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Attempts to Brute Force a Microsoft 365 User Account

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Attempts to Brute Force a Microsoft 365 User Account

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Identifies potential brute-force attempts against Microsoft 365 user accounts by detecting a high number of failed login attempts or login sources within a 30-minute window. Attackers may attempt to brute force user accounts to gain unauthorized access to Microsoft 365 services.

Rule type: esql

Rule indices: None

Severity: medium

Risk score: 47

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: Cloud
  • Domain: SaaS
  • Data Source: Microsoft 365
  • Use Case: Identity and Access Audit
  • Use Case: Threat Detection
  • Tactic: Credential Access

Version: 309

Rule authors:

  • Elastic
  • Willem D’Haese
  • Austin Songer

Rule license: Elastic License v2

Rule query

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from logs-o365.audit-*
// truncate the timestamp to a 30-minute window
| eval target_time_window = DATE_TRUNC(30 minutes, @timestamp)
| mv_expand event.category
| where event.dataset == "o365.audit"
  and event.category == "authentication"

  // filter only on Entra ID or Exchange audit logs in O365 integration
  and event.provider in ("AzureActiveDirectory", "Exchange")

  // filter only for UserLoginFailed or partial failures
  and event.action in ("UserLoginFailed", "PasswordLogonInitialAuthUsingPassword")

  // ignore specific logon errors
  and not o365.audit.LogonError in (
    "EntitlementGrantsNotFound",
    "UserStrongAuthEnrollmentRequired",
    "UserStrongAuthClientAuthNRequired",
    "InvalidReplyTo",
    "SsoArtifactExpiredDueToConditionalAccess",
    "PasswordResetRegistrationRequiredInterrupt",
    "SsoUserAccountNotFoundInResourceTenant",
    "UserStrongAuthExpired",
    "CmsiInterrupt"
)
  // filters out non user or application logins based on target
  and o365.audit.Target.Type in ("0", "2", "3", "5", "6", "10")

  // filters only for logins from user or application, ignoring oauth:token
  and to_lower(o365.audit.ExtendedProperties.RequestType) rlike "(.*)login(.*)"

// count the number of login sources and failed login attempts
| stats
  login_source_count = count(source.ip),
  failed_login_count = count(*) by target_time_window, o365.audit.UserId

// filter for users with more than 20 login sources or failed login attempts
| where (login_source_count >= 20 or failed_login_count >= 20)

Framework: MITRE ATT&CKTM

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