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Potential Modification of Accessibility Binaries

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Potential Modification of Accessibility Binaries

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Windows contains accessibility features that may be launched with a key combination before a user has logged in. An adversary can modify the way these programs are launched to get a command prompt or backdoor without logging in to the system.

Rule type: eql

Rule indices:

  • winlogbeat-*
  • logs-endpoint.events.process-*
  • logs-windows.sysmon_operational-*
  • endgame-*
  • logs-m365_defender.event-*

Severity: high

Risk score: 73

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Domain: Endpoint
  • OS: Windows
  • Use Case: Threat Detection
  • Tactic: Persistence
  • Resources: Investigation Guide
  • Data Source: Elastic Endgame
  • Data Source: Elastic Defend
  • Data Source: Sysmon
  • Data Source: Microsoft Defender for Endpoint

Version: 212

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Investigating Potential Modification of Accessibility Binaries

Adversaries may establish persistence and/or elevate privileges by executing malicious content triggered by accessibility features. Windows contains accessibility features that may be launched with a key combination before a user has logged in (ex: when the user is on the Windows logon screen). An adversary can modify the way these programs are launched to get a command prompt or backdoor without logging in to the system.

More details can be found here.

This rule looks for the execution of supposed accessibility binaries that don’t match any of the accessibility features binaries' original file names, which is likely a custom binary deployed by the attacker.

Note: This investigation guide uses the Osquery Markdown Plugin introduced in Elastic Stack version 8.5.0. Older Elastic Stack versions will display unrendered Markdown in this guide.

Possible investigation steps

  • Investigate the process execution chain (parent process tree) for unknown processes. Examine their executable files for prevalence, whether they are located in expected locations, and if they are signed with valid digital signatures.
  • Identify the user account that performed the action and whether it should perform this kind of action.
  • Contact the account and system owners and confirm whether they are aware of this activity.
  • Investigate other alerts associated with the user/host during the past 48 hours.
  • Assess whether this behavior is prevalent in the environment by looking for similar occurrences across hosts.
  • Examine the host for derived artifacts that indicate suspicious activities:
  • Analyze the file using a private sandboxed analysis system.
  • Observe and collect information about the following activities in both the sandbox and the alert subject host:
  • Attempts to contact external domains and addresses.
  • Use the Elastic Defend network events to determine domains and addresses contacted by the subject process by filtering by the process' process.entity_id.
  • Examine the DNS cache for suspicious or anomalous entries.
  • !{osquery{"label":"Osquery - Retrieve DNS Cache","query":"SELECT * FROM dns_cache"}}
  • Use the Elastic Defend registry events to examine registry keys accessed, modified, or created by the related processes in the process tree.
  • Examine the host services for suspicious or anomalous entries.
  • !{osquery{"label":"Osquery - Retrieve All Services","query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services"}}
  • !{osquery{"label":"Osquery - Retrieve Services Running on User Accounts","query":"SELECT description, display_name, name, path, pid, service_type, start_type, status, user_account FROM services WHERE\nNOT (user_account LIKE %LocalSystem OR user_account LIKE %LocalService OR user_account LIKE %NetworkService OR\nuser_account == null)\n"}}
  • !{osquery{"label":"Osquery - Retrieve Service Unsigned Executables with Virustotal Link","query":"SELECT concat(https://www.virustotal.com/gui/file/, sha1) AS VtLink, name, description, start_type, status, pid,\nservices.path FROM services JOIN authenticode ON services.path = authenticode.path OR services.module_path =\nauthenticode.path JOIN hash ON services.path = hash.path WHERE authenticode.result != trusted\n"}}
  • Retrieve the files' SHA-256 hash values using the PowerShell Get-FileHash cmdlet and search for the existence and reputation of the hashes in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.
  • Investigate potentially compromised accounts. Analysts can do this by searching for login events (for example, 4624) to the target host after the registry modification.

False positive analysis

  • This activity should not happen legitimately. The security team should address any potential benign true positive (B-TP), as this configuration can put the user and the domain at risk.

Response and remediation

  • Initiate the incident response process based on the outcome of the triage.
  • Isolate the involved host to prevent further post-compromise behavior.
  • If the triage identified malware, search the environment for additional compromised hosts.
  • Implement temporary network rules, procedures, and segmentation to contain the malware.
  • Stop suspicious processes.
  • Immediately block the identified indicators of compromise (IoCs).
  • Inspect the affected systems for additional malware backdoors like reverse shells, reverse proxies, or droppers that attackers could use to reinfect the system.
  • Remove and block malicious artifacts identified during triage.
  • Run a full antimalware scan. This may reveal additional artifacts left in the system, persistence mechanisms, and malware components.
  • Investigate credential exposure on systems compromised or used by the attacker to ensure all compromised accounts are identified. Reset passwords for these accounts and other potentially compromised credentials, such as email, business systems, and web services.
  • Determine the initial vector abused by the attacker and take action to prevent reinfection through the same vector.
  • Using the incident response data, update logging and audit policies to improve the mean time to detect (MTTD) and the mean time to respond (MTTR).

Rule query

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process where host.os.type == "windows" and event.type == "start" and
 process.parent.name : ("Utilman.exe", "winlogon.exe") and user.name == "SYSTEM" and
 process.pe.original_file_name : "?*" and
 process.args :
    (
    "C:\\Windows\\System32\\osk.exe",
    "C:\\Windows\\System32\\Magnify.exe",
    "C:\\Windows\\System32\\Narrator.exe",
    "C:\\Windows\\System32\\Sethc.exe",
    "utilman.exe",
    "ATBroker.exe",
    "DisplaySwitch.exe",
    "sethc.exe"
    )
 and not process.pe.original_file_name in
    (
    "osk.exe",
    "sethc.exe",
    "utilman2.exe",
    "DisplaySwitch.exe",
    "ATBroker.exe",
    "ScreenMagnifier.exe",
    "SR.exe",
    "Narrator.exe",
    "magnify.exe",
    "MAGNIFY.EXE"
    )

/* uncomment once in winlogbeat to avoid bypass with rogue process with matching pe original file name */
/* and process.code_signature.subject_name == "Microsoft Windows" and process.code_signature.status == "trusted" */

Framework: MITRE ATT&CKTM

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