Device-UserAddedasLocalAdmin.kql

//Detect when an admin adds another user to the local administrators group on a device and optionally query IdentityInfo to return the UPN of the user added
DeviceEvents
| where TimeGenerated > ago(7d)
| where ActionType == "UserAccountAddedToLocalGroup"
| where AdditionalFields.GroupName == "Administrators"
// Exclude processes initiated by system as this detection is for end users adding groups
| where InitiatingProcessAccountSid != "S-1-5-18"
| project
    TimeGenerated,
    DeviceName,
    AccountSid,
    Actor=InitiatingProcessAccountName
//Join query to IdentityInfo table to match the AccountSid
//if you do not use the IdentityInfo table remove everything below this line
| join kind=inner (
    IdentityInfo
    | where TimeGenerated > ago (21d)
    | summarize arg_max (TimeGenerated, *) by AccountUPN)
    on $left.AccountSid == $right.AccountSID
| project
    TimeGenerated,
    DeviceName,
    ['User Added']=AccountUPN,
    ['User Added Sid']=AccountSID,
    Actor


//Use the below for the equivalent query in M365 advanced hunting
DeviceEvents
| where Timestamp > ago(30d)
| where ActionType == "UserAccountAddedToLocalGroup"
| where AdditionalFields contains "Administrator"
| where InitiatingProcessAccountSid != "S-1-5-18"
| project DeviceName, Actor=InitiatingProcessAccountName, AccountSid
| join kind=inner (
IdentityInfo
)
on $left.AccountSid==$right.OnPremSid
| project DeviceName, Actor, AccountSid, UserAdded=AccountUpn