GCP Compute IAM Policy Update Detection

Severity: medium
Log types: GCP.AuditLog
Reference: https://cloud.google.com/compute/docs/access/iam
Source: github.com/panther-labs/panther-analysis

This rule detects updates to IAM policies for Compute Disks, Images, and Snapshots.

Rule body yaml

AnalysisType: rule
DedupPeriodMinutes: 60
DisplayName: GCP Compute IAM Policy Update Detection
Enabled: true
Filename: gcp_compute_set_iam_policy.py
RuleID: "GCP.Compute.IAM.Policy.Update"
Severity: Medium
LogTypes:
  - GCP.AuditLog
Description: >
  This rule detects updates to IAM policies for Compute Disks, Images, and Snapshots.
Runbook: >
  Ensure that the IAM policy update was expected. Unauthorized changes can lead to security risks.
Reference: https://cloud.google.com/compute/docs/access/iam
Tests:
  - Name: IAM policy update on a Compute Disk
    LogType: GCP.AuditLog
    ExpectedResult: true
    Log:
      {
        "insertId": "1abcd23efg456",
        "labels": {
          "compute.googleapis.com/root_trigger_id": "trigger-id-1"
        },
        "logName": "projects/test-project/logs/cloudaudit.googleapis.com%2Factivity",
        "protoPayload": {
          "@type": "type.googleapis.com/google.cloud.audit.AuditLog",
          "authenticationInfo": {
            "principalEmail": "user@example.com",
            "principalSubject": "serviceAccount:user@example.com",
            "serviceAccountKeyName": "//iam.googleapis.com/projects/test-project/serviceAccounts/user@example.com/keys/key-id"
          },
          "authorizationInfo": [
            {
              "granted": true,
              "permission": "compute.disks.setIamPolicy",
              "resource": "projects/test-project/zones/us-central1-a/disks/disk-1",
              "resourceAttributes": {
                "name": "projects/test-project/zones/us-central1-a/disks/disk-1",
                "service": "compute",
                "type": "compute.disks"
              }
            }
          ],
          "methodName": "v1.compute.disks.setIamPolicy",
          "request": {
            "@type": "type.googleapis.com/compute.disks.setIamPolicy",
            "policy": {
              "bindings": [
                {
                  "members": [
                    "user:anonymized@example.com"
                  ],
                  "role": "roles/owner"
                }
              ]
            }
          },
          "requestMetadata": {
            "callerIP": "192.0.2.1",
            "callerSuppliedUserAgent": "google-cloud-sdk",
            "destinationAttributes": {},
            "requestAttributes": {
              "auth": {},
              "time": "2023-10-01T12:34:56.789Z"
            }
          },
          "resourceLocation": {
            "currentLocations": [
              "us-central1-a"
            ]
          },
          "resourceName": "projects/test-project/zones/us-central1-a/disks/disk-1",
          "serviceName": "compute.googleapis.com"
        },
        "receiveTimestamp": "2023-10-01T12:34:57.123Z",
        "resource": {
          "labels": {
            "disk_id": "disk-id-1",
            "project_id": "test-project",
            "zone": "us-central1-a"
          },
          "type": "gce_disk"
        },
        "severity": "NOTICE",
        "timestamp": "2023-10-01T12:34:56.789Z"
      }
  - Name: IAM policy update on a Compute Image
    LogType: GCP.AuditLog
    ExpectedResult: true
    Log:
      {
        "insertId": "2hijk34lmn789",
        "labels": {
          "compute.googleapis.com/root_trigger_id": "trigger-id-2"
        },
        "logName": "projects/test-project/logs/cloudaudit.googleapis.com%2Factivity",
        "protoPayload": {
          "@type": "type.googleapis.com/google.cloud.audit.AuditLog",
          "authenticationInfo": {
            "principalEmail": "user@example.com",
            "principalSubject": "serviceAccount:user@example.com",
            "serviceAccountKeyName": "//iam.googleapis.com/projects/test-project/serviceAccounts/user@example.com/keys/key-id"
          },
          "authorizationInfo": [
            {
              "granted": true,
              "permission": "compute.images.setIamPolicy",
              "resource": "projects/test-project/global/images/image-1",
              "resourceAttributes": {
                "name": "projects/test-project/global/images/image-1",
                "service": "compute",
                "type": "compute.images"
              }
            }
          ],
          "methodName": "v1.compute.images.setIamPolicy",
          "request": {
            "@type": "type.googleapis.com/compute.images.setIamPolicy",
            "policy": {
              "bindings": [
                {
                  "members": [
                    "user:anonymized@example.com"
                  ],
                  "role": "roles/owner"
                }
              ]
            }
          },
          "requestMetadata": {
            "callerIP": "192.0.2.2",
            "callerSuppliedUserAgent": "google-cloud-sdk",
            "destinationAttributes": {},
            "requestAttributes": {
              "auth": {},
              "time": "2023-10-01T12:45:56.789Z"
            }
          },
          "resourceLocation": {
            "currentLocations": [
              "us-central1-a"
            ]
          },
          "resourceName": "projects/test-project/global/images/image-1",
          "serviceName": "compute.googleapis.com"
        },
        "receiveTimestamp": "2023-10-01T12:45:57.123Z",
        "resource": {
          "labels": {
            "image_id": "image-id-1",
            "project_id": "test-project"
          },
          "type": "gce_image"
        },
        "severity": "NOTICE",
        "timestamp": "2023-10-01T12:45:56.789Z"
      }
  - Name: Non-IAM policy update on a Compute Disk
    LogType: GCP.AuditLog
    ExpectedResult: false
    Log:
      {
        "insertId": "4stuv78wxy345",
        "labels": {
          "compute.googleapis.com/root_trigger_id": "trigger-id-4"
        },
        "logName": "projects/test-project/logs/cloudaudit.googleapis.com%2Factivity",
        "protoPayload": {
          "@type": "type.googleapis.com/google.cloud.audit.AuditLog",
          "authenticationInfo": {
            "principalEmail": "user@example.com",
            "principalSubject": "serviceAccount:user@example.com",
            "serviceAccountKeyName": "//iam.googleapis.com/projects/test-project/serviceAccounts/user@example.com/keys/key-id"
          },
          "authorizationInfo": [
            {
              "granted": true,
              "permission": "compute.disks.create",
              "resource": "projects/test-project/zones/us-central1-a/disks/disk-2",
              "resourceAttributes": {
                "name": "projects/test-project/zones/us-central1-a/disks/disk-2",
                "service": "compute",
                "type": "compute.disks"
              }
            }
          ],
          "methodName": "v1.compute.disks.create",
          "requestMetadata": {
            "callerIP": "192.0.2.4",
            "callerSuppliedUserAgent": "google-cloud-sdk",
            "destinationAttributes": { },
            "requestAttributes": {
              "auth": { },
              "time": "2023-10-01T13:00:00.000Z"
            }
          },
          "resourceLocation": {
            "currentLocations": [
              "us-central1-a"
            ]
          },
          "resourceName": "projects/test-project/zones/us-central1-a/disks/disk-2",
          "serviceName": "compute.googleapis.com"
        },
        "receiveTimestamp": "2023-10-01T13:00:01.000Z",
        "resource": {
          "labels": {
            "disk_id": "disk-id-2",
            "project_id": "test-project",
            "zone": "us-central1-a"
          },
          "type": "gce_disk"
        },
        "severity": "NOTICE",
        "timestamp": "2023-10-01T13:00:00.000Z"
      }

Detection logic

Condition

protoPayload.response.error is_null
protoPayload.methodName in ["v1.compute.disks.setIamPolicy", "v1.compute.images.setIamPolicy", "v1.compute.snapshots.setIamPolicy"]

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	`v1.compute.disks.setIamPolicy` `v1.compute.images.setIamPolicy` `v1.compute.snapshots.setIamPolicy`
`protoPayload.response.error`	is_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
`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)