AWS S3 Bucket Policy Modified

Severity: informational
Compliance: CIS 3.8; Stratus Red Team aws.defense-evasion.cloudtrail-lifecycle-rule, aws.exfiltration.s3-backdoor-bucket-policy
Log types: AWS.CloudTrail
Tags: AWS, Identity & Access Management, Exfiltration:Exfiltration Over Web Service
Reference: https://docs.aws.amazon.com/AmazonS3/latest/dev/using-iam-policies.html
Source: github.com/panther-labs/panther-analysis

An S3 Bucket was modified.

MITRE ATT&CK coverage

Tactic	Techniques
Exfiltration	`T1567` Exfiltration Over Web Service

Rules detecting the same action

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

ASL AWS Defense Evasion PutBucketLifecycle (Splunk)
AWS CloudTrail Retention Lifecycle Too Short (Panther)
AWS Defense Evasion PutBucketLifecycle (Splunk)
AWS S3 Bucket Expiration Lifecycle Configuration Added (Elastic)
AWS S3 Bucket Replicated to Another Account (Elastic)
AWSCloudTrail - S3 bucket exposed via ACL (Kusto)
AWSCloudTrail - S3 bucket exposed via policy (Kusto)

Rule body yaml

AnalysisType: rule
Filename: aws_s3_bucket_policy_modified.py
RuleID: "AWS.S3.BucketPolicyModified"
DisplayName: "AWS S3 Bucket Policy Modified"
Enabled: true
LogTypes:
  - AWS.CloudTrail
Tags:
  - AWS
  - Identity & Access Management
  - Exfiltration:Exfiltration Over Web Service
Reports:
  CIS:
    - 3.8
  MITRE ATT&CK:
    - TA0010:T1567

  Stratus Red Team:
    - aws.defense-evasion.cloudtrail-lifecycle-rule
    - aws.exfiltration.s3-backdoor-bucket-policy
Severity: Info
DedupPeriodMinutes: 720 # 12 hours
Description: >
  An S3 Bucket was modified.
Runbook: https://docs.runpanther.io/alert-runbooks/built-in-rules/aws-s3-bucket-policy-modified
Reference: https://docs.aws.amazon.com/AmazonS3/latest/dev/using-iam-policies.html
SummaryAttributes:
  - eventName
  - userAgent
  - sourceIpAddress
  - p_any_aws_arns
Tests:
  - Name: S3 Bucket Policy Modified
    ExpectedResult: true
    Log:
      {
        "eventVersion": "1.05",
        "userIdentity":
          {
            "type": "AssumedRole",
            "principalId": "1111:tester",
            "arn": "arn:aws:sts::123456789012:assumed-role/tester",
            "accountId": "123456789012",
            "accessKeyId": "1",
            "sessionContext":
              {
                "attributes":
                  {
                    "mfaAuthenticated": "true",
                    "creationDate": "2019-01-01T00:00:00Z",
                  },
                "sessionIssuer":
                  {
                    "type": "Role",
                    "principalId": "1111",
                    "arn": "arn:aws:iam::123456789012:role/tester",
                    "accountId": "123456789012",
                    "userName": "tester",
                  },
              },
          },
        "eventTime": "2019-01-01T00:00:00Z",
        "eventSource": "s3.amazonaws.com",
        "eventName": "PutBucketAcl",
        "awsRegion": "us-west-2",
        "sourceIPAddress": "111.111.111.111",
        "userAgent": "Mozilla",
        "requestParameters":
          {
            "host": ["bucket.s3.us-west-2.amazonaws.com"],
            "bucketName": "bucket",
            "acl": [""],
            "x-amz-acl": ["private"],
          },
        "responseElements": null,
        "additionalEventData":
          {
            "SignatureVersion": "SigV4",
            "CipherSuite": "ECDHE-RSA-AES128-GCM-SHA256",
            "AuthenticationMethod": "AuthHeader",
          },
        "requestID": "1",
        "eventID": "1",
        "eventType": "AwsApiCall",
        "recipientAccountId": "123456789012",
      }
  - Name: S3 Bucket Policy Modified Error Response
    ExpectedResult: false
    Log:
      {
        "eventVersion": "1.05",
        "userIdentity":
          {
            "type": "AssumedRole",
            "principalId": "1111:tester",
            "arn": "arn:aws:sts::123456789012:assumed-role/tester",
            "accountId": "123456789012",
            "accessKeyId": "1",
            "sessionContext":
              {
                "attributes":
                  {
                    "mfaAuthenticated": "true",
                    "creationDate": "2019-01-01T00:00:00Z",
                  },
                "sessionIssuer":
                  {
                    "type": "Role",
                    "principalId": "1111",
                    "arn": "arn:aws:iam::123456789012:role/tester",
                    "accountId": "123456789012",
                    "userName": "tester",
                  },
              },
          },
        "eventTime": "2019-01-01T00:00:00Z",
        "eventSource": "s3.amazonaws.com",
        "errorCode": "AccessDenied",
        "eventName": "PutBucketAcl",
        "awsRegion": "us-west-2",
        "sourceIPAddress": "111.111.111.111",
        "userAgent": "Mozilla",
        "requestParameters":
          {
            "host": ["bucket.s3.us-west-2.amazonaws.com"],
            "bucketName": "bucket",
            "acl": [""],
            "x-amz-acl": ["private"],
          },
        "responseElements": null,
        "additionalEventData":
          {
            "SignatureVersion": "SigV4",
            "CipherSuite": "ECDHE-RSA-AES128-GCM-SHA256",
            "AuthenticationMethod": "AuthHeader",
          },
        "requestID": "1",
        "eventID": "1",
        "eventType": "AwsApiCall",
        "recipientAccountId": "123456789012",
      }
  - Name: S3 Bucket Policy Not Modified
    ExpectedResult: false
    Log:
      {
        "eventVersion": "1.05",
        "userIdentity":
          {
            "type": "AssumedRole",
            "principalId": "1111:tester",
            "arn": "arn:aws:sts::123456789012:assumed-role/tester",
            "accountId": "123456789012",
            "accessKeyId": "1",
            "sessionContext":
              {
                "attributes":
                  {
                    "mfaAuthenticated": "false",
                    "creationDate": "2019-01-01T00:00:00Z",
                  },
                "sessionIssuer":
                  {
                    "type": "Role",
                    "principalId": "1111",
                    "arn": "arn:aws:iam::123456789012:role/tester",
                    "accountId": "123456789012",
                    "userName": "tester",
                  },
              },
          },
        "eventTime": "2019-01-01T00:00:00Z",
        "eventSource": "s3.amazonaws.com",
        "eventName": "GetBucketPolicy",
        "awsRegion": "us-west-2",
        "sourceIPAddress": "111.111.111.111",
        "userAgent": "Mozilla",
        "requestParameters":
          {
            "host": ["bucket.s3.us-west-2.amazonaws.com"],
            "bucketName": "bucket",
            "policy": [""],
          },
        "responseElements": null,
        "additionalEventData":
          {
            "SignatureVersion": "SigV4",
            "CipherSuite": "ECDHE-RSA-AES128-GCM-SHA256",
            "AuthenticationMethod": "AuthHeader",
          },
        "requestID": "1",
        "eventID": "1",
        "eventType": "AwsApiCall",
        "recipientAccountId": "123456789012",
      }

Detection logic

Condition

eventName in ["PutBucketAcl", "PutBucketPolicy", "PutBucketCors", "PutBucketLifecycle", "PutBucketReplication", "DeleteBucketPolicy", "DeleteBucketCors", "DeleteBucketLifecycle", "DeleteBucketReplication"]
not (errorCode is_not_null or errorMessage is_not_null)

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)

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`	in	`DeleteBucketCors` `DeleteBucketLifecycle` `DeleteBucketPolicy` `DeleteBucketReplication` `PutBucketAcl` `PutBucketCors` `PutBucketLifecycle` `PutBucketPolicy` `PutBucketReplication`

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`

`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)