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

Back


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

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

ARM