GCP KMS Bulk Encryption by GCS Service Account

Severity: medium
Log types: GCP.AuditLog
Tags: GCP, Google Cloud KMS, Impact:Data Encrypted for Impact, Ransomware
Reference: https://cloud.google.com/kms/docs/reference/rest/v1/projects.locations.keyRings.cryptoKeys/encrypt
Source: github.com/panther-labs/panther-analysis

Detects bulk KMS encryption operations performed by the GCS service account. This pattern is indicative of a ransomware attack where an adversary directly calls the KMS Encrypt API using the GCS service account identity to encrypt data at scale, effectively holding data hostage. The threshold of 10+ encryption operations suggests automated bulk encryption rather than normal application behavior.

MITRE ATT&CK coverage

Tactic	Techniques
Impact	`T1486` Data Encrypted for Impact

Rule body yaml

AnalysisType: rule
Filename: gcp_kms_bulk_encryption.py
RuleID: "GCP.KMS.BulkEncryption"
DisplayName: "GCP KMS Bulk Encryption by GCS Service Account"
Enabled: true
Threshold: 10
DedupPeriodMinutes: 15
LogTypes:
  - GCP.AuditLog
Tags:
  - GCP
  - Google Cloud KMS
  - Impact:Data Encrypted for Impact
  - Ransomware
Reports:
  MITRE ATT&CK:
    - TA0040:T1486
Severity: Medium
Description: >
  Detects bulk KMS encryption operations performed by the GCS service account. This pattern
  is indicative of a ransomware attack where an adversary directly calls the KMS Encrypt API
  using the GCS service account identity to encrypt data at scale, effectively holding data
  hostage. The threshold of 10+ encryption operations suggests automated bulk encryption
  rather than normal application behavior.
Runbook: |
  1. Query GCP Audit logs for all KMS Encrypt API calls by the GCS service account in the 1 hour window around this alert
  2. Identify the total number of encryption operations and the rate of operations per minute
  3. Check if KMS IAM policies were recently modified to grant the GCS service account encryption permissions
  4. Investigate the source of these encryption calls and what data is being encrypted
  5. Search for related GCS operations (object rewrite, copy, bucket updates) in the same time window
  6. Look for other ransomware indicators (disabled KMS keys, bucket configuration changes) from this project in the past 24 hours
Reference: https://cloud.google.com/kms/docs/reference/rest/v1/projects.locations.keyRings.cryptoKeys/encrypt
SummaryAttributes:
  - severity
  - p_any_emails
Tests:
  - Name: KMS Encrypt by GCS Service Account - Success
    ExpectedResult: true
    Log:
      {
        "protoPayload":
          {
            "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "authenticationInfo":
              {
                "principalEmail": "service-111111111111-gs-project-accounts.iam.gserviceaccount.com",
              },
            "methodName": "Encrypt",
            "resourceName": "projects/test-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key",
            "serviceName": "cloudkms.googleapis.com",
            "status": {},
          },
        "resource":
          {
            "labels":
              {
                "crypto_key_id": "test-key",
                "key_ring_id": "test-keyring",
                "location": "us",
                "project_id": "test-project",
              },
            "type": "cloudkms_cryptokey",
          },
        "severity": "INFO",
        "timestamp": "2025-12-15 15:40:47.284488263",
      }
  - Name: KMS Encrypt by Regular Service Account
    ExpectedResult: false
    Log:
      {
        "protoPayload":
          {
            "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "authenticationInfo":
              { "principalEmail": "denethor@lotr.com" },
            "methodName": "Encrypt",
            "resourceName": "projects/test-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key",
            "serviceName": "cloudkms.googleapis.com",
            "status": {},
          },
        "resource":
          {
            "labels": { "project_id": "test-project" },
            "type": "cloudkms_cryptokey",
          },
        "severity": "INFO",
        "timestamp": "2025-12-15 15:40:47.284488263",
      }
  - Name: KMS Encrypt by GCS Service Account - Failed
    ExpectedResult: false
    Log:
      {
        "protoPayload":
          {
            "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "authenticationInfo":
              {
                "principalEmail": "service-111111111111-gs-project-accounts.iam.gserviceaccount.com",
              },
            "methodName": "Encrypt",
            "resourceName": "projects/test-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key",
            "serviceName": "cloudkms.googleapis.com",
            "status":
              {
                "code": 9,
                "message": "key is not enabled, current state is: DISABLED.",
              },
          },
        "resource":
          {
            "labels": { "project_id": "test-project" },
            "type": "cloudkms_cryptokey",
          },
        "severity": "ERROR",
        "timestamp": "2025-12-15 15:40:47.284488263",
      }

Detection logic

Condition

protoPayload.methodName eq "Encrypt"
protoPayload.serviceName eq "cloudkms.googleapis.com"
protoPayload.authenticationInfo.principalEmail contains "gs-project-accounts.iam.gserviceaccount.com"
severity ne "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
`protoPayload.authenticationInfo.principalEmail`	contains	`gs-project-accounts.iam.gserviceaccount.com`
`protoPayload.methodName`	eq	`Encrypt`
`protoPayload.serviceName`	eq	`cloudkms.googleapis.com`
`severity`	ne	`ERROR`

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`
`kms_key`	`protoPayload.resourceName`
`key_ring`	`resource.labels.key_ring_id`
`crypto_key`	`resource.labels.crypto_key_id`
`project`	`resource.labels.project_id`
`status`	`protoPayload.status`
`location`	`resource.labels.location`

`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)