Encrypted Microsoft Office files from untrusted sender

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

Detects encrypted Microsoft Office document attachments (Word, Excel, PowerPoint, Access) from untrusted senders or high-trust senders failing DMARC authentication, which may indicate an effort to bypass security scanning.

Threat classification

Sublime's own taxonomy (not MITRE ATT&CK).

Category	Values
Attack types	BEC/Fraud, Callback Phishing, Credential Phishing, Extortion, Malware/Ransomware, Spam
Tactics and techniques	Encryption, Evasion

Event coverage

Message attribute
attachments (collection)
headers (collection)
headers.hops (collection)
sender.email
type

Rule body MQL

type.inbound
and any(attachments,
        (
          // Word documents
          strings.iends_with(.file_name, ".doc")
          or strings.iends_with(.file_name, ".docx")
          or strings.iends_with(.file_name, ".docm")
          or .content_type == "application/msword"
          or .content_type == "application/vnd.openxmlformats-officedocument.wordprocessingml.document"
          or .file_type in ("doc", "docx")
          or 

          // Excel documents
          strings.iends_with(.file_name, ".xls")
          or strings.iends_with(.file_name, ".xlsx")
          or strings.iends_with(.file_name, ".xlsm")
          or .content_type == "application/vnd.ms-excel"
          or .content_type == "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
          or .file_type in ("xls", "xlsx")
          or 

          // PowerPoint documents
          strings.iends_with(.file_name, ".ppt")
          or strings.iends_with(.file_name, ".pptx")
          or strings.iends_with(.file_name, ".pptm")
          or .content_type == "application/vnd.ms-powerpoint"
          or .content_type == "application/vnd.openxmlformats-officedocument.presentationml.presentation"
          or .file_type in ("ppt", "pptx")
          or 

          // Access documents
          strings.iends_with(.file_name, ".accdb")
          or strings.iends_with(.file_name, ".mdb")
          or .content_type == "application/msaccess"
          or .content_type == "application/x-msaccess"
          or .content_type == "application/vnd.ms-access"
        )
        and any(file.explode(.),
                any(.scan.yara.matches, .name == 'aes_encryption_keywords')
        )
)
// Negating high-trust sender domains unless they fail DMARC authentication
and (
  (
    sender.email.domain.root_domain in $high_trust_sender_root_domains
    and (
      any(distinct(headers.hops, .authentication_results.dmarc is not null),
          strings.ilike(.authentication_results.dmarc, "*fail")
      )
    )
  )
  or sender.email.domain.root_domain not in $high_trust_sender_root_domains
)

Detection logic

Scope: inbound message.

inbound message
any of attachments where all hold:
- any of:
  - .file_name ends with '.doc'
  - .file_name ends with '.docx'
  - .file_name ends with '.docm'
  - .content_type is 'application/msword'
  - .content_type is 'application/vnd.openxmlformats-officedocument.wordprocessingml.document'
  - .file_type in ('doc', 'docx')
  - .file_name ends with '.xls'
  - .file_name ends with '.xlsx'
  - .file_name ends with '.xlsm'
  - .content_type is 'application/vnd.ms-excel'
  - .content_type is 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet'
  - .file_type in ('xls', 'xlsx')
  - .file_name ends with '.ppt'
  - .file_name ends with '.pptx'
  - .file_name ends with '.pptm'
  - .content_type is 'application/vnd.ms-powerpoint'
  - .content_type is 'application/vnd.openxmlformats-officedocument.presentationml.presentation'
  - .file_type in ('ppt', 'pptx')
  - .file_name ends with '.accdb'
  - .file_name ends with '.mdb'
  - .content_type is 'application/msaccess'
  - .content_type is 'application/x-msaccess'
  - .content_type is 'application/vnd.ms-access'
- any of file.explode(.) where:
  - any of .scan.yara.matches where:
    
    .name is 'aes_encryption_keywords'
any of:
- all of:
  - sender.email.domain.root_domain in $high_trust_sender_root_domains
  - any of distinct(headers.hops) where:
    
    .authentication_results.dmarc matches '*fail'
- sender.email.domain.root_domain not in $high_trust_sender_root_domains

Inspects: attachments[].content_type, attachments[].file_name, attachments[].file_type, headers.hops, headers.hops[].authentication_results.dmarc, sender.email.domain.root_domain, type.inbound. Sensors: file.explode, strings.iends_with, strings.ilike. Reference lists: $high_trust_sender_root_domains.

Indicators matched (28)

Field	Match	Value
`strings.iends_with`	suffix	`.doc`
`strings.iends_with`	suffix	`.docx`
`strings.iends_with`	suffix	`.docm`
`attachments[].content_type`	equals	`application/msword`
`attachments[].content_type`	equals	`application/vnd.openxmlformats-officedocument.wordprocessingml.document`
`attachments[].file_type`	member	`doc`
`attachments[].file_type`	member	`docx`
`strings.iends_with`	suffix	`.xls`
`strings.iends_with`	suffix	`.xlsx`
`strings.iends_with`	suffix	`.xlsm`
`attachments[].content_type`	equals	`application/vnd.ms-excel`
`attachments[].content_type`	equals	`application/vnd.openxmlformats-officedocument.spreadsheetml.sheet`

16 more

`attachments[].file_type`	member	`xls`
`attachments[].file_type`	member	`xlsx`
`strings.iends_with`	suffix	`.ppt`
`strings.iends_with`	suffix	`.pptx`
`strings.iends_with`	suffix	`.pptm`
`attachments[].content_type`	equals	`application/vnd.ms-powerpoint`
`attachments[].content_type`	equals	`application/vnd.openxmlformats-officedocument.presentationml.presentation`
`attachments[].file_type`	member	`ppt`
`attachments[].file_type`	member	`pptx`
`strings.iends_with`	suffix	`.accdb`
`strings.iends_with`	suffix	`.mdb`
`attachments[].content_type`	equals	`application/msaccess`
`attachments[].content_type`	equals	`application/x-msaccess`
`attachments[].content_type`	equals	`application/vnd.ms-access`
`file.explode(attachments[])[].scan.yara.matches[].name`	equals	`aes_encryption_keywords`
`strings.ilike`	substring	`*fail`

`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)