Suspicious recipients pattern with no Compauth pass and suspicious content

Severity: medium
Type: rule
Source: github.com/sublime-security/sublime-rules

Detects messages with undisclosed recipients (likely all bcc), where the Compauth verdict is not 'pass', and ML has identified suspicious language or credential phishing links.

Event coverage

Message attribute
body
body.current_thread
headers (collection)
headers.auth_summary
headers.hops (collection)
recipients
recipients.to (collection)
sender.email
type

Rule body MQL

type.inbound
and (
  length(recipients.to) == 0
  or all(recipients.to, .display_name == "Undisclosed recipients")
)
and length(recipients.cc) == 0
and length(recipients.bcc) == 0
and 2 of (
  (
    any(headers.hops,
        .authentication_results.compauth.verdict is not null
        and .authentication_results.compauth.verdict not in ("pass", "softpass")
    )
  ),
  (
    any(ml.nlu_classifier(body.current_thread.text).intents,
        .name in ("bec", "cred_theft", "advance_fee") and .confidence == "high"
    )
  ),
  (
    any(body.links,
        any([ml.link_analysis(.)],
            .credphish.disposition == "phishing"
            and .credphish.confidence in ("high")
        )
    )
  )
)
and (
  profile.by_sender().prevalence in ("new", "outlier")
  or (
    profile.by_sender().any_messages_malicious_or_spam
    and not profile.by_sender().any_messages_benign
  )
)
// negate highly trusted sender domains unless they fail DMARC authentication
and (
  (
    sender.email.domain.root_domain in $high_trust_sender_root_domains
    and not headers.auth_summary.dmarc.pass
  )
  or sender.email.domain.root_domain not in $high_trust_sender_root_domains
)

Detection logic

Scope: inbound message.

Detects messages with undisclosed recipients (likely all bcc), where the Compauth verdict is not 'pass', and ML has identified suspicious language or credential phishing links.

inbound message
any of:
- length(recipients.to) is 0
- all of recipients.to where:
  - .display_name is 'Undisclosed recipients'
length(recipients.cc) is 0
length(recipients.bcc) is 0
at least 2 of:
- any of headers.hops where all hold:
  - .authentication_results.compauth.verdict is set
  - .authentication_results.compauth.verdict not in ('pass', 'softpass')
- any of ml.nlu_classifier(body.current_thread.text).intents where all hold:
  - .name in ('bec', 'cred_theft', 'advance_fee')
  - .confidence is 'high'
- any of body.links where:
  - any of [ml.link_analysis(.)] where all hold:
    
    .credphish.disposition is 'phishing'
    .credphish.confidence in ('high')
any of:
- profile.by_sender().prevalence in ('new', 'outlier')
- all of:
  - profile.by_sender().any_messages_malicious_or_spam
  - not:
    
    profile.by_sender().any_messages_benign
any of:
- all of:
  - sender.email.domain.root_domain in $high_trust_sender_root_domains
  - not:
    
    headers.auth_summary.dmarc.pass
- sender.email.domain.root_domain not in $high_trust_sender_root_domains

Inspects: body.current_thread.text, body.links, headers.auth_summary.dmarc.pass, headers.hops, headers.hops[].authentication_results.compauth.verdict, recipients.bcc, recipients.cc, recipients.to, recipients.to[].display_name, sender.email.domain.root_domain, type.inbound. Sensors: ml.link_analysis, ml.nlu_classifier, profile.by_sender. Reference lists: $high_trust_sender_root_domains.

Indicators matched (9)

Field	Match	Value
`recipients.to[].display_name`	equals	`Undisclosed recipients`
`headers.hops[].authentication_results.compauth.verdict`	member	`pass`
`headers.hops[].authentication_results.compauth.verdict`	member	`softpass`
`ml.nlu_classifier(body.current_thread.text).intents[].name`	member	`bec`
`ml.nlu_classifier(body.current_thread.text).intents[].name`	member	`cred_theft`
`ml.nlu_classifier(body.current_thread.text).intents[].name`	member	`advance_fee`
`ml.nlu_classifier(body.current_thread.text).intents[].confidence`	equals	`high`
`[ml.link_analysis(body.links[])][].credphish.disposition`	equals	`phishing`
`[ml.link_analysis(body.links[])][].credphish.confidence`	member	`high`

`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)

Suspicious recipients pattern with no Compauth pass and suspicious content

Event coverage

Rule body MQL

Detection logic

Indicators matched (9)

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