name: Detect Exchange Web Shell
id: 8c14eeee-2af1-4a4b-bda8-228da0f4862a
version: 4
date: '2022-09-30'
author: Michael Haag, Shannon Davis, David Dorsey, Splunk
type: TTP
datamodel:
- Endpoint
description: 'The following query identifies suspicious .aspx created in 3 paths identified
  by Microsoft as known drop locations for Exchange exploitation related to HAFNIUM
  group and recently disclosed vulnerablity named ProxyShell and ProxyNotShell. Paths include: `\HttpProxy\owa\auth\`,
  `\inetpub\wwwroot\aspnet_client\`, and `\HttpProxy\OAB\`. Upon triage, the suspicious
  .aspx file will likely look obvious on the surface. inspect the contents for script
  code inside. Identify additional log sources, IIS included, to review source and
  other potential exploitation. It is often the case that a particular threat is only
  applicable to a specific subset of systems in your environment. Typically analytics
  to detect those threats are written without the benefit of being able to only target
  those systems as well. Writing analytics against all systems when those behaviors
  are limited to identifiable subsets of those systems is suboptimal. Consider the
  case ProxyShell vulnerability on Microsoft Exchange Servers. With asset information,
  a hunter can limit their analytics to systems that have been identified as Exchange
  servers. A hunter may start with the theory that the exchange server is communicating
  with new systems that it has not previously. If this theory is run against all publicly
  facing systems, the amount of noise it will generate will likely render this theory
  untenable. However, using the asset information to limit this analytic to just the
  Exchange servers will reduce the noise allowing the hunter to focus only on the
  systems where this behavioral change is relevant.'
search: '| tstats `security_content_summariesonly` count FROM datamodel=Endpoint.Processes
  where Processes.process_name=System  by _time span=1h Processes.process_id Processes.process_name
  Processes.dest | `drop_dm_object_name(Processes)` | join process_guid, _time [|
  tstats `security_content_summariesonly` count min(_time) as firstTime max(_time)
  as lastTime FROM datamodel=Endpoint.Filesystem where Filesystem.file_path IN ("*\\HttpProxy\\owa\\auth\\*",
  "*\\inetpub\\wwwroot\\aspnet_client\\*", "*\\HttpProxy\\OAB\\*") Filesystem.file_name IN( "*.aspx", "*.ashx")
  by _time span=1h Filesystem.dest Filesystem.file_create_time Filesystem.file_name
  Filesystem.file_path | `drop_dm_object_name(Filesystem)` | fields _time dest file_create_time
  file_name file_path process_name process_path process] | dedup file_create_time
  | table dest file_create_time, file_name, file_path, process_name | `detect_exchange_web_shell_filter`'
how_to_implement: To successfully implement this search you need to be ingesting information
  on process that include the name of the process responsible for the changes from
  your endpoints into the `Endpoint` datamodel in the `Processes` node and `Filesystem`
  node.
known_false_positives: The query is structured in a way that `action` (read, create)
  is not defined. Review the results of this query, filter, and tune as necessary.
  It may be necessary to generate this query specific to your endpoint product.
references:
- https://raw.githubusercontent.com/Azure/Azure-Sentinel/master/Sample%20Data/Feeds/MSTICIoCs-ExchangeServerVulnerabilitiesDisclosedMarch2021.csv
- https://www.zerodayinitiative.com/blog/2021/8/17/from-pwn2own-2021-a-new-attack-surface-on-microsoft-exchange-proxyshell
- https://www.youtube.com/watch?v=FC6iHw258RI
- https://www.huntress.com/blog/rapid-response-microsoft-exchange-servers-still-vulnerable-to-proxyshell-exploit#what-should-you-do
tags:
  analytic_story:
  - HAFNIUM Group
  - ProxyShell
  - CISA AA22-257A
  - ProxyNotShell
  confidence: 90
  context:
  - Source:Endpoint
  - Stage:Execution
  dataset:
  - https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1505.003/windows-sysmon_proxylogon.log
  impact: 90
  kill_chain_phases:
  - Exploitation
  message: A file - $file_name$ was written to disk that is related to IIS exploitation
    previously performed by HAFNIUM. Review further file modifications on endpoint
    $dest$ by user $user$.
  mitre_attack_id:
  - T1505
  - T1505.003
  - T1190
  observable:
  - name: user
    type: User
    role:
    - Victim
  - name: dest
    type: Hostname
    role:
    - Victim
  - name: file_name
    type: File Name
    role:
    - Victim
  product:
  - Splunk Enterprise
  - Splunk Enterprise Security
  - Splunk Cloud
  required_fields:
  - _time
  - Filesystem.file_path
  - Filesystem.process_id
  - Filesystem.file_name
  - Filesystem.file_hash
  - Filesystem.user
  risk_score: 81
  security_domain: endpoint
  supported_tas:
  - Splunk_TA_microsoft_sysmon
  asset_type: Endpoint