Detect presence of uncommon user agents in web requests ASIM Web Session

Back


Id	2d50d937-d7f2-4c05-b151-9af7f9ec747e
Rulename	Detect presence of uncommon user agents in web requests (ASIM Web Session)
Description	This rule assists in detecting rare user agents, which may indicate web browsing activity by an unconventional process different from the usual ones. The rule specifically searches for UserAgent strings that have not been seen in the past 14 days. This query will perform better when run over summarized data
Severity	Medium
Tactics	InitialAccess
Techniques	T1190 T1133
Kind	Scheduled
Query frequency	1h
Query period	14d
Trigger threshold	0
Trigger operator	gt
Source Uri	https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Web Session Essentials/Analytic Rules/RareUserAgentDetected.yaml
Version	1.0.1
Arm template	2d50d937-d7f2-4c05-b151-9af7f9ec747e.json

KQL

let lookBack = 14d;
let timeframe = 1h;
// calculate avg. eps(events per second)
let eps = materialize(_Im_WebSession(starttime=ago(1d))
    | project TimeGenerated
    | summarize AvgPerSec = count() / 3600 by bin(TimeGenerated, 1h)
    | summarize round(avg(AvgPerSec))
    );
let summarizationexist  = (
    union isfuzzy=true 
        (
        WebSession_Summarized_SrcInfo_CL
        | where EventTime_t > ago(1d) 
        | project v = int(2)
        ),
        (
        print int(1) 
        | project v = print_0
        )
    | summarize maxv = max(v)
    | extend sumexist = (maxv > 1)
    );
let allUserAgents = union isfuzzy=true
        (
        (datatable(exists: int, sumexist: bool)[1, false]
        | where toscalar(eps) > 1000
        | join (summarizationexist) on sumexist)
        | join (
            _Im_WebSession(starttime=todatetime(ago(2d)), endtime=bin(ago(timeframe), 1h), eventresult='Success')
            | where isnotempty(HttpUserAgent)
            | summarize UserAgentsSet =  make_set(HttpUserAgent) // This could contain upto 10,48,576 unique values
            | extend exists=int(1)
            )
            on exists
        | project-away exist*, maxv, sum*
        ),
        (
        (datatable(exists: int, sumexist: bool)[1, false]
        | where toscalar(eps) between (501 .. 1000)
        | join (summarizationexist) on sumexist)
        | join (
            _Im_WebSession(starttime=todatetime(ago(3d)), endtime=bin(ago(timeframe), 1h), eventresult='Success')
            | where isnotempty(HttpUserAgent)
            | summarize UserAgentsSet = make_set(HttpUserAgent) // This could contain upto 10,48,576 unique values
            | extend exists=int(1)
            )
            on exists
        | project-away exist*, maxv, sum*
        ),
        (
        (datatable(exists: int, sumexist: bool)[1, false]
        | where toscalar(eps) <= 500
        | join (summarizationexist) on sumexist)
        | join (
            _Im_WebSession(starttime=todatetime(ago(4d)), endtime=bin(ago(timeframe), 1h), eventresult='Success')
            | where isnotempty(HttpUserAgent)
            | summarize UserAgentsSet = make_set(HttpUserAgent) // This could contain upto 10,48,576 unique values
            | extend exists=int(1)
            )
            on exists
        | project-away exist*, maxv, sum*
        ),
        (
        WebSession_Summarized_SrcInfo_CL
        | where EventTime_t between (ago(14d) .. ago(timeframe))
            and isnotempty(HttpUserAgent_s)
            and EventResult_s =~ 'Success'
        | summarize UserAgentsSet = make_set(HttpUserAgent_s) // This could contain upto 10,48,576 unique values
        );
_Im_WebSession (starttime=bin(ago(timeframe), 1h), eventresult='Success')
| project
    SrcIpAddr,
    SrcUsername,
    SrcHostname,
    DstIpAddr,
    DstPortNumber,
    Url,
    HttpUserAgent,
    TimeGenerated
| where isnotempty(HttpUserAgent) and HttpUserAgent !in~ (allUserAgents)
| summarize
    EventCount=count(),
    EventStartTime=min(TimeGenerated),
    EventEndTime=max(TimeGenerated)
    by
    SrcIpAddr,
    SrcUsername,
    SrcHostname,
    DstIpAddr,
    Url,
    HttpUserAgent,
    DstPortNumber
| extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")

YAML

queryPeriod: 14d
description: |
    'This rule assists in detecting rare user agents, which may indicate web browsing activity by an unconventional process different from the usual ones. The rule specifically searches for UserAgent strings that have not been seen in the past 14 days. This query will perform better when run over summarized data'
version: 1.0.1
query: |
  let lookBack = 14d;
  let timeframe = 1h;
  // calculate avg. eps(events per second)
  let eps = materialize(_Im_WebSession(starttime=ago(1d))
      | project TimeGenerated
      | summarize AvgPerSec = count() / 3600 by bin(TimeGenerated, 1h)
      | summarize round(avg(AvgPerSec))
      );
  let summarizationexist  = (
      union isfuzzy=true 
          (
          WebSession_Summarized_SrcInfo_CL
          | where EventTime_t > ago(1d) 
          | project v = int(2)
          ),
          (
          print int(1) 
          | project v = print_0
          )
      | summarize maxv = max(v)
      | extend sumexist = (maxv > 1)
      );
  let allUserAgents = union isfuzzy=true
          (
          (datatable(exists: int, sumexist: bool)[1, false]
          | where toscalar(eps) > 1000
          | join (summarizationexist) on sumexist)
          | join (
              _Im_WebSession(starttime=todatetime(ago(2d)), endtime=bin(ago(timeframe), 1h), eventresult='Success')
              | where isnotempty(HttpUserAgent)
              | summarize UserAgentsSet =  make_set(HttpUserAgent) // This could contain upto 10,48,576 unique values
              | extend exists=int(1)
              )
              on exists
          | project-away exist*, maxv, sum*
          ),
          (
          (datatable(exists: int, sumexist: bool)[1, false]
          | where toscalar(eps) between (501 .. 1000)
          | join (summarizationexist) on sumexist)
          | join (
              _Im_WebSession(starttime=todatetime(ago(3d)), endtime=bin(ago(timeframe), 1h), eventresult='Success')
              | where isnotempty(HttpUserAgent)
              | summarize UserAgentsSet = make_set(HttpUserAgent) // This could contain upto 10,48,576 unique values
              | extend exists=int(1)
              )
              on exists
          | project-away exist*, maxv, sum*
          ),
          (
          (datatable(exists: int, sumexist: bool)[1, false]
          | where toscalar(eps) <= 500
          | join (summarizationexist) on sumexist)
          | join (
              _Im_WebSession(starttime=todatetime(ago(4d)), endtime=bin(ago(timeframe), 1h), eventresult='Success')
              | where isnotempty(HttpUserAgent)
              | summarize UserAgentsSet = make_set(HttpUserAgent) // This could contain upto 10,48,576 unique values
              | extend exists=int(1)
              )
              on exists
          | project-away exist*, maxv, sum*
          ),
          (
          WebSession_Summarized_SrcInfo_CL
          | where EventTime_t between (ago(14d) .. ago(timeframe))
              and isnotempty(HttpUserAgent_s)
              and EventResult_s =~ 'Success'
          | summarize UserAgentsSet = make_set(HttpUserAgent_s) // This could contain upto 10,48,576 unique values
          );
  _Im_WebSession (starttime=bin(ago(timeframe), 1h), eventresult='Success')
  | project
      SrcIpAddr,
      SrcUsername,
      SrcHostname,
      DstIpAddr,
      DstPortNumber,
      Url,
      HttpUserAgent,
      TimeGenerated
  | where isnotempty(HttpUserAgent) and HttpUserAgent !in~ (allUserAgents)
  | summarize
      EventCount=count(),
      EventStartTime=min(TimeGenerated),
      EventEndTime=max(TimeGenerated)
      by
      SrcIpAddr,
      SrcUsername,
      SrcHostname,
      DstIpAddr,
      Url,
      HttpUserAgent,
      DstPortNumber
  | extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")  
tactics:
- InitialAccess
triggerOperator: gt
eventGroupingSettings:
  aggregationKind: AlertPerResult
alertDetailsOverride:
  alertDisplayNameFormat: User '{{SrcUsername}}' with IP '{{SrcIpAddr}}' has been observed accessing URL '{{Url}}' using a rare user agent.
  alertDescriptionFormat: The user agent '{{HttpUserAgent}}' has not been observed in the past 14 days. Conduct research on the user agent string to determine if it is associated with a known legitimate bot or if it is potentially linked to malicious activity. The URL is associated with the IP address '{{DstIpAddr}}'.
id: 2d50d937-d7f2-4c05-b151-9af7f9ec747e
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Web Session Essentials/Analytic Rules/RareUserAgentDetected.yaml
customDetails:
  EventStartTime: EventStartTime
  EventEndTime: EventEndTime
  DstPortNumber: DstPortNumber
  EventCount: EventCount
  HttpUserAgent: HttpUserAgent
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
triggerThreshold: 0
kind: Scheduled
tags:
- SchemaVersion: 0.2.6
  Schema: WebSession
status: Available
relevantTechniques:
- T1190
- T1133
name: Detect presence of uncommon user agents in web requests (ASIM Web Session)
severity: Medium
requiredDataConnectors: []
queryFrequency: 1h

ARM