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]),"")

entityMappings:
- entityType: File
  fieldMappings:
  - identifier: Name
    columnName: FileWithdualextension
- entityType: URL
  fieldMappings:
  - identifier: Url
    columnName: Url
- entityType: IP
  fieldMappings:
  - identifier: Address
    columnName: SrcIpAddr
- entityType: Account
  fieldMappings:
  - identifier: Name
    columnName: Name
  - identifier: UPNSuffix
    columnName: UPNSuffix
- entityType: Host
  fieldMappings:
  - identifier: HostName
    columnName: SrcHostname
name: Detect potential presence of a malicious file with a double extension (ASIM Web Session)
tactics:
- DefenseEvasion
- Persistence
- CommandAndControl
severity: Medium
triggerThreshold: 0
relevantTechniques:
- T1036
- T1505
- T1071
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.'
id: 6a71687f-00cf-44d3-93fc-8cbacc7b5615
version: 1.0.1
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Web Session Essentials/Analytic Rules/PossibleMaliciousDoubleExtension.yaml
customDetails:
  EventCount: EventCount
  EventEndTime: EventEndTime
  EventStartTime: EventStartTime
  DstHostname: DstHostname
queryFrequency: 1h
eventGroupingSettings:
  aggregationKind: AlertPerResult
triggerOperator: gt
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]),"")  
tags:
- SchemaVersion: 0.2.6
  Schema: WebSession
requiredDataConnectors: []
status: Available
queryPeriod: 1h
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
kind: Scheduled

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "workspace": {
      "type": "String"
    }
  },
  "resources": [
    {
      "apiVersion": "2024-01-01-preview",
      "id": "[concat(resourceId('Microsoft.OperationalInsights/workspaces/providers', parameters('workspace'), 'Microsoft.SecurityInsights'),'/alertRules/6a71687f-00cf-44d3-93fc-8cbacc7b5615')]",
      "kind": "Scheduled",
      "name": "[concat(parameters('workspace'),'/Microsoft.SecurityInsights/6a71687f-00cf-44d3-93fc-8cbacc7b5615')]",
      "properties": {
        "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"
        },
        "alertRuleTemplateName": "6a71687f-00cf-44d3-93fc-8cbacc7b5615",
        "customDetails": {
          "DstHostname": "DstHostname",
          "EventCount": "EventCount",
          "EventEndTime": "EventEndTime",
          "EventStartTime": "EventStartTime"
        },
        "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.'\n",
        "displayName": "Detect potential presence of a malicious file with a double extension (ASIM Web Session)",
        "enabled": true,
        "entityMappings": [
          {
            "entityType": "File",
            "fieldMappings": [
              {
                "columnName": "FileWithdualextension",
                "identifier": "Name"
              }
            ]
          },
          {
            "entityType": "URL",
            "fieldMappings": [
              {
                "columnName": "Url",
                "identifier": "Url"
              }
            ]
          },
          {
            "entityType": "IP",
            "fieldMappings": [
              {
                "columnName": "SrcIpAddr",
                "identifier": "Address"
              }
            ]
          },
          {
            "entityType": "Account",
            "fieldMappings": [
              {
                "columnName": "Name",
                "identifier": "Name"
              },
              {
                "columnName": "UPNSuffix",
                "identifier": "UPNSuffix"
              }
            ]
          },
          {
            "entityType": "Host",
            "fieldMappings": [
              {
                "columnName": "SrcHostname",
                "identifier": "HostName"
              }
            ]
          }
        ],
        "eventGroupingSettings": {
          "aggregationKind": "AlertPerResult"
        },
        "OriginalUri": "https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Web Session Essentials/Analytic Rules/PossibleMaliciousDoubleExtension.yaml",
        "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\n_Im_WebSession (starttime=ago(1h), eventresult='Success')\n| where HttpRequestMethod in~ (\"POST\", \"PUT\") \n| project\n    Url,\n    SrcIpAddr,\n    SrcUsername,\n    SrcHostname,\n    DstIpAddr,\n    DstPortNumber,\n    DstHostname,\n    TimeGenerated\n| extend requestedFileName=tostring(split(tostring(parse_url(Url)[\"Path\"]), '/')[-1])\n| extend FileWithdualextension = extract(@'([\\w-]+\\.\\w+\\.\\w+)$', 1, requestedFileName, typeof(string))\n| extend SecondExt = tostring(split(FileWithdualextension, '.')[-1])\n| where strcat('.', SecondExt) in~ (common_file_ext_list) // Second extension is mostly from the common files\n| summarize\n    EventCount=count(),\n    EventStartTime=min(TimeGenerated),\n    EventEndTime=max(TimeGenerated)\n    by\n    SrcIpAddr,\n    Url,\n    FileWithdualextension,\n    SrcUsername,\n    SrcHostname,\n    DstIpAddr,\n    DstPortNumber,\n    DstHostname\n| extend Name = iif(SrcUsername contains \"@\", tostring(split(SrcUsername,'@',0)[0]),SrcUsername), UPNSuffix = iif(SrcUsername contains \"@\",tostring(split(SrcUsername,'@',1)[0]),\"\")\n",
        "queryFrequency": "PT1H",
        "queryPeriod": "PT1H",
        "severity": "Medium",
        "status": "Available",
        "subTechniques": [],
        "suppressionDuration": "PT1H",
        "suppressionEnabled": false,
        "tactics": [
          "CommandAndControl",
          "DefenseEvasion",
          "Persistence"
        ],
        "tags": [
          {
            "Schema": "WebSession",
            "SchemaVersion": "0.2.6"
          }
        ],
        "techniques": [
          "T1036",
          "T1071",
          "T1505"
        ],
        "templateVersion": "1.0.1",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 0
      },
      "type": "Microsoft.OperationalInsights/workspaces/providers/alertRules"
    }
  ]
}