New Rule: Credential Phishing: Fake Password Expiration

credential_phishing_fake_password_expiration.yml

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+name: "Credential Phishing: Fake Password Expiration from New and Unsolicited sender"
+description: "This rule looks for password expiration verbiage in the subject and body. It requires between 1 and 9 links in the body. The rule uses NLU in addition to statically specified term anchors. High trust senders are also negated. "
+type: "rule"
+severity: "medium"
+source: |
+  type.inbound
+
+  // no attachments
+  and length(attachments) == 0
+
+  // body contains expire, expiration, loose, lose 
+  and regex.icontains(body.current_thread.text, '(expir(e)?ation|lo(o)?se)')
+
+  // subject or body contains account or access
+  and any([subject.subject, body.current_thread.text],
+          regex.icontains(body.current_thread.text, "account|access")
+  )
+
+  // subject or body must contains password
+  and any([subject.subject, body.current_thread.text],
+          regex.icontains(body.current_thread.text, '\bpassword\b')
+  )
+  and any(ml.nlu_classifier(body.current_thread.text).intents,
+          .name == "cred_theft" and .confidence == "high"
+  )
+
+  // sender is new and unsolicited and no previous malicious or spam flaggs
+  and (
+    (
+      profile.by_sender().prevalence in ("new", "outlier")
+      and not profile.by_sender().solicited
+    )
+    or profile.by_sender().any_messages_malicious_or_spam
+  )
+
+  // no false positives
+  and not profile.by_sender().any_false_positives
+
+  // 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
+  )
+
+  // body length between 600 and 2000
+  and 600 < length(body.current_thread.text) < 2000
+
+  // not a reply
+  and (
+    length(headers.references) == 0
+    or not any(headers.hops, any(.fields, strings.ilike(.name, "In-Reply-To")))
+  )
+
+attack_types:
+  - "Credential Phishing"
+tactics_and_techniques:
+  - "Social engineering"
+detection_methods:
+  - "Content analysis"
+  - "Natural Language Understanding"
+  - "Sender analysis"