Gsuite Email Suspicious Subject With Attachment

Status: production
Severity: low
Group by: "destination{}.address", "destination{}.service", "source.address", num_message_attachments, phase, severity, subject
Author: Teoderick Contreras, Splunk
Source: github.com/splunk/security_content

The following analytic identifies Gsuite emails with suspicious subjects and attachments commonly used in spear phishing attacks. It leverages Gsuite email logs, focusing on specific keywords in the subject line and known malicious file types in attachments. This activity is significant for a SOC as spear phishing is a prevalent method for initial compromise, often leading to further malicious actions. If confirmed malicious, this activity could result in unauthorized access, data exfiltration, or further malware deployment, posing a significant risk to the organization's security.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1566.001` Phishing: Spearphishing Attachment

Rule body splunk

name: Gsuite Email Suspicious Subject With Attachment
id: 8ef3971e-00f2-11ec-b54f-acde48001122
version: 10
creation_date: '2021-08-19'
modification_date: '2026-05-13'
author: Teoderick Contreras, Splunk
status: production
type: Anomaly
description: The following analytic identifies Gsuite emails with suspicious subjects and attachments commonly used in spear phishing attacks. It leverages Gsuite email logs, focusing on specific keywords in the subject line and known malicious file types in attachments. This activity is significant for a SOC as spear phishing is a prevalent method for initial compromise, often leading to further malicious actions. If confirmed malicious, this activity could result in unauthorized access, data exfiltration, or further malware deployment, posing a significant risk to the organization's security.
data_source:
    - G Suite Gmail
search: |-
    `gsuite_gmail` num_message_attachments > 0 subject IN ("*dhl*", "* ups *", "*delivery*", "*parcel*", "*label*", "*invoice*", "*postal*", "* fedex *", "* usps *", "* express *", "*shipment*", "*Banking/Tax*","*shipment*", "*new order*") attachment{}.file_extension_type IN ("doc", "docx", "xls", "xlsx", "ppt", "pptx", "pdf", "zip", "rar", "html","htm","hta")
      | rex field=source.from_header_address "[^@]+@(?<source_domain>[^@]+)"
      | rex field=destination{}.address "[^@]+@(?<dest_domain>[^@]+)"
      | where not source_domain="internal_test_email.com" and dest_domain="internal_test_email.com"
      | eval phase="plan"
      | eval severity="medium"
      | stats count min(_time) as firstTime max(_time) as lastTime values(attachment{}.file_extension_type) as email_attachments, values(attachment{}.sha256) as attachment_sha256, values(payload_size) as payload_size
        BY destination{}.service num_message_attachments subject
           destination{}.address source.address phase
           severity
      | `security_content_ctime(firstTime)`
      | `security_content_ctime(lastTime)`
      | `gsuite_email_suspicious_subject_with_attachment_filter`
how_to_implement: To successfully implement this search, you need to be ingesting logs related to gsuite having the file attachment metadata like file type, file extension, source email, destination email, num of attachment and etc.
known_false_positives: normal user or normal transaction may contain the subject and file type attachment that this detection try to search.
references:
    - https://www.redhat.com/en/topics/devops/what-is-devsecops
    - https://www.mandiant.com/resources/top-words-used-in-spear-phishing-attacks
drilldown_searches:
    - name: View the detection results for - "$destination{}.address$"
      search: '%original_detection_search% | search  destination{}.address = "$destination{}.address$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$destination{}.address$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$destination{}.address$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
intermediate_findings:
    entities:
        - field: destination{}.address
          type: user
          score: 20
          message: Suspicious email from $source.address$ to $destination{}.address$
threat_objects:
    - field: source.address
      type: email_address
analytic_story:
    - Dev Sec Ops
asset_type: GSuite
mitre_attack_id:
    - T1566.001
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: endpoint
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1566.001/gsuite_susp_subj/gsuite_susp_subj_attach.log
          source: http:gsuite
          sourcetype: gsuite:gmail:bigquery
      test_type: unit

Stages and Predicates

Stage 1: `search`

`gsuite_gmail` num_message_attachments > 0 subject IN ("*dhl*", "* ups *", "*delivery*", "*parcel*", "*label*", "*invoice*", "*postal*", "* fedex *", "* usps *", "* express *", "*shipment*", "*Banking/Tax*","*shipment*", "*new order*") attachment{}.file_extension_type IN ("doc", "docx", "xls", "xlsx", "ppt", "pptx", "pdf", "zip", "rar", "html","htm","hta")

Stage 2: `rex`

| rex field=source.from_header_address "[^@]+@(?<source_domain>[^@]+)"

Stage 3: `rex`

| rex field=destination{}.address "[^@]+@(?<dest_domain>[^@]+)"

Stage 4: `where`

| where not source_domain="internal_test_email.com" and dest_domain="internal_test_email.com"

Stage 5: `eval`

| eval phase="plan"

Stage 6: `eval`

| eval severity="medium"

Stage 7: `stats`

| stats count min(_time) as firstTime max(_time) as lastTime values(attachment{}.file_extension_type) as email_attachments, values(attachment{}.sha256) as attachment_sha256, values(payload_size) as payload_size
    BY destination{}.service num_message_attachments subject
       destination{}.address source.address phase
       severity

Stage 8: `search`

| `security_content_ctime(firstTime)`

Stage 9: `search`

| `security_content_ctime(lastTime)`

Stage 10: `search`

| `gsuite_email_suspicious_subject_with_attachment_filter`

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`source_domain`	eq	`"internal_test_email.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
`attachment{}.file_extension_type`	in	`"doc"` `"docx"` `"hta"` `"htm"` `"html"` `"pdf"` `"ppt"` `"pptx"` `"rar"` `"xls"` `"xlsx"` `"zip"`
`dest_domain`	eq	`"internal_test_email.com"`
`num_message_attachments`	gt	`0`
`sourcetype`	eq	`gsuite:gmail:bigquery`
`subject`	in	`"* express "` `" fedex "` `" ups "` `" usps "` `"Banking/Tax"` `"delivery"` `"dhl"` `"invoice"` `"label"` `"new order"` `"parcel"` `"postal"` `"shipment*"`

`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)

Gsuite Email Suspicious Subject With Attachment

MITRE ATT&CK coverage

Rule body splunk

Stages and Predicates

Stage 1: `search`

Stage 2: `rex`

Stage 3: `rex`

Stage 4: `where`

Stage 5: `eval`

Stage 6: `eval`

Stage 7: `stats`

Stage 8: `search`

Stage 9: `search`

Stage 10: `search`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Gsuite Email Suspicious Subject With Attachment

MITRE ATT&CK coverage

Rule body splunk

Stages and Predicates

Stage 1: search

Stage 2: rex

Stage 3: rex

Stage 4: where

Stage 5: eval

Stage 6: eval

Stage 7: stats

Stage 8: search

Stage 9: search

Stage 10: search

Exclusions

Indicators

Stage 1: `search`

Stage 2: `rex`

Stage 3: `rex`

Stage 4: `where`

Stage 5: `eval`

Stage 6: `eval`

Stage 7: `stats`

Stage 8: `search`

Stage 9: `search`

Stage 10: `search`