GCP GCS IAM Permission Changes

Severity: low
Group by: resource.labels.project_id
Compliance: CIS 2.1
Log types: GCP.AuditLog
Tags: GCP, Google Cloud Storage, Collection:Data From Cloud Storage Object
Reference: https://cloud.google.com/storage/docs/access-control/iam-permissions
Source: github.com/panther-labs/panther-analysis

Monitoring changes to Cloud Storage bucket permissions may reduce time to detect and correct permissions on sensitive Cloud Storage bucket and objects inside the bucket.

MITRE ATT&CK coverage

Tactic	Techniques
Collection	`T1530` Data from Cloud Storage

Rules detecting the same action

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

Detect New Open GCP Storage Buckets (Splunk)
GCP Storage Bucket Permissions Modification (Elastic)
GCS Bucket Made Public (Panther)

Rule body yaml

AnalysisType: rule
Filename: gcp_gcs_iam_changes.py
RuleID: "GCP.GCS.IAMChanges"
DisplayName: "GCP GCS IAM Permission Changes"
Enabled: true
DedupPeriodMinutes: 60
LogTypes:
  - GCP.AuditLog
Tags:
  - GCP
  - Google Cloud Storage
  - Collection:Data From Cloud Storage Object
Reports:
  CIS:
    - 2.10
  MITRE ATT&CK:
    - TA0009:T1530
Severity: Low
Description: >
  Monitoring changes to Cloud Storage bucket permissions may reduce time to detect and correct permissions on sensitive Cloud Storage bucket and objects inside the bucket.
Runbook: Validate the GCS bucket change was safe.
Reference: https://cloud.google.com/storage/docs/access-control/iam-permissions
SummaryAttributes:
  - severity
  - p_any_ip_addresses
  - p_any_domain_names
Tests:
  - Name: GCS IAM Change
    ExpectedResult: true
    Log:
      {
        "protoPayload":
          {
            "@type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "status": {},
            "authenticationInfo": { "principalEmail": "user@runpanther.io" },
            "requestMetadata":
              {
                "callerIp": "136.24.229.58",
                "callerSuppliedUserAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/81.0.4044.138 Safari/537.36,gzip(gfe)",
                "requestAttributes":
                  { "time": "2020-05-15T04:28:42.243082428Z", "auth": {} },
                "destinationAttributes": {},
              },
            "serviceName": "storage.googleapis.com",
            "methodName": "storage.setIamPermissions",
            "authorizationInfo":
              [
                {
                  "resource": "projects/_/buckets/jacks-test-bucket",
                  "permission": "storage.buckets.setIamPolicy",
                  "granted": true,
                  "resourceAttributes": {},
                },
              ],
            "resourceName": "projects/_/buckets/jacks-test-bucket",
            "serviceData":
              {
                "@type": "type.googleapis.com/google.iam.v1.logging.AuditData",
                "policyDelta":
                  {
                    "bindingDeltas":
                      [
                        {
                          "action": "ADD",
                          "role": "roles/storage.objectViewer",
                          "member": "allUsers",
                        },
                      ],
                  },
              },
            "resourceLocation": { "currentLocations": ["us"] },
          },
        "insertId": "15cp9rve72xt1",
        "resource":
          {
            "type": "gcs_bucket",
            "labels":
              {
                "bucket_name": "jacks-test-bucket",
                "project_id": "western-verve-123456",
                "location": "us",
              },
          },
        "timestamp": "2020-05-15T04:28:42.237027213Z",
        "severity": "NOTICE",
        "logName": "projects/western-verve-123456/logs/cloudaudit.googleapis.com%2Factivity",
        "receiveTimestamp": "2020-05-15T04:28:42.900626148Z",
      }

Detection logic

Condition

resource.type eq "gcs_bucket"
protoPayload.methodName eq "storage.setIamPermissions"

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
`protoPayload.methodName`	eq	`storage.setIamPermissions`
`resource.type`	eq	`gcs_bucket`

`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)