Microsoft Sentinel Analytic Rules
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Anomalous User Agent connection attempt

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Idf845881e-2500-44dc-8ed7-b372af3e1e25
RulenameAnomalous User Agent connection attempt
DescriptionIdentifies connection attempts (success or fail) from clients with very short or very long User Agent strings and with less than 100 connection attempts.
SeverityLow
TacticsInitialAccess
TechniquesT1190
Required data connectorsAzureMonitor(IIS)
KindScheduled
Query frequency1d
Query period1d
Trigger threshold0
Trigger operatorgt
Source Urihttps://github.com/Azure/Azure-Sentinel/blob/master/Detections/W3CIISLog/AnomomlousUserAgentConnection.yaml
Version1.0.3
Arm templatef845881e-2500-44dc-8ed7-b372af3e1e25.json
Deploy To Azure
let short_uaLength = 5;
let long_uaLength = 1000;
let c_threshold = 100;
W3CIISLog
// Exclude local IPs as these create noise
| where cIP !startswith "192.168." and cIP != "::1"
| where isnotempty(csUserAgent) and csUserAgent !in~ ("-", "MSRPC") and (string_size(csUserAgent) <= short_uaLength or string_size(csUserAgent) >= long_uaLength)
| extend csUserAgent_size = string_size(csUserAgent)
| summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), ConnectionCount = count() by Computer, sSiteName, sPort, csUserAgent, csUserAgent_size, csUserName , csMethod, csUriStem, sIP, cIP, scStatus, scSubStatus, scWin32Status
| where ConnectionCount < c_threshold
| extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
| extend AccountName = tostring(split(csUserName, "@")[0]), AccountUPNSuffix = tostring(split(csUserName, "@")[1])
OriginalUri: https://github.com/Azure/Azure-Sentinel/blob/master/Detections/W3CIISLog/AnomomlousUserAgentConnection.yaml
query: |
  let short_uaLength = 5;
  let long_uaLength = 1000;
  let c_threshold = 100;
  W3CIISLog
  // Exclude local IPs as these create noise
  | where cIP !startswith "192.168." and cIP != "::1"
  | where isnotempty(csUserAgent) and csUserAgent !in~ ("-", "MSRPC") and (string_size(csUserAgent) <= short_uaLength or string_size(csUserAgent) >= long_uaLength)
  | extend csUserAgent_size = string_size(csUserAgent)
  | summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), ConnectionCount = count() by Computer, sSiteName, sPort, csUserAgent, csUserAgent_size, csUserName , csMethod, csUriStem, sIP, cIP, scStatus, scSubStatus, scWin32Status
  | where ConnectionCount < c_threshold
  | extend HostName = tostring(split(Computer, ".")[0]), DomainIndex = toint(indexof(Computer, '.'))
  | extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)
  | extend AccountName = tostring(split(csUserName, "@")[0]), AccountUPNSuffix = tostring(split(csUserName, "@")[1])  
description: |
    'Identifies connection attempts (success or fail) from clients with very short or very long User Agent strings and with less than 100 connection attempts.'
severity: Low
requiredDataConnectors:
- dataTypes:
  - W3CIISLog
  connectorId: AzureMonitor(IIS)
name: Anomalous User Agent connection attempt
triggerThreshold: 0
metadata:
  source:
    kind: Community
  categories:
    domains:
    - Security - Threat Protection
  author:
    name: Microsoft Security Research
  support:
    tier: Community
tactics:
- InitialAccess
version: 1.0.3
relevantTechniques:
- T1190
triggerOperator: gt
entityMappings:
- entityType: Account
  fieldMappings:
  - columnName: csUserName
    identifier: FullName
  - columnName: AccountName
    identifier: Name
  - columnName: AccountUPNSuffix
    identifier: UPNSuffix
- entityType: Host
  fieldMappings:
  - columnName: Computer
    identifier: FullName
  - columnName: HostName
    identifier: HostName
  - columnName: HostNameDomain
    identifier: NTDomain
- entityType: IP
  fieldMappings:
  - columnName: cIP
    identifier: Address
id: f845881e-2500-44dc-8ed7-b372af3e1e25
kind: Scheduled
queryFrequency: 1d
queryPeriod: 1d
{
  "$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/f845881e-2500-44dc-8ed7-b372af3e1e25')]",
      "kind": "Scheduled",
      "name": "[concat(parameters('workspace'),'/Microsoft.SecurityInsights/f845881e-2500-44dc-8ed7-b372af3e1e25')]",
      "properties": {
        "alertRuleTemplateName": "f845881e-2500-44dc-8ed7-b372af3e1e25",
        "customDetails": null,
        "description": "'Identifies connection attempts (success or fail) from clients with very short or very long User Agent strings and with less than 100 connection attempts.'\n",
        "displayName": "Anomalous User Agent connection attempt",
        "enabled": true,
        "entityMappings": [
          {
            "entityType": "Account",
            "fieldMappings": [
              {
                "columnName": "csUserName",
                "identifier": "FullName"
              },
              {
                "columnName": "AccountName",
                "identifier": "Name"
              },
              {
                "columnName": "AccountUPNSuffix",
                "identifier": "UPNSuffix"
              }
            ]
          },
          {
            "entityType": "Host",
            "fieldMappings": [
              {
                "columnName": "Computer",
                "identifier": "FullName"
              },
              {
                "columnName": "HostName",
                "identifier": "HostName"
              },
              {
                "columnName": "HostNameDomain",
                "identifier": "NTDomain"
              }
            ]
          },
          {
            "entityType": "IP",
            "fieldMappings": [
              {
                "columnName": "cIP",
                "identifier": "Address"
              }
            ]
          }
        ],
        "OriginalUri": "https://github.com/Azure/Azure-Sentinel/blob/master/Detections/W3CIISLog/AnomomlousUserAgentConnection.yaml",
        "query": "let short_uaLength = 5;\nlet long_uaLength = 1000;\nlet c_threshold = 100;\nW3CIISLog\n// Exclude local IPs as these create noise\n| where cIP !startswith \"192.168.\" and cIP != \"::1\"\n| where isnotempty(csUserAgent) and csUserAgent !in~ (\"-\", \"MSRPC\") and (string_size(csUserAgent) <= short_uaLength or string_size(csUserAgent) >= long_uaLength)\n| extend csUserAgent_size = string_size(csUserAgent)\n| summarize StartTimeUtc = min(TimeGenerated), EndTimeUtc = max(TimeGenerated), ConnectionCount = count() by Computer, sSiteName, sPort, csUserAgent, csUserAgent_size, csUserName , csMethod, csUriStem, sIP, cIP, scStatus, scSubStatus, scWin32Status\n| where ConnectionCount < c_threshold\n| extend HostName = tostring(split(Computer, \".\")[0]), DomainIndex = toint(indexof(Computer, '.'))\n| extend HostNameDomain = iff(DomainIndex != -1, substring(Computer, DomainIndex + 1), Computer)\n| extend AccountName = tostring(split(csUserName, \"@\")[0]), AccountUPNSuffix = tostring(split(csUserName, \"@\")[1])\n",
        "queryFrequency": "P1D",
        "queryPeriod": "P1D",
        "severity": "Low",
        "subTechniques": [],
        "suppressionDuration": "PT1H",
        "suppressionEnabled": false,
        "tactics": [
          "InitialAccess"
        ],
        "techniques": [
          "T1190"
        ],
        "templateVersion": "1.0.3",
        "triggerOperator": "GreaterThan",
        "triggerThreshold": 0
      },
      "type": "Microsoft.OperationalInsights/workspaces/providers/alertRules"
    }
  ]
}