Callback phishing: AOL senders with suspicious HTML template or PDF attachment

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

Detects a specific behavioral pattern from AOL senders using consistent HTML templates and PDF attachment characteristics. The pattern includes particular Yahoo-style formatting with specific font families, and standardized PDF metadata when attachments are present.

Threat classification

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

Category	Values
Attack types	Callback Phishing
Tactics and techniques	Free email provider, Social engineering

Event coverage

Message attribute
attachments (collection)
body.html
headers (collection)
recipients
sender.email
type

Rule body MQL

type.inbound
// sender is aol
and sender.email.domain.root_domain == "aol.com"

// there is a single recipient
and length(recipients.to) == 1
and length(recipients.cc) == 0
and length(recipients.bcc) == 0

// is not a reply
and headers.in_reply_to is null

// this is another way to limit replies but need to account for 
// a behavior where Yahoo/Aol seems to include it's own message ID as a reference
and length(headers.references) == 1

// all message-id values end in mail.yahoo.com, some benign messages use @aol.com
and strings.iends_with(headers.message_id, '@mail.yahoo.com>')

// All are using the legit Yahoo X-Mailer
// this removes AOL sent from iphones and other non AOL client
and strings.ends_with(headers.mailer, 'AolMailNorrin')

// exclude common recipients which interact with aol addresses that are commonly (but not always) benign 
// recipeint exclusions are suggested for other recipients
and (
  // no attachment
  (
    length(attachments) == 0
    // if there are no attachments focus on the HTML template being observed
    and (

      // yahoo div followed by <br>
      // // verdana, helvetica, sans-serif;
      strings.icontains(body.html.raw,
                        "<div class=\"yahoo-style-wrap\" style=\"font-family:verdana, helvetica, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><br>\r\n"
      )
      // // Helvetica Neue, Helvetica, Arial, sans-serif;
      or strings.icontains(body.html.raw,
                           "<div class=\"yahoo-style-wrap\" style=\"font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><br>\r\n"
      )
      // yahoo div followed by doctype 
      // // verdana, helvetica, sans-serif;
      or strings.icontains(body.html.raw,
                           "<div class=\"yahoo-style-wrap\" style=\"font-family:verdana, helvetica, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><!DOCTYPE html>"
      )
      // // Helvetica Neue, Helvetica, Arial, sans-serif;
      or strings.icontains(body.html.raw,
                           "<div class=\"yahoo-style-wrap\" style=\"font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><!DOCTYPE html>"
      )
    )
  )
  or (
    // if there is an attachment, there can be only one
    length(attachments) == 1
    and all(attachments,
            // it's a PDF, 
            .file_type == "pdf"
            and (
              // static content_id value for the attachments
              (
                .content_id == '<@yahoo.com>' or .content_id == '@yahoo.com'
              )
              // created by observed static PDF details
              or (

                // 
                // This rule makes use of a beta feature and is subject to change without notice
                // using the beta feature in custom rules is not suggested until it has been formally released
                // 
                beta.parse_exif(.).creator == "wkhtmltopdf 0.12.6"
                and beta.parse_exif(.).title is null
                and beta.parse_exif(.).producer == 'Qt 4.8.7'
              )
            )
    )
  )
)

Detection logic

Scope: inbound message.

inbound message
sender.email.domain.root_domain is 'aol.com'
length(recipients.to) is 1
length(recipients.cc) is 0
length(recipients.bcc) is 0
headers.in_reply_to is missing
length(headers.references) is 1
headers.message_id ends with '@mail.yahoo.com>'
headers.mailer ends with 'AolMailNorrin'
any of:
- all of:
  - length(attachments) is 0
  - body.html.raw contains any of 4 patterns
    
    <div class=\"yahoo-style-wrap\" style=\"font-family:verdana, helvetica, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><br>\r\n
    <div class=\"yahoo-style-wrap\" style=\"font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><br>\r\n
    <div class=\"yahoo-style-wrap\" style=\"font-family:verdana, helvetica, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><!DOCTYPE html>
    <div class=\"yahoo-style-wrap\" style=\"font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><!DOCTYPE html>
- all of:
  - length(attachments) is 1
  - all of attachments where all hold:
    
    .file_type is 'pdf'
    any of:
    
    any of:
    
    .content_id is '<@yahoo.com>'
    .content_id is '@yahoo.com'
    all of:
    
    beta.parse_exif(.).creator is 'wkhtmltopdf 0.12.6'
    beta.parse_exif(.).title is missing
    beta.parse_exif(.).producer is 'Qt 4.8.7'

Inspects: attachments[].content_id, attachments[].file_type, body.html.raw, headers.in_reply_to, headers.mailer, headers.message_id, headers.references, recipients.bcc, recipients.cc, recipients.to, sender.email.domain.root_domain, type.inbound. Sensors: beta.parse_exif, strings.ends_with, strings.icontains, strings.iends_with.

Indicators matched (10)

Field	Match	Value
`sender.email.domain.root_domain`	equals	`aol.com`
`strings.iends_with`	suffix	`@mail.yahoo.com>`
`strings.ends_with`	suffix	`AolMailNorrin`
`strings.icontains`	substring	`<div class=\"yahoo-style-wrap\" style=\"font-family:verdana, helvetica, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><br>\r\n`
`strings.icontains`	substring	`<div class=\"yahoo-style-wrap\" style=\"font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><br>\r\n`
`strings.icontains`	substring	`<div class=\"yahoo-style-wrap\" style=\"font-family:verdana, helvetica, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><!DOCTYPE html>`
`strings.icontains`	substring	`<div class=\"yahoo-style-wrap\" style=\"font-family:Helvetica Neue, Helvetica, Arial, sans-serif;font-size:18px;\">\r\n<div dir=\"ltr\" data-setdir=\"false\"><!DOCTYPE html>`
`attachments[].file_type`	equals	`pdf`
`attachments[].content_id`	equals	`<@yahoo.com>`
`attachments[].content_id`	equals	`@yahoo.com`

`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)