GCP GCS Bulk Object Rewrite Operation

Severity: medium
Group by: protoPayload.authenticationInfo.principalEmail, resource.labels.bucket_name
Entities: emails, ip_addresses, usernames
Compliance: CIS 2.1
Log types: GCP.AuditLog
Tags: GCP, Google Cloud Storage, Collection:Data From Cloud Storage Object, Ransomware
Reference: https://cloud.google.com/storage/docs/gsutil/commands/rewrite
Source: github.com/panther-labs/panther-analysis

Detects GCS object rewrite operations which may indicate ransomware operations attempting to rewrite data in the same bucket with an attacker-controlled encryption key. Attackers with compromised credentials can use gsutil rewrite commands to replace existing encryption keys on cloud storage objects, effectively encrypting data for ransom. This detection focuses on identifying suspicious re-encryption activity through the 'gsutil rewrite -k' command patterns in user agent strings, with a threshold of 10 events.

MITRE ATT&CK coverage

Tactic	Techniques
Collection	`T1530` Data from Cloud Storage
Exfiltration	`T1537` Transfer Data to Cloud Account
Impact	`T1486` Data Encrypted for Impact

Rules detecting the same action

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

GCP GCS Ransom Note Upload (Panther)

Rule body yaml

AnalysisType: rule
Filename: gcp_gcs_object_rewrite.py
RuleID: "GCP.GCS.BulkObjectRewrite"
DisplayName: "GCP GCS Bulk Object Rewrite Operation"
Enabled: true
Threshold: 10
LogTypes:
  - GCP.AuditLog
Tags:
  - GCP
  - Google Cloud Storage
  - Collection:Data From Cloud Storage Object
  - Ransomware
Reports:
  CIS:
    - 2.10
  MITRE ATT&CK:
    - TA0040:T1486  # Impact: Data Encrypted for Impact
    - TA0009:T1530  # Collection: Data from Cloud Storage Object
    - TA0010:T1537  # Exfiltration: Transfer Data to Cloud Account
Severity: Medium
Description: >
  Detects GCS object rewrite operations which may indicate ransomware operations attempting to rewrite data in the same bucket with an attacker-controlled encryption key.
  Attackers with compromised credentials can use gsutil rewrite commands to replace existing encryption keys on cloud storage objects,
  effectively encrypting data for ransom. This detection focuses on identifying suspicious re-encryption activity through the 'gsutil rewrite -k'
  command patterns in user agent strings, with a threshold of 10 events.
Runbook: |
  1. Query GCP Audit Logs for all storage.objects.create operations by the principalEmail in the 24 hours before and after the alert to determine the scope of rewrite operations
  2. Check if the callerIP is associated with known cloud provider IP ranges, VPN endpoints, or matches the user's typical access locations from the past 90 days
  3. Find other alerts or suspicious GCS activity from this principalEmail or bucket in the past 7 days to identify potential ransomware patterns
Reference: https://cloud.google.com/storage/docs/gsutil/commands/rewrite
SummaryAttributes:
  - severity
  - p_any_ip_addresses
  - p_any_domain_names
Tests:
  - Name: GCS Object Rewrite
    ExpectedResult: true
    Log:
      {
      "p_any_ip_addresses": [
        "1.2.3.4"
      ],
      "p_any_emails": [
        "frodo@lotr.com"
      ],
      "p_any_usernames": [
        "homer.simpson"
      ],
      "p_event_time": "2025-12-15 15:35:44.471503481",
      "p_log_type": "GCP.AuditLog",
      "p_parse_time": "2025-12-15 15:37:21.119003440",
      "p_row_id": "000000000088d76191e538b59ed49209",
      "p_schema_version": 0,
      "p_source_id": "bd7da315-647e-4eca-bcfe-083fab18f3f1",
      "p_source_label": "your-project-pubsub",
      "p_udm": {
        "source": {
          "address": "1.2.3.4",
          "ip": "1.2.3.4"
        },
        "user": {
          "email": "frodo@lotr.com"
        }
      },
      "insertId": "kua6lje2lauj",
      "logName": "projects/your-project/logs/cloudaudit.googleapis.com%2Fdata_access",
      "protoPayload": {
        "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
        "authenticationInfo": {
          "oauthInfo": {
            "oauthClientId": "111111111111-example1a2b3c4d5e6f7g8h9i0j.apps.googleusercontent.com"
          },
          "principalEmail": "frodo@lotr.com"
        },
        "authorizationInfo": [
          {
            "granted": true,
            "permission": "storage.objects.create",
            "resource": "projects/_/buckets/victim-bucket/objects/victim-file.txt",
            "resourceAttributes": {}
          },
          {
            "granted": true,
            "permission": "storage.objects.delete",
            "resource": "projects/_/buckets/victim-bucket/objects/victim-file.txt",
            "resourceAttributes": {}
          }
        ],
        "methodName": "storage.objects.create",
        "requestMetadata": {
          "callerIP": "1.2.3.4",
          "callerIp": "1.2.3.4",
          "callerSuppliedUserAgent": "apitools Python/3.13.7 gsutil/5.35 (linux) analytics/enabled interactive/True invocation-id/000000000098a6854caf369de05cd1c0 command/rewrite-k google-cloud-sdk/548.0.0,gzip(gfe)",
          "destinationAttributes": {},
          "requestAttributes": {
            "auth": {},
            "time": "2025-12-15T15:35:44.477602259Z"
          }
        },
        "resourceLocation": {
          "currentLocations": [
            "us"
          ]
        },
        "resourceName": "projects/_/buckets/victim-bucket/objects/victim-file.txt",
        "serviceData": {
          "@type": "type.googleapis.com/google.iam.v1.logging.AuditData",
          "at_sign_type": "type.googleapis.com/google.iam.v1.logging.AuditData",
          "policyDelta": {}
        },
        "serviceName": "storage.googleapis.com",
        "status": {}
      },
      "receiveTimestamp": "2025-12-15 15:35:45.157653388",
      "resource": {
        "labels": {
          "bucket_name": "victim-bucket",
          "location": "us",
          "project_id": "your-project"
        },
        "type": "gcs_bucket"
      },
      "severity": "INFO",
      "timestamp": "2025-12-15 15:35:44.471503481"
    }
  - Name: GCS Object Get
    ExpectedResult: false
    Log:
      {
      "p_any_ip_addresses": [
        "1.2.3.4"
      ],
      "p_any_emails": [
        "homer.simpson@example.com"
      ],
      "p_any_usernames": [
        "homer.simpson"
      ],
      "p_event_time": "2025-12-15 15:35:44.471503481",
      "p_log_type": "GCP.AuditLog",
      "p_parse_time": "2025-12-15 15:37:21.119003440",
      "p_row_id": "c6d39a8b9284ee95bfc3d7de2ad6f407",
      "p_schema_version": 0,
      "p_source_id": "bd7da315-647e-4eca-bcfe-083fab18f3f1",
      "p_source_label": "your-project-pubsub",
      "p_udm": {
        "source": {
          "address": "1.2.3.4",
          "ip": "1.2.3.4"
        },
        "user": {
          "email": "homer.simpson@example.com"
        }
      },
      "insertId": "kua6lje2lauj",
      "logName": "projects/your-project/logs/cloudaudit.googleapis.com%2Fdata_access",
      "protoPayload": {
        "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
        "authenticationInfo": {
          "oauthInfo": {
            "oauthClientId": "123456789012-example1a2b3c4d5e6f7g8h9i0j.apps.googleusercontent.com"
          },
          "principalEmail": "homer.simpson@example.com"
        },
        "authorizationInfo": [
          {
            "granted": true,
            "permission": "storage.objects.get",
            "resource": "projects/_/buckets/victim-bucket/objects/victim-file.txt",
            "resourceAttributes": {}
          }
        ],
        "methodName": "storage.objects.list",
        "requestMetadata": {
          "callerIP": "1.2.3.4",
          "callerIp": "1.2.3.4",
          "callerSuppliedUserAgent": "apitools Python/3.13.7 gsutil/5.35 (linux) analytics/enabled interactive/True invocation-id/8ad1636b4f184078859fe329c196a5c3 command/rewrite-k google-cloud-sdk/548.0.0,gzip(gfe)",
          "destinationAttributes": {},
          "requestAttributes": {
            "auth": {},
            "time": "2025-12-15T15:35:44.477602259Z"
          }
        },
        "resourceLocation": {
          "currentLocations": [
            "us"
          ]
        },
        "resourceName": "projects/_/buckets/victim-bucket/objects/victim-file.txt",
        "serviceData": {
          "@type": "type.googleapis.com/google.iam.v1.logging.AuditData",
          "at_sign_type": "type.googleapis.com/google.iam.v1.logging.AuditData",
          "policyDelta": {}
        },
        "serviceName": "storage.googleapis.com",
        "status": {}
      },
      "receiveTimestamp": "2025-12-15 15:35:45.157653388",
      "resource": {
        "labels": {
          "bucket_name": "victim-bucket",
          "location": "us",
          "project_id": "your-project"
        },
        "type": "gcs_bucket"
      },
      "severity": "INFO",
      "timestamp": "2025-12-15 15:35:44.471503481"
    }

Detection logic

Condition

protoPayload.serviceName eq "storage.googleapis.com"
protoPayload.methodName in "storage.objects.create"
protoPayload.resourceName not is_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.resourceName`	is_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
`protoPayload.methodName`	in	`storage.objects.create`
`protoPayload.serviceName`	eq	`storage.googleapis.com`

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
`actor`	`protoPayload.authenticationInfo.principalEmail`
`method`	`protoPayload.methodName`
`bucket`	`resource.labels.bucket_name`
`user_agent`	`protoPayload.requestMetadata.callerSuppliedUserAgent`
`source_ip`	`protoPayload.requestMetadata.callerIp`
`project`	`resource.labels.project_id`

`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)