Threshold: lt 100

Stage 6: `where`

| where ConnectionCount < c_threshold

Stage 7: `extend` (3 consecutive steps)

| extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
| extend AccountName = tostring(split(csUserName, "@")[0]), AccountUPNSuffix = tostring(split(csUserName, "@")[1])

Exclusions

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

Field	Kind	Excluded values
`cIP`	starts_with	`192.168.`
`csUserAgent`	in	`-`, `MSRPC`

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
`ConnectionCount`	lt	`100` transforms: `cased`
`cIP`	ne	`::1` transforms: `cased`
`csUserAgent`	ge	`1000` transforms: `strlen`
`csUserAgent`	is_not_null	(no value, null check)
`csUserAgent`	le	`5` transforms: `strlen`

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
`Computer`	`summarize`
`ConnectionCount`	`summarize`
`EndTimeUtc`	`summarize`
`StartTimeUtc`	`summarize`
`cIP`	`summarize`
`csMethod`	`summarize`
`csUriStem`	`summarize`
`csUserAgent`	`summarize`
`csUserAgent_size`	`summarize`
`csUserName`	`summarize`
`sIP`	`summarize`
`sPort`	`summarize`
`sSiteName`	`summarize`
`scStatus`	`summarize`
`scSubStatus`	`summarize`
`scWin32Status`	`summarize`
`DomainIndex`	`extend`
`HostName`	`extend`
`HostNameDomain`	`extend`
`AccountName`	`extend`
`AccountUPNSuffix`	`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)

Anomalous User Agent connection attempt

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `summarize`

Stage 6: `where`

Stage 7: `extend` (3 consecutive steps)

Exclusions

Indicators

Output fields

Keyboard shortcuts

Search operators

Anomalous User Agent connection attempt

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Parameters

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: extend

Stage 5: summarize

Stage 6: where

Stage 7: extend (3 consecutive steps)

Exclusions

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `where`

Stage 4: `extend`

Stage 5: `summarize`

Stage 6: `where`

Stage 7: `extend` (3 consecutive steps)