diff --git a/detection-rules/attachment_ics_non-gregorian.yml b/detection-rules/attachment_ics_non-gregorian.yml
new file mode 100644
index 00000000000..21b3b7e6acd
--- /dev/null
+++ b/detection-rules/attachment_ics_non-gregorian.yml
@@ -0,0 +1,21 @@
+name: "Attachment: ICS file with non-Gregorian calendar scale"
+description: "Detects ICS calendar attachments that use a non-standard calendar scale other than GREGORIAN, which may indicate malicious calendar files attempting to exploit calendar parsing vulnerabilities or bypass security filters."
+type: "rule"
+severity: "medium"
+source: |
+  type.inbound
+  and any(attachments,
+          (
+            .file_extension in~ ('ics')
+            or .content_type in ("application/ics", "text/calendar")
+          )
+          and not strings.icontains(beta.file.parse_ics(.).scale, 'GREGORIAN')
+  )
+attack_types:
+  - "Credential Phishing"
+tactics_and_techniques:
+  - "Evasion"
+detection_methods:
+  - "File analysis"
+  - "Content analysis"
+id: "9315bbf5-c710-5ead-8a8e-5c7f8fb118dc"