Detection: AWS EC2 Startup Script Modified on Stopped Instance

Created: 2026-10-07 Last Updated: 2026-10-07 ID: 84578867-02bf-475e-9815-6e8e0d798253 Author: Maria Jose Erquiaga Type: TTP Product: Splunk Enterprise Security

Description

A successful EC2 StopInstances request was followed within one hour by a successful ModifyInstanceAttribute request targeting userData on the same instance, by the same IAM principal. This may indicate an attempt to change instance startup behavior or establish persistence. These CloudTrail events do not confirm that the modified user-data script executed.

Search

 1`cloudtrail`
 2eventSource="ec2.amazonaws.com"
 3eventName IN ("StopInstances", "ModifyInstanceAttribute")
 4
 5
 6| eval user_arn='userIdentity.arn'
 7
 8
 9| eval instance_id=case(
10    eventName="StopInstances", 'requestParameters.instancesSet.items{}.instanceId',
11    eventName="ModifyInstanceAttribute", 'requestParameters.instanceId'
12  )
13
14
15| mvexpand instance_id
16
17
18| where isnotnull(eventID) AND isnotnull(instance_id) AND isnotnull(user_arn)
19    AND (isnull(errorCode) OR errorCode="" OR lower(errorCode)="success")
20
21
22| where eventName="StopInstances"
23    OR (eventName="ModifyInstanceAttribute" AND 'requestParameters.attribute'="userData")
24
25
26| dedup eventID instance_id
27
28
29| eval event_epoch=coalesce(strptime(eventTime, "%Y-%m-%dT%H:%M:%SZ"), _time)
30
31
32| sort 0 user_arn instance_id awsRegion event_epoch
33
34
35| streamstats current=f
36    last(eval(if(eventName="StopInstances", event_epoch, null()))) AS stop_epoch
37    last(eval(if(eventName="StopInstances", eventID, null()))) AS stop_event_id
38    BY user_arn instance_id awsRegion
39
40
41| where eventName="ModifyInstanceAttribute" AND isnotnull(stop_epoch)
42
43
44| eval elapsed_seconds=event_epoch-stop_epoch
45
46
47| where elapsed_seconds>0 AND elapsed_seconds<=3600
48
49
50| eval stop_time_utc=strftime(stop_epoch, "%Y-%m-%dT%H:%M:%SZ")
51
52
53| eval modification_time_utc=strftime(event_epoch, "%Y-%m-%dT%H:%M:%SZ")
54
55
56| rename eventID AS modification_event_id
57
58
59| table user_arn instance_id awsRegion stop_time_utc modification_time_utc
60        elapsed_seconds stop_event_id modification_event_id
61
62
63| `aws_ec2_startup_script_modified_on_stopped_instance_filter`

Data Source

Name	Platform	Sourcetype	Source
AWS CloudTrail	AWS	`'aws:cloudtrail'`	`'aws_cloudtrail'`

Macros Used

Name	Value
cloudtrail	`sourcetype=aws:cloudtrail`
aws_ec2_startup_script_modified_on_stopped_instance_filter	`search *`

aws_ec2_startup_script_modified_on_stopped_instance_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK

+ Kill Chain Phases

+ NIST

+ CIS

- Threat Actors

ID	Technique	Tactic
T1578.005	Modify Cloud Compute Configurations	Defense Impairment

Exploitation

DE.CM

CIS 10

Default Configuration

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

Setting	Value
Disabled	true
Cron Schedule	`0 * * * *`
Earliest Time	`-70m@m`
Latest Time	`-10m@m`
Schedule Window	`auto`
Creates Finding (Notable)	Yes
Rule Title	`%name%`
Rule Description	`%description%`
Notable Event Fields	user, dest
Creates Intermediate Finding (Risk Event)	No

TTP detections generate a Finding (Notable) and may generate Intermediate Findings (Risk Events) for associated entities.

Implementation

You must install Splunk AWS Add on and enable CloudTrail logs in your AWS Environment.

Known False Positives

Authorized administrators and infrastructure automation may stop instances and update their user data. Review the IAM principal, instance, change window, and approved automation before escalating.

Associated Analytic Story

Suspicious Cloud User Activities

Finding

Title	Entity Field	Entity Type	Risk Score
The IAM principal [$user_arn$] modified user data on stopped EC2 instance [$instance_id$]	user_arn	user	50

References

Detection Testing

Test Type	Status	Dataset	Source	Sourcetype
Validation	✅ Passing	N/A	N/A	N/A
Unit	✅ Passing	Dataset	`aws_cloudtrail`	`aws:cloudtrail`
Integration	✅ Passing	Dataset	`aws_cloudtrail`	`aws:cloudtrail`

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: 1