Detection: Azure AD Unusual Number of Failed Authentications From Ip

Description

The following analytic identifies a single source IP failing to authenticate with multiple valid users, potentially indicating a Password Spraying attack against an Azure Active Directory tenant. It uses Azure SignInLogs data and calculates the standard deviation for source IPs, applying the 3-sigma rule to detect unusual numbers of failed authentication attempts. This activity is significant as it may signal an adversary attempting to gain initial access or elevate privileges. If confirmed malicious, this could lead to unauthorized access, privilege escalation, and potential compromise of sensitive information.

Search

1 `azure_monitor_aad`  category=SignInLogs properties.status.errorCode=50126 properties.authenticationDetails{}.succeeded=false 
2| rename properties.* as * 
3| bucket span=5m _time 
4| stats  dc(userPrincipalName) AS unique_accounts values(userPrincipalName) as userPrincipalName by _time, ipAddress 
5| eventstats  avg(unique_accounts) as ip_avg, stdev(unique_accounts) as ip_std by ipAddress 
6| eval  upperBound=(ip_avg+ip_std*3) 
7| eval  isOutlier=if(unique_accounts > 10 and unique_accounts >= upperBound, 1,0) 
8| where isOutlier = 1 
9| `azure_ad_unusual_number_of_failed_authentications_from_ip_filter`

Data Source

Name	Platform	Sourcetype	Source	Supported App
Azure Active Directory	Azure	'azure:monitor:aad'	'Azure AD'	N/A

Macros Used

Annotations

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Implementation

You must install the latest version of Splunk Add-on for Microsoft Cloud Services from Splunkbase (https://splunkbase.splunk.com/app/3110/#/details). You must be ingesting Azure Active Directory events into your Splunk environment through an EventHub. This analytic was written to be used with the azure:monitor:aad sourcetype leveraging the Signin log category.

Known False Positives

A source Ip failing to authenticate with multiple users is not a common for legitimate behavior.

Associated Analytic Story

• Azure Active Directory Account Takeover

Risk Based Analytics (RBA)

References

• https://attack.mitre.org/techniques/T1110/003/

• https://docs.microsoft.com/en-us/security/compass/incident-response-playbook-password-spray

• https://www.cisa.gov/uscert/ncas/alerts/aa21-008a

• https://docs.microsoft.com/azure/active-directory/reports-monitoring/reference-sign-ins-error-codes

Detection Testing

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

Source: GitHub | Version: 3