GCP GCS Object Copied to Different Bucket

Severity: medium
Log types: GCP.AuditLog
Tags: GCP, Google Cloud Storage, Exfiltration:Transfer Data to Cloud Account, Ransomware
Reference: https://cloud.google.com/storage/docs/copying-renaming-moving-objects
Source: github.com/panther-labs/panther-analysis

Detects when GCS objects are copied from one bucket to a bucket in a different GCP project. Cross-project copies are more suspicious than same-project copies and can indicate data exfiltration where an adversary copies sensitive data to a project they control. The threshold of 50+ copy operations suggests bulk exfiltration rather than normal operations. This is detected by monitoring storage.objects.get operations that include a destination field in the metadata, indicating a copy operation.

MITRE ATT&CK coverage

Tactic	Techniques
Exfiltration	`T1537` Transfer Data to Cloud Account

Rule body yaml

AnalysisType: rule
Filename: gcp_gcs_object_exfiltration.py
RuleID: "GCP.GCS.ObjectExfiltration"
DisplayName: "GCP GCS Object Copied to Different Bucket"
Enabled: true
Threshold: 50
LogTypes:
  - GCP.AuditLog
Tags:
  - GCP
  - Google Cloud Storage
  - Exfiltration:Transfer Data to Cloud Account
  - Ransomware
Reports:
  MITRE ATT&CK:
    - TA0010:T1537
Severity: Medium
Description: >
  Detects when GCS objects are copied from one bucket to a bucket in a different GCP project.
  Cross-project copies are more suspicious than same-project copies and can indicate data
  exfiltration where an adversary copies sensitive data to a project they control. The
  threshold of 50+ copy operations suggests bulk exfiltration rather than normal operations.
  This is detected by monitoring storage.objects.get operations that include a destination
  field in the metadata, indicating a copy operation.
Runbook: |
  1. Query GCP Audit logs for all storage.objects.get operations with destination metadata by authenticationInfo:principalEmail in the 2 hours around this alert to identify the full scope of copy operations
  2. Check if the protoPayload:metadata:destination bucket belongs to the same project, a different project in the organization, or an external attacker-controlled project
  3. Review GCP Audit logs for IAM permission changes, service account key creation, or bucket policy modifications by this principal in the 24 hours before the first copy operation
Reference: https://cloud.google.com/storage/docs/copying-renaming-moving-objects
SummaryAttributes:
  - severity
  - p_any_ip_addresses
  - p_any_emails
Tests:
  - Name: Cross-Project Object Copy
    ExpectedResult: true
    Log:
      {
        "protoPayload":
          {
            "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "authenticationInfo":
              { "principalEmail": "denethor@lotr.com" },
            "authorizationInfo":
              [
                {
                  "granted": true,
                  "permission": "storage.objects.get",
                  "resource": "projects/_/buckets/source-bucket/objects/sensitive.txt",
                },
              ],
            "metadata":
              {
                "destination": "projects/attacker-project/buckets/exfil-bucket/objects/sensitive.txt",
                "requested_bytes": 12345,
              },
            "methodName": "storage.objects.get",
            "requestMetadata":
              {
                "callerIp": "1.2.3.4",
                "callerSuppliedUserAgent": "google-cloud-sdk gcloud/548.0.0 command/gcloud.storage.cp",
              },
            "resourceName": "projects/_/buckets/source-bucket/objects/sensitive.txt",
            "serviceName": "storage.googleapis.com",
            "status": {},
          },
        "resource":
          {
            "labels":
              {
                "bucket_name": "source-bucket",
                "location": "us",
                "project_id": "victim-project",
              },
            "type": "gcs_bucket",
          },
        "severity": "INFO",
        "timestamp": "2025-12-15 15:55:03.915792086",
      }
  - Name: Same-Project Different Bucket Copy
    ExpectedResult: false
    Log:
      {
        "protoPayload":
          {
            "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "authenticationInfo":
              { "principalEmail": "denethor@lotr.com" },
            "metadata":
              {
                "destination": "projects/_/buckets/backup-bucket/objects/file.txt",
                "requested_bytes": 100,
              },
            "methodName": "storage.objects.get",
            "resourceName": "projects/_/buckets/prod-bucket/objects/file.txt",
            "serviceName": "storage.googleapis.com",
            "status": {},
          },
        "resource":
          {
            "labels":
              {
                "bucket_name": "prod-bucket",
                "project_id": "test-project",
              },
            "type": "gcs_bucket",
          },
        "severity": "INFO",
        "timestamp": "2025-12-15 15:55:03.915792086",
      }
  - Name: Object Get Without Copy Operation
    ExpectedResult: false
    Log:
      {
        "protoPayload":
          {
            "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "authenticationInfo":
              { "principalEmail": "user@example.com" },
            "methodName": "storage.objects.get",
            "resourceName": "projects/_/buckets/test-bucket/objects/file.txt",
            "serviceName": "storage.googleapis.com",
            "status": {},
          },
        "resource":
          {
            "labels":
              {
                "bucket_name": "test-bucket",
                "project_id": "test-project",
              },
            "type": "gcs_bucket",
          },
        "severity": "INFO",
        "timestamp": "2025-12-15 15:55:03.915792086",
      }

Detection logic

Condition

not (protoPayload.methodName ne "storage.objects.get" or protoPayload.serviceName ne "storage.googleapis.com" or severity eq "ERROR" or protoPayload.metadata.destination is_null)
resource.labels.bucket_name is_not_null
resource.labels.project_id is_not_null

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
`protoPayload.metadata.destination`	is_null	(no value, null check)
`protoPayload.methodName`	ne	`storage.objects.get`
`protoPayload.serviceName`	ne	`storage.googleapis.com`
`severity`	eq	`ERROR`

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
`resource.labels.bucket_name`	is_not_null	(no value, null check)
`resource.labels.project_id`	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
`principal`	`protoPayload.authenticationInfo.principalEmail`
`source_project`	`resource.labels.project_id`
`source_bucket`	`resource.labels.bucket_name`
`destination_path`	`protoPayload.metadata.destination`
`source_object`	`protoPayload.resourceName`
`source_ip`	`protoPayload.requestMetadata.callerIp`
`user_agent`	`protoPayload.requestMetadata.callerSuppliedUserAgent`
`bytes_requested`	`protoPayload.metadata.requested_bytes`

`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)