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

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Id	6a71687f-00cf-44d3-93fc-8cbacc7b5615
Rulename	Detect potential presence of a malicious file with a double extension (ASIM Web Session)
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.
Severity	Medium
Tactics	DefenseEvasion Persistence CommandAndControl
Techniques	T1036 T1505 T1071
Kind	Scheduled
Query frequency	1h
Query period	1h
Trigger threshold	0
Trigger operator	gt
Source Uri	https://github.com/Azure/Azure-Sentinel/blob/master/Solutions/Web Session Essentials/Analytic Rules/PossibleMaliciousDoubleExtension.yaml
Version	1.0.1
Arm template	6a71687f-00cf-44d3-93fc-8cbacc7b5615.json

KQL

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]),"")

YAML

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

ARM

{
  "$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"
    }
  ]
}