Exchange Server Suspicious File Downloads

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Id	8955c0fb-3408-47b0-a3b9-a1faec41e427
Rulename	Exchange Server Suspicious File Downloads.
Description	This query looks for messages related to file downloads of suspicious file types on an Exchange Server. This could indicate attempted deployment of webshells. This query uses the Exchange HttpProxy AOBGeneratorLog, you will need to onboard this log as a custom log under the table http_proxy_oab_CL before using this query. This log is commonly found at C:\Program Files\Microsoft\Exchange Server\V15\Logging\OABGeneratorLog on the Exchange server. Details on collecting custom logs into Sentinel can be found here: https://learn.microsoft.com/azure/sentinel/connect-custom-logs
Severity	Medium
Tactics	InitialAccess
Techniques	T1190
Kind	Scheduled
Query frequency	1d
Query period	1d
Trigger threshold	0
Trigger operator	gt
Source Uri	https://github.com/Azure/Azure-Sentinel/blob/master/Detections/http_proxy_oab_CL/ExchagngeSuspiciousFileDownloads.yaml
Version	1.0.2
Arm template	8955c0fb-3408-47b0-a3b9-a1faec41e427.json

KQL

let scriptExtensions = dynamic([".php", ".jsp", ".js", ".aspx", ".asmx", ".asax", ".cfm", ".shtml"]);
http_proxy_oab_CL
| where RawData contains "Download failed and temporary file"
| extend File = extract("([^\\\\]*)(\\\\[^']*)",2,RawData)
| extend Extension = strcat(".",split(File, ".")[-1])
| extend InteractiveFile = iif(Extension in (scriptExtensions), "Yes", "No")
// Uncomment the following line to alert only on interactive file download type
//| where InteractiveFile =~ "Yes"
| extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)

YAML

OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Detections/http_proxy_oab_CL/ExchagngeSuspiciousFileDownloads.yaml
query: |
  let scriptExtensions = dynamic([".php", ".jsp", ".js", ".aspx", ".asmx", ".asax", ".cfm", ".shtml"]);
  http_proxy_oab_CL
  | where RawData contains "Download failed and temporary file"
  | extend File = extract("([^\\\\]*)(\\\\[^']*)",2,RawData)
  | extend Extension = strcat(".",split(File, ".")[-1])
  | extend InteractiveFile = iif(Extension in (scriptExtensions), "Yes", "No")
  // Uncomment the following line to alert only on interactive file download type
  //| where InteractiveFile =~ "Yes"
  | extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
  | extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)  
description: |
  'This query looks for messages related to file downloads of suspicious file types on an Exchange Server. This could indicate attempted deployment of webshells. 
  This query uses the Exchange HttpProxy AOBGeneratorLog, you will need to onboard this log as a custom log under the table http_proxy_oab_CL before using this query. 
  This log is commonly found at C:\Program Files\Microsoft\Exchange Server\V15\Logging\OABGeneratorLog on the Exchange server. Details on collecting custom logs into Sentinel
  can be found here: https://learn.microsoft.com/azure/sentinel/connect-custom-logs  
severity: Medium
requiredDataConnectors: []
name: Exchange Server Suspicious File Downloads.
triggerThreshold: 0
metadata:
  source:
    kind: Community
  categories:
    domains:
    - Application
  author:
    name: Microsoft Security Research
  support:
    tier: Community
tactics:
- InitialAccess
version: 1.0.2
relevantTechniques:
- T1190
triggerOperator: gt
entityMappings:
- entityType: Host
  fieldMappings:
  - columnName: Computer
    identifier: FullName
  - columnName: HostName
    identifier: HostName
  - columnName: HostNameDomain
    identifier: DnsDomain
id: 8955c0fb-3408-47b0-a3b9-a1faec41e427
kind: Scheduled
queryFrequency: 1d
queryPeriod: 1d

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/8955c0fb-3408-47b0-a3b9-a1faec41e427')]",
      "kind": "Scheduled",
      "name": "[concat(parameters('workspace'),'/Microsoft.SecurityInsights/8955c0fb-3408-47b0-a3b9-a1faec41e427')]",
      "properties": {
        "alertRuleTemplateName": "8955c0fb-3408-47b0-a3b9-a1faec41e427",
        "customDetails": null,
        "description": "'This query looks for messages related to file downloads of suspicious file types on an Exchange Server. This could indicate attempted deployment of webshells. \nThis query uses the Exchange HttpProxy AOBGeneratorLog, you will need to onboard this log as a custom log under the table http_proxy_oab_CL before using this query. \nThis log is commonly found at C:\\Program Files\\Microsoft\\Exchange Server\\V15\\Logging\\OABGeneratorLog on the Exchange server. Details on collecting custom logs into Sentinel\ncan be found here: https://learn.microsoft.com/azure/sentinel/connect-custom-logs\n",
        "displayName": "Exchange Server Suspicious File Downloads.",
        "enabled": true,
        "entityMappings": [
          {
            "entityType": "Host",
            "fieldMappings": [
              {
                "columnName": "Computer",
                "identifier": "FullName"
              },
              {
                "columnName": "HostName",
                "identifier": "HostName"
              },
              {
                "columnName": "HostNameDomain",
                "identifier": "DnsDomain"
              }
            ]
          }
        ],
        "OriginalUri": "https://github.com/Azure/Azure-Sentinel/blob/master/Detections/http_proxy_oab_CL/ExchagngeSuspiciousFileDownloads.yaml",
        "query": "let scriptExtensions = dynamic([\".php\", \".jsp\", \".js\", \".aspx\", \".asmx\", \".asax\", \".cfm\", \".shtml\"]);\nhttp_proxy_oab_CL\n| where RawData contains \"Download failed and temporary file\"\n| extend File = extract(\"([^\\\\\\\\]*)(\\\\\\\\[^']*)\",2,RawData)\n| extend Extension = strcat(\".\",split(File, \".\")[-1])\n| extend InteractiveFile = iif(Extension in (scriptExtensions), \"Yes\", \"No\")\n// Uncomment the following line to alert only on interactive file download type\n//| where InteractiveFile =~ \"Yes\"\n| extend HostName = tostring(split(Computer, \".\")[0]), DomainIndex = toint(indexof(Computer, '.'))\n| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)\n",
        "queryFrequency": "P1D",
        "queryPeriod": "P1D",
        "severity": "Medium",
        "subTechniques": [],
        "suppressionDuration": "PT1H",
        "suppressionEnabled": false,
        "tactics": [
          "InitialAccess"
        ],
        "techniques": [
          "T1190"
        ],
        "templateVersion": "1.0.2",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 0
      },
      "type": "Microsoft.OperationalInsights/workspaces/providers/alertRules"
    }
  ]
}