Attachment: HTML smuggling with fromCharCode and other signals

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

Recursively scans files and archives to detect HTML smuggling techniques.

Threat classification

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

Category	Values
Attack types	Credential Phishing, Malware/Ransomware
Tactics and techniques	Evasion, HTML smuggling, Scripting

Event coverage

Message attribute
attachments (collection)
type

Rule body MQL

type.inbound
and any(attachments,
        (
          .file_extension in~ ("html", "htm", "shtml", "dhtml")
          or .file_extension in~ $file_extensions_common_archives
          or .file_type == "html"
        )
        and any(file.explode(.),
                length(.scan.javascript.identifiers) < 100
                and "location" in .scan.javascript.identifiers
                and "charCodeAt" in .scan.javascript.identifiers
                and "fromCharCode" in .scan.javascript.identifiers
                and "indexOf" in .scan.javascript.identifiers
                and "try" in .scan.javascript.keywords
                and "catch" in .scan.javascript.keywords
        )
)

Detection logic

Scope: inbound message.

Recursively scans files and archives to detect HTML smuggling techniques.

inbound message
any of attachments where all hold:
- any of:
  - .file_extension in ('html', 'htm', 'shtml', 'dhtml')
  - .file_extension in $file_extensions_common_archives
  - .file_type is 'html'
- any of file.explode(.) where all hold:
  - length(.scan.javascript.identifiers) < 100
  - .scan.javascript.identifiers contains 'location'
  - .scan.javascript.identifiers contains 'charCodeAt'
  - .scan.javascript.identifiers contains 'fromCharCode'
  - .scan.javascript.identifiers contains 'indexOf'
  - .scan.javascript.keywords contains 'try'
  - .scan.javascript.keywords contains 'catch'

Inspects: attachments[].file_extension, attachments[].file_type, type.inbound. Sensors: file.explode. Reference lists: $file_extensions_common_archives.

Indicators matched (11)

Field	Match	Value
`attachments[].file_extension`	member	`html`
`attachments[].file_extension`	member	`htm`
`attachments[].file_extension`	member	`shtml`
`attachments[].file_extension`	member	`dhtml`
`attachments[].file_type`	equals	`html`
`file.explode(attachments[])[].scan.javascript.identifiers`	contains	`location`
`file.explode(attachments[])[].scan.javascript.identifiers`	contains	`charCodeAt`
`file.explode(attachments[])[].scan.javascript.identifiers`	contains	`fromCharCode`
`file.explode(attachments[])[].scan.javascript.identifiers`	contains	`indexOf`
`file.explode(attachments[])[].scan.javascript.keywords`	contains	`try`
`file.explode(attachments[])[].scan.javascript.keywords`	contains	`catch`

`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)