Attachment: EML with embedded Javascript in SVG file

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

Detects incoming messages containing EML attachments with embedded SVG files that contain malicious JavaScript code, including base64-encoded content and potentially harmful event handlers. The rule specifically watches for onload events, location redirects, error handlers, and iframe elements with base64 data URIs.

Threat classification

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

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

Event coverage

Message attribute
attachments (collection)
type

Rule body MQL

type.inbound
and any(attachments,
        (.content_type == "message/rfc822" or .file_extension =~ "eml")
        and (
          any(file.parse_eml(.).attachments,
              .file_extension in~ ("svg", "svgz")
              or .file_type == "svg"
              and (
                (
                  strings.ilike(file.parse_text(.,
                                                encodings=[
                                                  "ascii",
                                                  "utf8",
                                                  "utf16-le"
                                                ]
                                ).text,
                                "*onload*",
                                "*window.location.href*",
                                "*onerror*",
                                "*CDATA*",
                                "*<script*",
                                "*</script*",
                                "*atob*",
                                '*location.assign*',
                                '*decodeURIComponent*'
                  )
                  or regex.icontains(file.parse_text(.,
                                                     encodings=[
                                                       "ascii",
                                                       "utf8",
                                                       "utf16-le"
                                                     ]
                                     ).text,
                                     '<iframe[^\>]+src\s*=\s*\"data:[^\;]+;base64,'
                  )
                  or any(beta.scan_base64(file.parse_text(.).text,
                                          encodings=[
                                            "ascii",
                                            "utf8",
                                            "utf16-le"
                                          ]
                         ),
                         strings.ilike(.,
                                       "*onload*",
                                       "*window.location.href*",
                                       "*onerror*",
                                       "*CDATA*",
                                       "*<script*",
                                       "*</script*",
                                       "*atob*",
                                       '*location.assign*',
                                       '*decodeURIComponent*'
                         )
                  )
                )
                or (
                  (
                    .file_extension in $file_extensions_common_archives
                    or .file_type == "gz"
                    or .content_type == "application/x-gzip"
                  )
                  and any(file.explode(.),
                          (
                            .file_extension in~ ("svg", "svgz")
                            or .flavors.mime == "image/svg+xml"
                          )
                          and any(.scan.strings.strings,
                                  strings.ilike(.,
                                                "*onload*",
                                                "*window.location.href*",
                                                "*onerror*",
                                                "*CDATA*",
                                                "*<script*",
                                                "*</script*",
                                                "*atob*",
                                                "*location.assign*",
                                                "*decodeURIComponent*"
                                  )
                          )
                  )
                )
              )
          )
        )
)

Detection logic

Scope: inbound message.

inbound message
any of attachments where all hold:
- any of:
  - .content_type is 'message/rfc822'
  - .file_extension is 'eml'
- any of file.parse_eml(.).attachments where any holds:
  - .file_extension in ('svg', 'svgz')
  - all of:
    
    .file_type is 'svg'
    any of:
    
    any of:
    
    file.parse_text(., encodings=['ascii', 'utf8', 'utf16-le']).text matches any of 9 patterns
    
    *onload*
    *window.location.href*
    *onerror*
    *CDATA*
    *<script*
    *</script*
    *atob*
    *location.assign*
    *decodeURIComponent*
    file.parse_text(., encodings=['ascii', 'utf8', 'utf16-le']).text matches '<iframe[^\\>]+src\\s*=\\s*\\"data:[^\\;]+;base64,'
    any of beta.scan_base64(...) where:
    
    . matches any of 9 patterns
    
    *onload*
    *window.location.href*
    *onerror*
    *CDATA*
    *<script*
    *</script*
    *atob*
    *location.assign*
    *decodeURIComponent*
    all of:
    
    any of:
    
    .file_extension in $file_extensions_common_archives
    .file_type is 'gz'
    .content_type is 'application/x-gzip'
    any of file.explode(.) where all hold:
    
    any of:
    
    .file_extension in ('svg', 'svgz')
    .flavors.mime is 'image/svg+xml'
    any of .scan.strings.strings where:
    
    . matches any of 9 patterns
    
    *onload*
    *window.location.href*
    *onerror*
    *CDATA*
    *<script*
    *</script*
    *atob*
    *location.assign*
    *decodeURIComponent*

Inspects: attachments[].content_type, attachments[].file_extension, type.inbound. Sensors: beta.scan_base64, file.explode, file.parse_eml, file.parse_text, regex.icontains, strings.ilike. Reference lists: $file_extensions_common_archives.

Indicators matched (20)

Field	Match	Value
`attachments[].content_type`	equals	`message/rfc822`
`attachments[].file_extension`	equals	`eml`
`file.parse_eml(attachments[]).attachments[].file_extension`	member	`svg`
`file.parse_eml(attachments[]).attachments[].file_extension`	member	`svgz`
`file.parse_eml(attachments[]).attachments[].file_type`	equals	`svg`
`strings.ilike`	substring	`onload`
`strings.ilike`	substring	`window.location.href`
`strings.ilike`	substring	`onerror`
`strings.ilike`	substring	`CDATA`
`strings.ilike`	substring	`<script`
`strings.ilike`	substring	`</script`
`strings.ilike`	substring	`atob`

8 more

`strings.ilike`	substring	`location.assign`
`strings.ilike`	substring	`decodeURIComponent`
`regex.icontains`	regex	`<iframe[^\>]+src\s=\s\"data:[^\;]+;base64,`
`file.parse_eml(attachments[]).attachments[].file_type`	equals	`gz`
`file.parse_eml(attachments[]).attachments[].content_type`	equals	`application/x-gzip`
`file.explode(file.parse_eml(attachments[]).attachments[])[].file_extension`	member	`svg`
`file.explode(file.parse_eml(attachments[]).attachments[])[].file_extension`	member	`svgz`
`file.explode(file.parse_eml(attachments[]).attachments[])[].flavors.mime`	equals	`image/svg+xml`

`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)