Detect potential presence of a malicious file with a double extension ASIM Web Session

let common_file_ext_list = dynamic([".txt", ".xlsx", ".doc", ".docx", ".csv", ".pdf", ".png", ".jpg", ".jpeg"]); // Add list of common files as per your environment
_Im_WebSession (starttime=ago(1h), eventresult='Success')
| where HttpRequestMethod in~ ("POST", "PUT") 
| project
    Url,
    SrcIpAddr,
    SrcUsername,
    SrcHostname,
    DstIpAddr,
    DstPortNumber,
    DstHostname,
    TimeGenerated
| extend requestedFileName=tostring(split(tostring(parse_url(Url)["Path"]), '/')[-1])
| extend FileWithdualextension = extract(@'([\w-]+\.\w+\.\w+)$', 1, requestedFileName, typeof(string))
| extend SecondExt = tostring(split(FileWithdualextension, '.')[-1])
| where strcat('.', SecondExt) in~ (common_file_ext_list) // Second extension is mostly from the common files
| summarize
    EventCount=count(),
    EventStartTime=min(TimeGenerated),
    EventEndTime=max(TimeGenerated)
    by
    SrcIpAddr,
    Url,
    FileWithdualextension,
    SrcUsername,
    SrcHostname,
    DstIpAddr,
    DstPortNumber,
    DstHostname
| extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")

description: |
    'Double extension vulnerability is a significant concern in file uploads, as it can lead to various issues if an attacker successfully uploads a virus-infected file.'
version: 1.0.1
tactics:
- DefenseEvasion
- Persistence
- CommandAndControl
queryFrequency: 1h
query: |
  let common_file_ext_list = dynamic([".txt", ".xlsx", ".doc", ".docx", ".csv", ".pdf", ".png", ".jpg", ".jpeg"]); // Add list of common files as per your environment
  _Im_WebSession (starttime=ago(1h), eventresult='Success')
  | where HttpRequestMethod in~ ("POST", "PUT") 
  | project
      Url,
      SrcIpAddr,
      SrcUsername,
      SrcHostname,
      DstIpAddr,
      DstPortNumber,
      DstHostname,
      TimeGenerated
  | extend requestedFileName=tostring(split(tostring(parse_url(Url)["Path"]), '/')[-1])
  | extend FileWithdualextension = extract(@'([\w-]+\.\w+\.\w+)$', 1, requestedFileName, typeof(string))
  | extend SecondExt = tostring(split(FileWithdualextension, '.')[-1])
  | where strcat('.', SecondExt) in~ (common_file_ext_list) // Second extension is mostly from the common files
  | summarize
      EventCount=count(),
      EventStartTime=min(TimeGenerated),
      EventEndTime=max(TimeGenerated)
      by
      SrcIpAddr,
      Url,
      FileWithdualextension,
      SrcUsername,
      SrcHostname,
      DstIpAddr,
      DstPortNumber,
      DstHostname
  | extend Name = iif(SrcUsername contains "@", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains "@",tostring(split(SrcUsername,'@',1)[0]),"")  
status: Available
triggerOperator: gt
kind: Scheduled
triggerThreshold: 0
customDetails:
  EventStartTime: EventStartTime
  DstHostname: DstHostname
  EventCount: EventCount
  EventEndTime: EventEndTime
alertDetailsOverride:
  alertDescriptionFormat: User posted file '{{FileWithdualextension}}' which potentially contain dual extensions. This type of activity could be malicious and performed to bypass file upload filters or security measures implemented by the application. Destination server name this request was targetted to - '{{DstHostname}}'
  alertDisplayNameFormat: User '{{SrcUsername}}' with IP address '{{SrcIpAddr}}' has been observed with posting potentially risky dual extension file
relevantTechniques:
- T1036
- T1505
- T1071
id: 6a71687f-00cf-44d3-93fc-8cbacc7b5615
queryPeriod: 1h
entityMappings:
- fieldMappings:
  - identifier: Name
    columnName: FileWithdualextension
  entityType: File
- fieldMappings:
  - identifier: Url
    columnName: Url
  entityType: URL
- fieldMappings:
  - identifier: Address
    columnName: SrcIpAddr
  entityType: IP
- fieldMappings:
  - identifier: Name
    columnName: Name
  - identifier: UPNSuffix
    columnName: UPNSuffix
  entityType: Account
- fieldMappings:
  - identifier: HostName
    columnName: SrcHostname
  entityType: Host
eventGroupingSettings:
  aggregationKind: AlertPerResult
severity: Medium
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Web Session Essentials/Analytic Rules/PossibleMaliciousDoubleExtension.yaml
name: Detect potential presence of a malicious file with a double extension (ASIM Web Session)
tags:
- SchemaVersion: 0.2.6
  Schema: WebSession
requiredDataConnectors: []