AWS EC2 Startup Script Change

Severity: high
Group by: requestParameters.instanceId
Entities: aws_instance_ids
Compliance: Stratus Red Team aws.execution.ec2-user-data
Log types: AWS.CloudTrail
Reference: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/user-data.html#user-data-shell-scripts
Source: github.com/panther-labs/panther-analysis

Detects changes to the EC2 instance startup script. The shell script will be executed as root/SYSTEM every time the specific instances are booted up.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059` Command and Scripting Interpreter

Rules detecting the same action

Other rules on this platform that filter on the same API call or operation.

Rule body yaml

AnalysisType: rule
Description: Detects changes to the EC2 instance startup script. The shell script will be executed as root/SYSTEM every time the specific instances are booted up.
DisplayName: "AWS EC2 Startup Script Change"
Enabled: true
Filename: aws_ec2_startup_script_change.py
Reports:
  MITRE ATT&CK:
    - TA0002:T1059

  Stratus Red Team:
    - aws.execution.ec2-user-data
Reference: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/user-data.html#user-data-shell-scripts
Severity: High
Tests:
  - ExpectedResult: false
    Log:
      awsregion: us-east-1
      eventid: abc-123
      eventname: ModifyInstanceAttribute
      eventsource: ec2.amazonaws.com
      eventtime: "2022-07-17 04:50:23"
      eventtype: AwsApiCall
      eventversion: "1.08"
      p_any_aws_instance_ids:
        - testinstanceid
      p_event_time: "2022-07-17 04:50:23"
      p_log_type: AWS.CloudTrail
      p_parse_time: "2022-07-17 04:55:11.788"
      requestParameters:
        instanceId: testinstanceid
        instanceType:
          value: t3.nano
    Name: ModifyInstanceAttribute-NoUserData
  - ExpectedResult: true
    Log:
      awsRegion: us-east-2
      eventCategory: Management
      eventName: ModifyInstanceAttribute
      eventSource: ec2.amazonaws.com
      eventTime: "2022-09-30 15:11:25.000000000"
      eventType: AwsApiCall
      eventVersion: "1.08"
      managementEvent: true
      readOnly: false
      recipientAccountId: "0123456789"
      requestID: example-id
      requestParameters:
        instanceId: i-012345abcde
        userData: <sensitiveDataRemoved>
      responseElements:
        _return: true
        requestId: 012345abcdef
      sourceIPAddress: 1.2.3.4
      tlsDetails:
        cipherSuite: ECDHE-RSA-AES128-GCM-SHA256
        clientProvidedHostHeader: ec2.us-east-2.amazonaws.com
        tlsVersion: TLSv1.2
      userAgent: aws sdk
      userIdentity:
        accessKeyId: ABCDEXAMPLE123
        accountId: "0123456789"
        arn: arn:aws:sts::0123456789:assumed-role/Role-us-1/CodeBuild
        principalId: ABCDEXAMPLE:CodeBuild
        sessionContext:
          attributes:
            creationDate: "2022-09-30T14:52:26Z"
            mfaAuthenticated: "false"
          sessionIssuer:
            accountId: "0123456789"
            arn: arn:aws:iam::0123456789:role/CodeBuild-US-East
            principalId: AROAQUW22FGSKBTQ7R5HP
            type: Role
            userName: CodeBuild-US-East
          webIdFederationData: {}
        type: AssumedRole
    Name: ModifyInstanceAttributeUserdata
  - ExpectedResult: false
    Log:
      awsregion: us-east-1
      eventid: abc-123
      eventname: ModifyImageAttribute
      eventsource: ec2.amazonaws.com
      eventtime: "2022-07-17 04:50:23"
      eventtype: AwsApiCall
      eventversion: "1.08"
      p_any_aws_instance_ids:
        - testinstanceid
      p_event_time: "2022-07-17 04:50:23"
      p_log_type: AWS.CloudTrail
      p_parse_time: "2022-07-17 04:55:11.788"
      requestParameters:
        instanceId: testinstanceid
        instanceType:
          value: t3.nano
    Name: NoModifyInstanceAttribute
DedupPeriodMinutes: 60
LogTypes:
  - AWS.CloudTrail
RuleID: "AWS.EC2.Startup.Script.Change"
Threshold: 1

Detection logic

Condition

eventName eq "ModifyInstanceAttribute"
requestParameters.userData is_not_null

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`eventName`	eq	`ModifyInstanceAttribute`
`requestParameters.userData`	is_not_null	(no value, null check)

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

Field	Source
`eventName`
`eventSource`
`awsRegion`
`recipientAccountId`
`sourceIPAddress`
`userAgent`
`userIdentity`
`arn`	`userIdentity.arn`
`instanceId`	`requestParameters.instanceId`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)