GCP KMS Cross-Project Encryption

Severity: high
Log types: GCP.AuditLog
Tags: GCP, KMS, Encryption, Ransomware
Reference: https://cloud.google.com/kms/docs/encrypt-decrypt https://cloud.google.com/storage/docs/encryption/customer-managed-keys
Source: github.com/panther-labs/panther-analysis

Detects when a GCS service account in one project uses a KMS encryption key from a different project. This could indicate potential ransomware activity where an attacker is using their own KMS key to encrypt data in a victim's project, making it inaccessible without the attacker's key.

Rule body yaml

AnalysisType: rule
Filename: gcp_kms_cross_project_encryption.py
RuleID: "GCP.KMS.CrossProjectEncryption"
DisplayName: "GCP KMS Cross-Project Encryption"
Enabled: true
LogTypes:
  - GCP.AuditLog
Tags:
  - GCP
  - KMS
  - Encryption
  - Ransomware
Severity: High
Description: >
  Detects when a GCS service account in one project uses a KMS encryption key from a different project.
  This could indicate potential ransomware activity where an attacker is using their own KMS key to encrypt
  data in a victim's project, making it inaccessible without the attacker's key.
Runbook: |
  1. Query GCP audit logs for all KMS operations by the principal email in the 24 hours before and after this alert to understand the scope of encryption activity
  2. Check if the cross-project KMS key access is documented in approved service integrations or has been used by this service account in the past 90 days
  3. Find all storage operations (GCS object writes, rewrites) by this service account in the 1 hour window around the alert to identify potentially affected data
Reference: >
  https://cloud.google.com/kms/docs/encrypt-decrypt
  https://cloud.google.com/storage/docs/encryption/customer-managed-keys
SummaryAttributes:
  - protoPayload:authenticationInfo:principalEmail
  - protoPayload:resourceName
  - resource:labels:project_id
Tests:
  - Name: Cross-Project KMS Encryption Detected
    ExpectedResult: true
    Log:
      {
        "p_log_type": "GCP.AuditLog",
        "insertId": "test-insert-id-001",
        "logName": "projects/victim-project/logs/cloudaudit.googleapis.com%2Fdata_access",
        "protoPayload": {
          "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
          "authenticationInfo": {
            "principalEmail": "service-111111111111-gs-project-accounts.iam.gserviceaccount.com"
          },
          "authorizationInfo": [
            {
              "granted": true,
              "permission": "cloudkms.cryptoKeyVersions.useToEncrypt",
              "permissionType": "DATA_READ",
              "resource": "projects/victim-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key"
            }
          ],
          "methodName": "Encrypt",
          "request": {
            "@type": "type.googleapis.com/google.cloud.kms.v1.EncryptRequest",
            "name": "projects/victim-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key"
          },
          "resourceName": "projects/attacker-project/locations/us/keyRings/malicious-keyring/cryptoKeys/ransomware-key",
          "serviceName": "cloudkms.googleapis.com"
        },
        "resource": {
          "labels": {
            "crypto_key_id": "test-key",
            "key_ring_id": "test-keyring",
            "location": "us",
            "project_id": "victim-project"
          },
          "type": "cloudkms_cryptokey"
        },
        "severity": "INFO",
        "timestamp": "2025-12-02 19:41:27.745108384"
      }
  - Name: Same-Project KMS Encryption (No Alert)
    ExpectedResult: false
    Log:
      {
        "p_log_type": "GCP.AuditLog",
        "insertId": "test-insert-id-002",
        "logName": "projects/legitimate-project/logs/cloudaudit.googleapis.com%2Fdata_access",
        "protoPayload": {
          "at_sign_type": "type.googleapis.com/google.cloud.audit.AuditLog",
          "authenticationInfo": {
            "principalEmail": "service-111111111111-gs-project-accounts.iam.gserviceaccount.com"
          },
          "authorizationInfo": [
            {
              "granted": true,
              "permission": "cloudkms.cryptoKeyVersions.useToEncrypt",
              "permissionType": "DATA_READ",
              "resource": "projects/legitimate-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key"
            }
          ],
          "methodName": "Encrypt",
          "request": {
            "@type": "type.googleapis.com/google.cloud.kms.v1.EncryptRequest",
            "name": "projects/legitimate-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key"
          },
          "resourceName": "projects/legitimate-project/locations/us/keyRings/test-keyring/cryptoKeys/test-key",
          "serviceName": "cloudkms.googleapis.com"
        },
        "resource": {
          "labels": {
            "crypto_key_id": "test-key",
            "project_id": "legitimate-project"
          },
          "type": "cloudkms_cryptokey"
        },
        "severity": "INFO",
        "timestamp": "2025-12-02 19:41:27.745108384"
      }

Detection logic

Condition

not (protoPayload.serviceName ne "cloudkms.googleapis.com" or protoPayload.methodName ne "Encrypt" or protoPayload.authenticationInfo.principalEmail not contains "gs-project-accounts.iam.gserviceaccount.com")

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

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`

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
`project`	`resource.labels.project_id`
`principal`	`protoPayload.authenticationInfo.principalEmail`
`caller_ip`	`protoPayload.requestMetadata.callerIP`
`methodName`	`protoPayload.methodName`
`resourceName`	`protoPayload.resourceName`
`serviceName`	`protoPayload.serviceName`

`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)