Service abuse: HelloSign from an unsolicited sender address

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

Detects messages from HelloSign in which the document originates from a newly observed email address. The email address is extracted from across multiple message components, including HTML body templates and email header fields.

Threat classification

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

Category	Values
Attack types	Credential Phishing
Tactics and techniques	Social engineering, Free file host, Evasion

Event coverage

Message attribute
body.html
headers (collection)
headers.auth_summary
headers.hops (collection)
sender.email
type

Rule body MQL

type.inbound
and sender.email.domain.domain == "mail.hellosign.com"
and headers.auth_summary.spf.pass
and headers.auth_summary.dmarc.pass
and (
  // extract the sender out of the body html template
  (
    // if the sender_email is available in the body
    regex.icontains(body.html.raw,
                    '<th class="action-item--action[^\>]+\>\s*[^\<]*\((?P<sender_email>[^\)]+)\).*?</th>'
    )
    // check that the sender email has not been observed previously
    and all(regex.iextract(body.html.raw,
                           '<th class="action-item--action[^\>]+\>\s*[^\<]*\((?P<sender_email>[^\"]+@(?P<sender_domain>[^\"]+))\).*?</th>'
            ),
            .named_groups["sender_domain"] not in $org_domains
            and .named_groups["sender_email"] not in $recipient_emails
            and .named_groups["sender_email"] not in $sender_emails
            and not (
              .named_groups["sender_domain"] not in $free_email_providers
              and .named_groups["sender_domain"] in $recipient_domains
              and .named_groups["sender_domain"] in $sender_domains
            )
    )
  )

  // extract the sender out of header hops if it's there
  or any(headers.hops,
         any(.fields,
             .name == "X-Mailgun-Variables"
             and strings.icontains(.value, 'on_behalf_of_email')
             and all(regex.iextract(.value,
                                    '\"on_behalf_of_email": \"(?P<sender_email>[^\"]+@(?P<sender_domain>[^\"]+))\",'
                     ),
                     .named_groups["sender_domain"] not in $org_domains
                     and .named_groups["sender_email"] not in $recipient_emails
                     and .named_groups["sender_email"] not in $sender_emails
                     and not (
                       .named_groups["sender_domain"] not in $free_email_providers
                       and .named_groups["sender_domain"] in $recipient_domains
                       and .named_groups["sender_domain"] in $sender_domains
                     )
             )
         )
  )

  // extract the sender from the "reply to sender" element withn the body.html.raw
  or (
    regex.icontains(body.html.raw,
                    '<a href="mailto:[^\?]+\?[^\"]+\"[^\>]+\>(?:<img[^\>]+\>)?\s*Reply to sender<\/a>'
    )
    and all(regex.iextract(body.html.raw,
                           '<a href="mailto:(?P<sender_email>[^\?]+@(?P<sender_domain>[^\?]+))\?[^\"]+\"[^\>]+\>(?:<img[^\>]+\>)?\s*Reply to sender<\/a>'
            ),
            .named_groups["sender_domain"] not in $org_domains
            and .named_groups["sender_email"] not in $recipient_emails
            and .named_groups["sender_email"] not in $sender_emails
            and not (
              .named_groups["sender_domain"] not in $free_email_providers
              and .named_groups["sender_domain"] in $recipient_domains
              and .named_groups["sender_domain"] in $sender_domains
            )
    )
  )
)

Detection logic

Scope: inbound message.

inbound message
sender.email.domain.domain is 'mail.hellosign.com'
headers.auth_summary.spf.pass
headers.auth_summary.dmarc.pass
any of:
- all of:
  - body.html.raw matches '<th class="action-item--action[^\\>]+\\>\\s*[^\\<]*\$(?P<sender_email>[^\$]+)\\).*?</th>'
  - all of regex.iextract(body.html.raw) where all hold:
    
    .named_groups['sender_domain'] not in $org_domains
    .named_groups['sender_email'] not in $recipient_emails
    .named_groups['sender_email'] not in $sender_emails
    not:
    
    all of:
    
    .named_groups['sender_domain'] not in $free_email_providers
    .named_groups['sender_domain'] in $recipient_domains
    .named_groups['sender_domain'] in $sender_domains
- any of headers.hops where:
  - any of .fields where all hold:
    
    .name is 'X-Mailgun-Variables'
    .value contains 'on_behalf_of_email'
    all of regex.iextract(.value) where all hold:
    
    .named_groups['sender_domain'] not in $org_domains
    .named_groups['sender_email'] not in $recipient_emails
    .named_groups['sender_email'] not in $sender_emails
    not:
    
    all of:
    
    .named_groups['sender_domain'] not in $free_email_providers
    .named_groups['sender_domain'] in $recipient_domains
    .named_groups['sender_domain'] in $sender_domains
- all of:
  - body.html.raw matches '<a href="mailto:[^\\?]+\\?[^\\"]+\\"[^\\>]+\\>(?:<img[^\\>]+\\>)?\\s*Reply to sender<\\/a>'
  - all of regex.iextract(body.html.raw) where all hold:
    
    .named_groups['sender_domain'] not in $org_domains
    .named_groups['sender_email'] not in $recipient_emails
    .named_groups['sender_email'] not in $sender_emails
    not:
    
    all of:
    
    .named_groups['sender_domain'] not in $free_email_providers
    .named_groups['sender_domain'] in $recipient_domains
    .named_groups['sender_domain'] in $sender_domains

Inspects: body.html.raw, headers.auth_summary.dmarc.pass, headers.auth_summary.spf.pass, headers.hops, headers.hops[].fields, headers.hops[].fields[].name, headers.hops[].fields[].value, sender.email.domain.domain, type.inbound. Sensors: regex.icontains, regex.iextract, strings.icontains. Reference lists: $free_email_providers, $org_domains, $recipient_domains, $recipient_emails, $sender_domains, $sender_emails.

Indicators matched (8)

Field	Match	Value
`sender.email.domain.domain`	equals	`mail.hellosign.com`
`regex.icontains`	regex	`<th class="action-item--action[^\>]+\>\s[^\<]$(?P<sender_email>[^$]+)\).*?</th>`
`regex.iextract`	regex	`<th class="action-item--action[^\>]+\>\s[^\<]$(?P<sender_email>[^\"]+@(?P<sender_domain>[^\"]+))$.*?</th>`
`headers.hops[].fields[].name`	equals	`X-Mailgun-Variables`
`strings.icontains`	substring	`on_behalf_of_email`
`regex.iextract`	regex	`\"on_behalf_of_email": \"(?P<sender_email>[^\"]+@(?P<sender_domain>[^\"]+))\",`
`regex.icontains`	regex	`<a href="mailto:[^\?]+\?[^\"]+\"[^\>]+\>(?:<img[^\>]+\>)?\s*Reply to sender<\/a>`
`regex.iextract`	regex	`<a href="mailto:(?P<sender_email>[^\?]+@(?P<sender_domain>[^\?]+))\?[^\"]+\"[^\>]+\>(?:<img[^\>]+\>)?\s*Reply to sender<\/a>`

`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)