GCP IAM serviceAccounts signBlob

Severity: high
Log types: GCP.AuditLog
Reference: https://rhinosecuritylabs.com/gcp/privilege-escalation-google-cloud-platform-part-1/
Source: github.com/panther-labs/panther-analysis

The iam.serviceAccounts.signBlob permission "allows signing of arbitrary payloads" in GCP. This means we can create a signed blob that requests an access token from the Service Account we are targeting.

MITRE ATT&CK coverage

Tactic	Techniques
Privilege Escalation	`T1548` Abuse Elevation Control Mechanism

Rule body yaml

AnalysisType: rule
Filename: gcp_iam_service_accounts_sign_blob.py
RuleID: "GCP.IAM.serviceAccounts.signBlob"
DisplayName: "GCP IAM serviceAccounts signBlob"
Enabled: true
LogTypes:
  - GCP.AuditLog
Reports:
  MITRE ATT&CK:
    - TA0004:T1548
Severity: High
Description:
  The iam.serviceAccounts.signBlob permission "allows signing of arbitrary payloads" in GCP.
  This means we can create a signed blob that requests an access token from the Service Account we are targeting.
Reference: https://rhinosecuritylabs.com/gcp/privilege-escalation-google-cloud-platform-part-1/
Tests:
  - Name: iam.serviceAccounts.signBlob granted
    ExpectedResult: true
    Log:
      {
        "protoPayload":
          {
            "@type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "status": {},
            "authenticationInfo":
              {
                "principalEmail": "some-project@company.iam.gserviceaccount.com",
                "serviceAccountKeyName": "//iam.googleapis.com/projects/some-project/serviceAccounts/some-project@company.iam.gserviceaccount.com/keys/a378358365ff3d22e9c1a72fecf4605ddff76b47",
                "principalSubject": "serviceAccount:some-project@company.iam.gserviceaccount.com",
              },
            "requestMetadata":
              {
                "callerIp": "1.2.3.4",
                "requestAttributes":
                  { "time": "2024-02-26T17:15:16.327542536Z", "auth": {} },
                "destinationAttributes": {},
              },
            "serviceName": "iamcredentials.googleapis.com",
            "methodName": "SignJwt",
            "authorizationInfo":
              [
                {
                  "permission": "iam.serviceAccounts.signBlob",
                  "granted": true,
                  "resourceAttributes": {},
                },
              ],
            "resourceName": "projects/-/serviceAccounts/114885146936855121342",
            "request":
              {
                "name": "projects/-/serviceAccounts/some-project@company.iam.gserviceaccount.com",
                "@type": "type.googleapis.com/google.iam.credentials.v1.SignJwtRequest",
              },
          },
        "insertId": "1hu88qbef4d2o",
        "resource":
          {
            "type": "service_account",
            "labels":
              {
                "project_id": "some-project",
                "unique_id": "114885146936855121342",
                "email_id": "some-project@company.iam.gserviceaccount.com",
              },
          },
        "timestamp": "2024-02-26T17:15:16.314854637Z",
        "severity": "INFO",
        "logName": "projects/some-project/logs/cloudaudit.googleapis.com%2Fdata_access",
        "receiveTimestamp": "2024-02-26T17:15:17.100020459Z",
      }
  - Name: iam.serviceAccounts.signBlob not granted
    ExpectedResult: false
    Log:
      {
        "protoPayload":
          {
            "@type": "type.googleapis.com/google.cloud.audit.AuditLog",
            "status": {},
            "authenticationInfo":
              {
                "principalEmail": "some-project@company.iam.gserviceaccount.com",
                "serviceAccountKeyName": "//iam.googleapis.com/projects/some-project/serviceAccounts/some-project@company.iam.gserviceaccount.com/keys/a378358365ff3d22e9c1a72fecf4605ddff76b47",
                "principalSubject": "serviceAccount:some-project@company.iam.gserviceaccount.com",
              },
            "requestMetadata":
              {
                "callerIp": "1.2.3.4",
                "requestAttributes":
                  { "time": "2024-02-26T17:15:16.327542536Z", "auth": {} },
                "destinationAttributes": {},
              },
            "serviceName": "iamcredentials.googleapis.com",
            "methodName": "SignJwt",
            "authorizationInfo":
              [
                {
                  "permission": "iam.serviceAccounts.signBlob",
                  "granted": false,
                  "resourceAttributes": {},
                },
              ],
            "resourceName": "projects/-/serviceAccounts/114885146936855121342",
            "request":
              {
                "name": "projects/-/serviceAccounts/some-project@company.iam.gserviceaccount.com",
                "@type": "type.googleapis.com/google.iam.credentials.v1.SignJwtRequest",
              },
          },
        "insertId": "1hu88qbef4d2o",
        "resource":
          {
            "type": "service_account",
            "labels":
              {
                "project_id": "some-project",
                "unique_id": "114885146936855121342",
                "email_id": "some-project@company.iam.gserviceaccount.com",
              },
          },
        "timestamp": "2024-02-26T17:15:16.314854637Z",
        "severity": "INFO",
        "logName": "projects/some-project/logs/cloudaudit.googleapis.com%2Fdata_access",
        "receiveTimestamp": "2024-02-26T17:15:17.100020459Z",
      }

Detection logic

Condition

protoPayload.authorizationInfo is_not_null
protoPayload.authorizationInfo array_any

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.authorizationInfo`	is_not_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)