S3 Object Encrypted with External KMS Key

Severity: high
Log types: AWS.CloudTrail
Tags: AWS, S3, Ransomware, Impact:Data Destruction
Reference: https://rhinosecuritylabs.com/aws/s3-ransomware-part-1-attack-vector/
Source: github.com/panther-labs/panther-analysis

Detects when an S3 object is copied with a KMS key belonging to an account ID different than the bucket owner's account ID. This technique is used in S3 ransomware attacks where attackers encrypt objects with their own KMS key from an attacker-controlled AWS account, making the data inaccessible to the original owner. This is often a precursor to ransom demands or permanent data loss.

MITRE ATT&CK coverage

Tactic	Techniques
Impact	`T1486` Data Encrypted for Impact

Rule body yaml

AnalysisType: rule
Filename: aws_s3_copy_object_cross_account_kms.py
RuleID: "AWS.S3.CopyObject.CrossAccount.Encryption.KMS"
DisplayName: "S3 Object Encrypted with External KMS Key"
Enabled: true
LogTypes:
  - AWS.CloudTrail
Tags:
  - AWS
  - S3
  - Ransomware
  - Impact:Data Destruction
Reports:
  MITRE ATT&CK:
    - TA0040:T1486 # Impact: Data Encrypted for Impact
Severity: High
Description: >
  Detects when an S3 object is copied with a KMS key belonging to an account ID different than
  the bucket owner's account ID. This technique is used in S3
  ransomware attacks where attackers encrypt objects with their own KMS key from
  an attacker-controlled AWS account, making the data inaccessible to the original
  owner. This is often a precursor to ransom demands or permanent data loss.
Runbook: |
  1. Query CloudTrail for all CopyObject events by the userIdentity:arn in the 24 hours before and after the alert to identify all affected objects in the requestParameters:bucketName
  2. Check if the KMS key ARN from resources field belongs to an external account ID that appears in any legitimate cross-account operations in the past 90 days
  3. Find all S3 GetObject and ListBucket events by this user on the source bucket in the 1 hour before the first CopyObject to check if the attacker performed reconnaissance
Reference: https://rhinosecuritylabs.com/aws/s3-ransomware-part-1-attack-vector/
Tests:
  - Name: In-Place Copy with KMS Key Change
    ExpectedResult: true
    Log:
      {
        "eventVersion": "1.08",
        "userIdentity": {
          "type": "AssumedRole",
          "principalId": "AAAAAAAAAAAAAAAAAAAAA:attacker",
          "arn": "arn:aws:sts::111111111111:assumed-role/sample-role-elastic-rhodes/sample-role-brave-yalow-role-jolly-banzai-role-intelligent-brahmagupta-role-admiring-cori-role-beautiful-keldysh-role-hopeful-chaplygin",
          "accountId": "111111111111",
          "accessKeyId": "ASIA-MOCKACCESSKEYID-1"
        },
        "eventTime": "2024-01-15T10:45:23Z",
        "eventSource": "s3.amazonaws.com",
        "eventName": "CopyObject",
        "awsRegion": "us-east-1",
        "sourceIPAddress": "1.2.3.4",
        "userAgent": "aws-cli/2.13.0 Python/3.11.4",
        "requestParameters": {
          "bucketName": "victim-data-bucket",
          "key": "important-file.txt",
          "x-amz-copy-source": "victim-data-bucket/important-file.txt",
          "x-amz-server-side-encryption": "aws:kms",
          "x-amz-server-side-encryption-aws-kms-key-id": "arn:aws:kms:us-east-1:999999999999:key/attacker-key-id"
        },
        "responseElements": null,
        "requestID": "ABC123DEF456",
        "eventID": "12345678-1234-1234-1234-111111111111",
        "readOnly": false,
        "resources": [
          {
            "type": "AWS::S3::Object",
            "ARN": "arn:aws:s3:::sample-bucket-quizzical-yalow/important-file.txt"
          },
          {
            "accountId": "999999999999",
            "type": "AWS::KMS::Key",
            "ARN": "arn:aws:kms:us-east-1:999999999999:key/attacker-key-id"
          }
        ],
        "eventType": "AwsApiCall",
        "managementEvent": false,
        "recipientAccountId": "111111111111"
      }
  - Name: Copy with Same Account Owner
    ExpectedResult: false
    Log:
      {
        "eventVersion": "1.08",
        "userIdentity": {
          "type": "AssumedRole",
          "principalId": "AAAAAAAAAAAAAAAAAAAAA:attacker",
          "arn": "arn:aws:sts::111111111111:assumed-role/sample-role-elastic-rhodes/sample-role-beautiful-keldysh-role-hopeful-chaplygin",
          "accountId": "111111111111",
          "accessKeyId": "ASIA-MOCKACCESSKEYID-1"
        },
        "eventTime": "2024-01-15T10:45:23Z",
        "eventSource": "s3.amazonaws.com",
        "eventName": "CopyObject",
        "awsRegion": "us-east-1",
        "sourceIPAddress": "1.2.3.4",
        "userAgent": "aws-cli/2.13.0 Python/3.11.4",
        "requestParameters": {
          "bucketName": "victim-data-bucket",
          "key": "important-file.txt",
          "x-amz-copy-source": "victim-data-bucket/important-file.txt",
          "x-amz-server-side-encryption": "aws:kms",
          "x-amz-server-side-encryption-aws-kms-key-id": "arn:aws:kms:us-east-1:111111111111:key/developer"
        },
        "responseElements": null,
        "requestID": "ABC123DEF456",
        "eventID": "12345678-1234-1234-1234-111111111111",
        "readOnly": false,
        "resources": [
          {
            "type": "AWS::S3::Object",
            "ARN": "arn:aws:s3:::sample-bucket-quizzical-yalow/important-file.txt"
          },
          {
            "accountId": "111111111111",
            "type": "AWS::KMS::Key",
            "ARN": "arn:aws:kms:us-east-1:111111111111:key/attacker-key-id"
          }
        ],
        "eventType": "AwsApiCall",
        "managementEvent": false,
        "recipientAccountId": "111111111111"
      }

Detection logic

Condition

not (eventName ne "CopyObject" or errorCode is_not_null or errorMessage is_not_null)
requestParameters.x-amz-server-side-encryption-aws-kms-key-id starts_with "arn:aws:kms:"

This rule also runs imperative logic the parser cannot express as a filter; the conditions above are the structured part it could extract.

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`errorCode`	is_not_null	(no value, null check)
`errorMessage`	is_not_null	(no value, null check)
`eventName`	ne	`CopyObject`

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
`requestParameters.x-amz-server-side-encryption-aws-kms-key-id`	starts_with	`arn:aws:kms:`

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`
`actor_user`
`bucketName`	`requestParameters.bucketName`

`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)