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Potential Modification of Accessibility Binaries

Elastic Stack Serverless Security

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.*
  • logs-windows.*

Severity: high

Risk score: 73

Runs every: 5m

Searches indices from: now-9m (https://www.elastic.co/guide/en/elasticsearch/reference/current/common-options.html#date-math[Date Math format], see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Elastic
  • Host
  • Windows
  • Threat Detection
  • Persistence
  • has_guide

Version: 101

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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.

  • 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.
  • Retrieve the file and determine if it is malicious:
    • Use a private sandboxed malware analysis system to perform analysis.
      • Observe and collect information about the following activities:
        • Attempts to contact external domains and addresses.
        • File and registry access, modification, and creation activities.
        • Service creation and launch activities.
        • Scheduled task creation.
    • Use the PowerShell Get-FileHash cmdlet to get the files' SHA-256 hash values.
      • Search for the existence and reputation of the hashes in resources like VirusTotal, Hybrid-Analysis, CISCO Talos, Any.run, etc.
  • 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.
  • 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

process where event.type == "start" and
 process.parent.name : ("Utilman.exe", "winlogon.exe") and user.name == "SYSTEM" 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