GCP GCE Image Open To Public

Severity: high
Type: Alert
Author: Google Cloud Security
Source: github.com/chronicle/detection-rules

Detect when GCP Compute Image is opened to public by adding allAuthenticatedUsers to the access policy.

MITRE ATT&CK coverage

Tactic	Techniques
Exfiltration	`T1537` Transfer Data to Cloud Account

Event coverage

Provider	Event	Title
GCP-compute.googleapis.com	compute-images-setIamPolicy	Set image IAM policy

Rule body yaral

/*
 * Copyright 2024 Google LLC
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

 rule gcp_gce_image_open_to_public {

  meta:
    author = "Google Cloud Security"
    description = "Detect when GCP Compute Image is opened to public by adding allAuthenticatedUsers to the access policy."
    rule_id = "mr_4a1525be-9e17-41d6-ba67-a11058ee4d3c"
    rule_name = "GCP GCE Image Open To Public"
    mitre_attack_tactic = "Exfiltration"
    mitre_attack_technique = "Transfer Data to Cloud Account"
    mitre_attack_url = "https://attack.mitre.org/techniques/T1537/"
    mitre_attack_version = "v14.1"
    type = "Alert"
    data_source = "GCP Cloud Audit"
    platform = "GCP"
    severity = "High"
    priority = "High"

  events:
    $gcp.metadata.event_type = "USER_RESOURCE_UPDATE_PERMISSIONS"
    $gcp.metadata.log_type = "GCP_CLOUDAUDIT"
    $gcp.metadata.vendor_name = "Google Cloud Platform"
    $gcp.metadata.product_event_type = "beta.compute.images.setIamPolicy"
    $gcp.target.application = "compute.googleapis.com"
    $gcp.security_result.action = "ALLOW"
    any $gcp.target.resource.attribute.labels.value = "allAuthenticatedUsers"
    $gcp.target.resource.resource_subtype = "gce_image"

  outcome:
    $risk_score = max(75)
    $mitre_attack_tactic = "Exfiltration"
    $mitre_attack_technique = "Transfer Data to Cloud Account"
    $mitre_attack_technique_id = "T1537"
    $event_count = count_distinct($gcp.metadata.id)
    $network_http_user_agent = array_distinct($gcp.network.http.user_agent)
    $principal_ip = array_distinct($gcp.principal.ip)
    $principal_ip_country = array_distinct($gcp.principal.ip_geo_artifact.location.country_or_region)
    $principal_ip_state = array_distinct($gcp.principal.ip_geo_artifact.location.state)
    $principal_user_id = $gcp.principal.user.userid
    $principal_user_display_name = $gcp.principal.user.user_display_name
    $target_resource_name = $gcp.target.resource.name
    $event_name = $gcp.metadata.product_event_type
    $target_email_addresses = array_distinct($gcp.target.user.email_addresses)

  condition:
    $gcp
}

Detection logic

Fires when at least one $gcp event.

Events

`$gcp` `USER_RESOURCE_UPDATE_PERMISSIONS`

metadata.log_type = "GCP_CLOUDAUDIT"
metadata.product_event_type = "beta.compute.images.setIamPolicy"
target.application = "compute.googleapis.com"
security_result.action = "ALLOW"
target.resource.attribute.labels.value = "allAuthenticatedUsers"
target.resource.resource_subtype = "gce_image"

Outcome

Fields the detection emits on a match. $risk_score drives alerting; Chronicle surfaces the rest on the detection.

Field	Expression
`risk_score`	`max(75)`
`event_count`	`count_distinct($gcp.metadata.id)`
`network_http_user_agent`	`array_distinct($gcp.network.http.user_agent)`
`principal_ip`	`array_distinct($gcp.principal.ip)`
`principal_ip_country`	`array_distinct($gcp.principal.ip_geo_artifact.location.country_or_region)`
`principal_ip_state`	`array_distinct($gcp.principal.ip_geo_artifact.location.state)`
`principal_user_id`	`$gcp.principal.user.userid`
`principal_user_display_name`	`$gcp.principal.user.user_display_name`
`target_resource_name`	`$gcp.target.resource.name`
`event_name`	`$gcp.metadata.product_event_type`
`target_email_addresses`	`array_distinct($gcp.target.user.email_addresses)`

`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)

GCP GCE Image Open To Public

MITRE ATT&CK coverage

Event coverage

Rule body yaral

Detection logic

Events

`$gcp` `USER_RESOURCE_UPDATE_PERMISSIONS`

Outcome

Keyboard shortcuts

Search operators

GCP GCE Image Open To Public

MITRE ATT&CK coverage

Event coverage

Rule body yaral

Detection logic

Events

$gcp USER_RESOURCE_UPDATE_PERMISSIONS

Outcome

`$gcp` `USER_RESOURCE_UPDATE_PERMISSIONS`