Shadow Credentials Added to Account

Group by: DedupEntity
Author: FalconForce
Source: github.com/FalconForceTeam/FalconFriday

This query searches for modifications to the 'msDS-KeyCredentialLink' property in Active Directory, introduced in Windows Server 2016. There are two different events which contain information to detect such changes 5136 and 4662. This detection uses the 5136, which is the preferred event to use.

Known false positives

The property can also be used for legitimate purposes. However, the legitimate use of the property is limited in most environments.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1098` Account Manipulation
Privilege Escalation	`T1098` Account Manipulation `T1484` Domain or Tenant Policy Modification

References

Telemetry coverage

Provider	Record / event type
Security-Auditing	Event ID 5136: A directory service object was modified.

Rule body

let timeframe = 2*1h;
let RuleId = "0275";
let DedupFields = dynamic(["TimeGenerated", "SubjectUserName", "Computer"]);
SecurityEvent
| where ingestion_time() >= ago(timeframe)
| where EventID == 5136
| extend AttributeName = extract("<Data Name=\"AttributeLDAPDisplayName\">(.*?)</Data>", 1, EventData)
| extend ObjectDN = extract("<Data Name=\"ObjectDN\">(.*?)</Data>", 1, EventData)
| extend SubjectUserName = extract("<Data Name=\"SubjectUserName\">(.*?)</Data>", 1, EventData)
| where AttributeName has "msDS-KeyCredentialLink"
| where not(SubjectUserName endswith "$" and ObjectDN startswith strcat("CN=", replace_string(SubjectUserName, "$", ""), ",")) // Machine account changing its own msDS-KeyCredentialLink.
| extend HostName=tostring(split(Computer,".")[0]),DnsDomain=iif(Computer contains ".", substring(Computer, indexof(Computer, ".") + 1, strlen(Computer)),"")
| extend AccountName=iif(Account contains @"\",tostring(split(Account,@"\")[1]),Account),AccountDomain=iif(Account contains @"\",tostring(split(Account,@"\")[0]),"")
// Begin environment-specific filter.
// End environment-specific filter.
// Begin de-duplication logic.
| extend DedupFieldValues=pack_all()
| mv-apply e=DedupFields to typeof(string) on (
    extend DedupValue=DedupFieldValues[tostring(e)]
    | order by e // Sorting is required to ensure make_list is deterministic.
    | summarize DedupValues=make_list(DedupValue)
)
| extend DedupEntity=strcat_array(DedupValues, "|")
| project-away DedupFieldValues, DedupValues
| join kind=leftanti (
    SecurityAlert
    | where AlertName has RuleId and ProviderName has "ASI"
    | where TimeGenerated >= ago(timeframe)
    | extend DedupEntity = tostring(parse_json(tostring(parse_json(ExtendedProperties)["Custom Details"])).DedupEntity[0])
    | project DedupEntity
) on DedupEntity
// End de-duplication logic.

Stages and Predicates

Parameters

let timeframe = 2*1h;
let RuleId = "0275";
let DedupFields = dynamic(["TimeGenerated", "SubjectUserName", "Computer"]);

Stage 1: `source`

SecurityEvent

Stage 2: `where`

| where ingestion_time() >= ago(timeframe)

Stage 3: `where`

| where EventID == 5136

Stage 4: `extend` (3 consecutive steps)

| extend AttributeName = extract("<Data Name=\"AttributeLDAPDisplayName\">(.*?)</Data>", 1, EventData)
| extend ObjectDN = extract("<Data Name=\"ObjectDN\">(.*?)</Data>", 1, EventData)
| extend SubjectUserName = extract("<Data Name=\"SubjectUserName\">(.*?)</Data>", 1, EventData)

Stage 5: `where`

| where AttributeName has "msDS-KeyCredentialLink"

Stage 6: `where`

| where not(SubjectUserName endswith "$" and ObjectDN startswith strcat("CN=", replace_string(SubjectUserName, "$", ""), ","))

Stage 7: `extend` (3 consecutive steps)

| extend HostName=tostring(split(Computer,".")[0]),DnsDomain=iif(Computer contains ".", substring(Computer, indexof(Computer, ".") + 1, strlen(Computer)),"")
| extend AccountName=iif(Account contains @"\",tostring(split(Account,@"\")[1]),Account),AccountDomain=iif(Account contains @"\",tostring(split(Account,@"\")[0]),"")
| extend DedupFieldValues=pack_all()

Stage 8: `kusto:mv-apply`

| mv-apply e=DedupFields to typeof(string) on (
    extend DedupValue=DedupFieldValues[tostring(e)]
    | order by e
    | summarize DedupValues=make_list(DedupValue)
)

Stage 9: `extend`

| extend DedupEntity=strcat_array(DedupValues, "|")

Stage 10: `project-away`

| project-away DedupFieldValues, DedupValues

Stage 11: `join` (negated)

| join kind=leftanti (
    SecurityAlert
    | where AlertName has RuleId and ProviderName has "ASI"
    | where TimeGenerated >= ago(timeframe)
    | extend DedupEntity = tostring(parse_json(tostring(parse_json(ExtendedProperties)["Custom Details"])).DedupEntity[0])
    | project DedupEntity
) on DedupEntity

Stage 12: `summarize` aggregation inside the `mv-apply` branch

summarize

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`SubjectUserName`	ends_with	`$`	`excludes:SubjectUserName` `field:"SubjectUserName" value:"$"`
`AlertName`	match	`0275`	`excludes:AlertName` `field:"AlertName" value:"0275"`
`ProviderName`	match	`ASI`	`excludes:ProviderName` `field:"ProviderName" value:"ASI"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`AttributeName`	match	`msDS-KeyCredentialLink` transforms: `term`	`field:"AttributeName" kind:match value:"msDS-KeyCredentialLink"`
`EventID`	eq	`5136` corpus 45 (splunk 24, kusto 18, elastic 3)	`field:"EventID" kind:eq value:"5136"`

Shadow Credentials Added to Account

Known false positives

MITRE ATT&CK coverage

References

Telemetry coverage

Rule body

Stages and Predicates

Parameters

Stage 1: source

Stage 2: where

Stage 3: where

Stage 4: extend (3 consecutive steps)

Stage 5: where

Stage 6: where

Stage 7: extend (3 consecutive steps)

Stage 8: kusto:mv-apply

Stage 9: extend

Stage 10: project-away

Stage 11: join (negated)

Stage 12: summarize aggregation inside the mv-apply branch