Detection rules › Sublime MQL
Attachment: RFC822 containing suspicious file sharing language with links from untrusted sender
This rule identifies messages with an RFC822 attachment contains language indicative of suspicious file-sharing activity. It checks both the original sender and the nested sender against highly trusted domains. The original message is unsolicited, and has not been previously flagged as a false positive.
Threat classification
Sublime's own taxonomy (not MITRE ATT&CK).
| Category | Values |
|---|---|
| Attack types | Credential Phishing |
| Tactics and techniques | Evasion, Social engineering |
Event coverage
Rule body MQL
type.inbound
and any(attachments,
.file_type == "unknown"
and (.content_type == "message/rfc822" or .file_extension in ('eml'))
and regex.contains(file.parse_eml(.).subject.subject,
'(shared.{0,30}with you|View Document)'
)
and 0 < length(file.parse_eml(.).body.links) < 10
and file.parse_eml(.).sender.email.email not in $recipient_emails
// exclude bounce backs & read receipts
and not strings.like(file.parse_eml(.).sender.email.local_part,
"*postmaster*",
"*mailer-daemon*",
"*administrator*"
)
and not regex.imatch(file.parse_eml(.).subject.subject,
"(undeliverable|read:).*"
)
and not any(file.parse_eml(.).attachments,
.content_type == "message/delivery-status"
)
// negate highly trusted sender domains in the nested eml unless they fail DMARC
and (
(
file.parse_eml(.).sender.email.domain.root_domain in $high_trust_sender_root_domains
and (
any(distinct(file.parse_eml(.).headers.hops,
.authentication_results.dmarc is not null
),
strings.ilike(.authentication_results.dmarc, "*fail")
)
)
)
or file.parse_eml(.).sender.email.domain.root_domain not in $high_trust_sender_root_domains
)
// negate org domains that passed dmarc
and not file.parse_eml(.).sender.email.domain.root_domain in $org_domains
)
// negate highly trusted sender domains unless they fail DMARC authentication
and (
(
sender.email.domain.root_domain in $high_trust_sender_root_domains
and (
any(distinct(headers.hops, .authentication_results.dmarc is not null),
strings.ilike(.authentication_results.dmarc, "*fail")
)
)
)
or sender.email.domain.root_domain not in $high_trust_sender_root_domains
)
and (
not profile.by_sender().solicited
or (
profile.by_sender().any_messages_malicious_or_spam
and not profile.by_sender().any_messages_benign
)
)
and not profile.by_sender().any_messages_benign
Detection logic
Scope: inbound message.
This rule identifies messages with an RFC822 attachment contains language indicative of suspicious file-sharing activity. It checks both the original sender and the nested sender against highly trusted domains. The original message is unsolicited, and has not been previously flagged as a false positive.
- inbound message
any of
attachmentswhere all hold:- .file_type is 'unknown'
any of:
- .content_type is 'message/rfc822'
- .file_extension in ('eml')
- file.parse_eml(.).subject.subject matches '(shared.{0,30}with you|View Document)'
all of:
- length(file.parse_eml(.).body.links) > 0
- length(file.parse_eml(.).body.links) < 10
- file.parse_eml(.).sender.email.email not in $recipient_emails
not:
file.parse_eml(.).sender.email.local_part matches any of 3 patterns
*postmaster**mailer-daemon**administrator*
not:
- file.parse_eml(.).subject.subject matches '(undeliverable|read:).*'
not:
any of
file.parse_eml(.).attachmentswhere:- .content_type is 'message/delivery-status'
any of:
all of:
- file.parse_eml(.).sender.email.domain.root_domain in $high_trust_sender_root_domains
any of
distinct(...)where:- .authentication_results.dmarc matches '*fail'
- file.parse_eml(.).sender.email.domain.root_domain not in $high_trust_sender_root_domains
not:
- file.parse_eml(.).sender.email.domain.root_domain in $org_domains
any of:
all of:
- sender.email.domain.root_domain in $high_trust_sender_root_domains
any of
distinct(headers.hops)where:- .authentication_results.dmarc matches '*fail'
- sender.email.domain.root_domain not in $high_trust_sender_root_domains
any of:
not:
- profile.by_sender().solicited
all of:
- profile.by_sender().any_messages_malicious_or_spam
not:
- profile.by_sender().any_messages_benign
not:
- profile.by_sender().any_messages_benign
Inspects: attachments[].content_type, attachments[].file_extension, attachments[].file_type, headers.hops, headers.hops[].authentication_results.dmarc, sender.email.domain.root_domain, type.inbound. Sensors: file.parse_eml, profile.by_sender, regex.contains, regex.imatch, strings.ilike, strings.like. Reference lists: $high_trust_sender_root_domains, $org_domains, $recipient_emails.
Indicators matched (10)
| Field | Match | Value |
|---|---|---|
attachments[].file_type | equals | unknown |
attachments[].content_type | equals | message/rfc822 |
attachments[].file_extension | member | eml |
regex.contains | regex | (shared.{0,30}with you|View Document) |
strings.like | substring | *postmaster* |
strings.like | substring | *mailer-daemon* |
strings.like | substring | *administrator* |
regex.imatch | regex | (undeliverable|read:).* |
file.parse_eml(attachments[]).attachments[].content_type | equals | message/delivery-status |
strings.ilike | substring | *fail |