Attachment: DocX embedded binary

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

This rule is designed to detect sophisticated phishing attacks that deliver binary payloads through MS office open XML files. It identifies malicious documents containing embedded scripts or objects, either encoded in base64 or using specific JavaScript functions like createObjectURL or msSaveOrOpenBlob, which are indicative of attempts to download and execute a binary payload.

Threat classification

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

Category	Values
Attack types	Malware/Ransomware
Tactics and techniques	Evasion

Event coverage

Message attribute
attachments (collection)
type

Rule body MQL

type.inbound
and any(attachments,
        (
          .file_extension in~ $file_extensions_macros
          or .file_extension in~ $file_extensions_common_archives
          or .content_type == "application/zip"
          or (
            .file_extension is null
            and .file_type == "unknown"
            and .content_type == "application/octet-stream"
            and .size < 100000000
          )
        )
        and any(file.explode(.),
                .file_extension in~ (
                  "doc",
                  "docm",
                  "docx",
                  "dot",
                  "dotm",
                  "xls",
                  "xlsx",
                  "xlsm",
                  "xlm",
                  "xlsb",
                  "xlt",
                  "xltm",
                  "ppt",
                  "pptx",
                  "pptm",
                  "ppsm"
                )
                and (
                  any(.flavors.yara, . == "base64_pe")
                  // The malicious file to be downloaded and run with the data URI may not always be portable executable
                  or any(.scan.strings.strings,
                         strings.ilike(., "*.createObjectURL(*)*")
                  )
                  or any(.scan.strings.strings,
                         strings.ilike(., "*.msSaveOrOpenBlob(*)*")
                  )
                )
        )
)

Detection logic

Scope: inbound message.

inbound message
any of attachments where all hold:
- any of:
  - .file_extension in $file_extensions_macros
  - .file_extension in $file_extensions_common_archives
  - .content_type is 'application/zip'
  - all of:
    
    .file_extension is missing
    .file_type is 'unknown'
    .content_type is 'application/octet-stream'
    .size < 100000000
- any of file.explode(.) where all hold:
  - .file_extension in ('doc', 'docm', 'docx', 'dot', 'dotm', 'xls', 'xlsx', 'xlsm', 'xlm', 'xlsb', 'xlt', 'xltm', 'ppt', 'pptx', 'pptm', 'ppsm')
  - any of:
    
    any of .flavors.yara where:
    
    . is 'base64_pe'
    any of .scan.strings.strings where:
    
    . matches '*.createObjectURL(*)*'
    any of .scan.strings.strings where:
    
    . matches '*.msSaveOrOpenBlob(*)*'

Inspects: attachments[].content_type, attachments[].file_extension, attachments[].file_type, attachments[].size, type.inbound. Sensors: file.explode, strings.ilike. Reference lists: $file_extensions_common_archives, $file_extensions_macros.

Indicators matched (22)

Field	Match	Value
`attachments[].content_type`	equals	`application/zip`
`attachments[].file_type`	equals	`unknown`
`attachments[].content_type`	equals	`application/octet-stream`
`file.explode(attachments[])[].file_extension`	member	`doc`
`file.explode(attachments[])[].file_extension`	member	`docm`
`file.explode(attachments[])[].file_extension`	member	`docx`
`file.explode(attachments[])[].file_extension`	member	`dot`
`file.explode(attachments[])[].file_extension`	member	`dotm`
`file.explode(attachments[])[].file_extension`	member	`xls`
`file.explode(attachments[])[].file_extension`	member	`xlsx`
`file.explode(attachments[])[].file_extension`	member	`xlsm`
`file.explode(attachments[])[].file_extension`	member	`xlm`

10 more

`file.explode(attachments[])[].file_extension`	member	`xlsb`
`file.explode(attachments[])[].file_extension`	member	`xlt`
`file.explode(attachments[])[].file_extension`	member	`xltm`
`file.explode(attachments[])[].file_extension`	member	`ppt`
`file.explode(attachments[])[].file_extension`	member	`pptx`
`file.explode(attachments[])[].file_extension`	member	`pptm`
`file.explode(attachments[])[].file_extension`	member	`ppsm`
`file.explode(attachments[])[].flavors.yara[]`	equals	`base64_pe`
`strings.ilike`	substring	`.createObjectURL()*`
`strings.ilike`	substring	`.msSaveOrOpenBlob()*`

`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)