Attachment: Encrypted PDF with credential theft body

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

Attached PDF is encrypted, and email body contains credential theft language. Seen in-the-wild impersonating e-fax services.

Threat classification

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

Category	Values
Attack types	Credential Phishing
Tactics and techniques	Encryption, Evasion, PDF, Social engineering

Event coverage

Message attribute
attachments (collection)
body
body.current_thread
body.previous_threads (collection)
headers (collection)
headers.auth_summary
recipients
recipients.to (collection)
sender.email
subject
type

Rule body MQL

type.inbound
and any(filter(attachments, .file_type == "pdf"),
        any(file.explode(.),
            any(.scan.exiftool.fields, .key == "Encryption")
            or (
              .scan.entropy.entropy > 7
              and any(.scan.strings.strings, strings.icontains(., "/Encrypt"))
            )
        )
        // Encrypted PDFs do not have child nodes with any data
        and all(filter(file.explode(.), .depth > 0), .size == 0)
)
and (
  any(ml.nlu_classifier(body.current_thread.text).intents,
      .name == "cred_theft" and .confidence in ("medium", "high")
  )
  or any(ml.nlu_classifier(beta.ocr(file.message_screenshot()).text).intents,
         .name == "cred_theft" and .confidence in ("medium", "high")
  )
  or regex.icontains(body.current_thread.text,
                     'PDF\s*(?:Access|Preview|Unlock|Decrypt|passcode)',
                     '(Access|Preview|Unlock|Decrypt|Pass)\s*(?:word|code)\s*(?:\S+\s+){0,3}PDF\s*is?\s*:',
                     'This\s+(?:file|document|pdf)\s+is\s+(?:password[-\s]?)\s+protected\.\s*The\s+password\s+is\s*:?',
                     '(?:Access|Preview|Unlock|Decrypt)\s+(?:\S+\s+){0,3}PDF(?:\S+\s+){0,3}pass(?:word|code)'
  )
  or (
    (
      length(body.current_thread.text) <= 10
      or (body.current_thread.text is null)
    )
    and any(body.previous_threads,
            regex.icontains(.text,
                            'PDF\s*(?:Access|Preview|Unlock|Decrypt|passcode)',
                            '(Access|Preview|Unlock|Decrypt|Pass)\s*(?:word|code)\s*(?:\S+\s+){0,3}PDF\s*is?\s*:',
                            'This\s+(?:file|document|pdf)\s+is\s+(?:password[-\s]?)\s+protected\.\s*The\s+password\s+is\s*:?',
                            '(?:Access|Preview|Unlock|Decrypt)\s+(?:\S+\s+){0,3}PDF(?:\S+\s+){0,3}pass(?:word|code)'
            )
    )
  )
)
// not forwards/replies
and not (
  (length(headers.references) > 0 or headers.in_reply_to is not null)
  and (subject.is_forward or subject.is_reply)
  and length(body.previous_threads) >= 1
)
and (
  (
    profile.by_sender_email().prevalence in ("new", "outlier")
    and not profile.by_sender_email().solicited
  )
  or (
    profile.by_sender_email().any_messages_malicious_or_spam
    and not profile.by_sender_email().any_messages_benign
  )
  or (
    length(recipients.to) == 0
    or all(recipients.to,
           strings.ilike(.display_name, "undisclosed?recipients")
    )
  )
  or (
    length(recipients.to) == 1
    and any(recipients.to, .email.email == sender.email.email)
  )
)
// negate highly trusted sender domains unless they fail DMARC authentication
and (
  (
    sender.email.domain.root_domain in $high_trust_sender_root_domains
    and not headers.auth_summary.dmarc.pass
  )
  or sender.email.domain.root_domain not in $high_trust_sender_root_domains
)

Detection logic

Scope: inbound message.

Attached PDF is encrypted, and email body contains credential theft language. Seen in-the-wild impersonating e-fax services.

inbound message
any of filter(attachments) where all hold:
- any of file.explode(.) where any holds:
  - any of .scan.exiftool.fields where:
    
    .key is 'Encryption'
  - all of:
    
    .scan.entropy.entropy > 7
    any of .scan.strings.strings where:
    
    . contains '/Encrypt'
- all of filter(...) where:
  - .size is 0
any of:
- any of ml.nlu_classifier(body.current_thread.text).intents where all hold:
  - .name is 'cred_theft'
  - .confidence in ('medium', 'high')
- any of ml.nlu_classifier(beta.ocr(file.message_screenshot()).text).intents where all hold:
  - .name is 'cred_theft'
  - .confidence in ('medium', 'high')
- body.current_thread.text matches any of 4 patterns
  - PDF\s*(?:Access|Preview|Unlock|Decrypt|passcode)
  - (Access|Preview|Unlock|Decrypt|Pass)\s*(?:word|code)\s*(?:\S+\s+){0,3}PDF\s*is?\s*:
  - This\s+(?:file|document|pdf)\s+is\s+(?:password[-\s]?)\s+protected\.\s*The\s+password\s+is\s*:?
  - (?:Access|Preview|Unlock|Decrypt)\s+(?:\S+\s+){0,3}PDF(?:\S+\s+){0,3}pass(?:word|code)
- all of:
  - any of:
    
    length(body.current_thread.text) ≤ 10
    body.current_thread.text is missing
  - any of body.previous_threads where:
    
    .text matches any of 4 patterns
    
    PDF\s*(?:Access|Preview|Unlock|Decrypt|passcode)
    (Access|Preview|Unlock|Decrypt|Pass)\s*(?:word|code)\s*(?:\S+\s+){0,3}PDF\s*is?\s*:
    This\s+(?:file|document|pdf)\s+is\s+(?:password[-\s]?)\s+protected\.\s*The\s+password\s+is\s*:?
    (?:Access|Preview|Unlock|Decrypt)\s+(?:\S+\s+){0,3}PDF(?:\S+\s+){0,3}pass(?:word|code)
not:
- all of:
  - any of:
    
    length(headers.references) > 0
    headers.in_reply_to is set
  - any of:
    
    subject.is_forward
    subject.is_reply
  - length(body.previous_threads) ≥ 1
any of:
- all of:
  - profile.by_sender_email().prevalence in ('new', 'outlier')
  - not:
    
    profile.by_sender_email().solicited
- all of:
  - profile.by_sender_email().any_messages_malicious_or_spam
  - not:
    
    profile.by_sender_email().any_messages_benign
- any of:
  - length(recipients.to) is 0
  - all of recipients.to where:
    
    .display_name matches 'undisclosed?recipients'
- all of:
  - length(recipients.to) is 1
  - any of recipients.to where:
    
    .email.email is sender.email.email
any of:
- all of:
  - sender.email.domain.root_domain in $high_trust_sender_root_domains
  - not:
    
    headers.auth_summary.dmarc.pass
- sender.email.domain.root_domain not in $high_trust_sender_root_domains

Inspects: attachments[].file_type, body.current_thread.text, body.previous_threads, body.previous_threads[].text, headers.auth_summary.dmarc.pass, headers.in_reply_to, headers.references, recipients.to, recipients.to[].display_name, recipients.to[].email.email, sender.email.domain.root_domain, sender.email.email, subject.is_forward, subject.is_reply, type.inbound. Sensors: beta.ocr, file.explode, file.message_screenshot, ml.nlu_classifier, profile.by_sender_email, regex.icontains, strings.icontains, strings.ilike. Reference lists: $high_trust_sender_root_domains.

Indicators matched (14)

Field	Match	Value
`attachments[].file_type`	equals	`pdf`
`file.explode(filter(attachments)[])[].scan.exiftool.fields[].key`	equals	`Encryption`
`strings.icontains`	substring	`/Encrypt`
`ml.nlu_classifier(body.current_thread.text).intents[].name`	equals	`cred_theft`
`ml.nlu_classifier(body.current_thread.text).intents[].confidence`	member	`medium`
`ml.nlu_classifier(body.current_thread.text).intents[].confidence`	member	`high`
`ml.nlu_classifier(beta.ocr(file.message_screenshot()).text).intents[].name`	equals	`cred_theft`
`ml.nlu_classifier(beta.ocr(file.message_screenshot()).text).intents[].confidence`	member	`medium`
`ml.nlu_classifier(beta.ocr(file.message_screenshot()).text).intents[].confidence`	member	`high`
`regex.icontains`	regex	`PDF\s*(?:Access\|Preview\|Unlock\|Decrypt\|passcode)`
`regex.icontains`	regex	`(Access\|Preview\|Unlock\|Decrypt\|Pass)\s(?:word\|code)\s(?:\S+\s+){0,3}PDF\sis?\s:`
`regex.icontains`	regex	`This\s+(?:file\|document\|pdf)\s+is\s+(?:password[-\s]?)\s+protected\.\sThe\s+password\s+is\s:?`

2 more

`regex.icontains`	regex	`(?:Access\|Preview\|Unlock\|Decrypt)\s+(?:\S+\s+){0,3}PDF(?:\S+\s+){0,3}pass(?:word\|code)`
`strings.ilike`	substring	`undisclosed?recipients`

`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)