Corelight - SMTP Email containing NON Ascii Characters within the Subject

Status: available
Severity: low
Time window: 1h
Source: github.com/Azure/Azure-Sentinel

'Detects where an emails contain non ascii characters within the Subject.'

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1566` Phishing

Rule body kusto

id: 50c61708-9824-46f3-87cf-22490796fae2
name: Corelight - SMTP Email containing NON Ascii Characters within the Subject
description: |
  'Detects where an emails contain non ascii characters within the Subject.'
severity: Low
status: Available
requiredDataConnectors:
  - connectorId: Corelight
    dataTypes:
      - Corelight_v2_smtp
      - corelight_smtp
queryFrequency: 1h
queryPeriod: 1h
triggerOperator: gt
triggerThreshold: 0
tactics:
  - InitialAccess
relevantTechniques:
  - T1566 
query: |
  corelight_smtp
  | where subject hasprefix  @'\=?utf-16'
  | summarize recipients = dcount(_to)
  | extend k = 1
  | join (corelight_smtp
          | where subject hasprefix  @'\=?utf-16'
          | summarize by _to
          | extend k = 1) on k
  | where recipients > 1
entityMappings:
  - entityType: MailMessage
    fieldMappings:
      - identifier: Recipient
        columnName: _to
version: 2.1.0
kind: Scheduled

Stages and Predicates

Stage 1: `source`

corelight_smtp

Stage 2: `where`

| where subject hasprefix  @'\=?utf-16'

Stage 3: `summarize`

| summarize recipients = dcount(_to)

Threshold: gt 1

Stage 4: `extend`

| extend k = 1

Stage 5: `join`

| join (corelight_smtp
        | where subject hasprefix  @'\=?utf-16'
        | summarize by _to
        | extend k = 1) on k

Stage 6: `where`

| where recipients > 1

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
`recipients`	gt	`1` transforms: `cased`
`subject`	starts_with	`\=?utf-16`

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
`recipients`	`summarize`
`k`	`extend`

`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)

Corelight - SMTP Email containing NON Ascii Characters within the Subject

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Stage 1: `source`

Stage 2: `where`

Stage 3: `summarize`

Stage 4: `extend`

Stage 5: `join`

Stage 6: `where`

Indicators

Output fields

Keyboard shortcuts

Search operators

Corelight - SMTP Email containing NON Ascii Characters within the Subject

MITRE ATT&CK coverage

Rule body kusto

Stages and Predicates

Stage 1: source

Stage 2: where

Stage 3: summarize

Stage 4: extend

Stage 5: join

Stage 6: where

Indicators

Output fields

Stage 1: `source`

Stage 2: `where`

Stage 3: `summarize`

Stage 4: `extend`

Stage 5: `join`

Stage 6: `where`