headers_arc_seal_validation_failure.yml

name: "Headers: ARC-Seal validation failure after initial success"
description: "Detects messages with ARC-Seal headers showing an initial validation success (cv=none) followed by a validation failure (cv=fail), indicating potential authentication bypass or message tampering."
type: "rule"
severity: "medium"
source: |
  type.inbound
  and any(headers.hops,
          any(filter(.fields, .name == "ARC-Seal" and .value != ""),
              strings.starts_with(.value, "i=1")
              and strings.contains(.value, "cv=none;")
          )
  )
  and any(headers.hops,
          any(filter(.fields, .name == "ARC-Seal" and .value != ""),
              regex.extract(.value, 'i=(?P<number>\d);')[0].named_groups["number"] >= "2"
              and strings.contains(.value, "cv=fail;")
          )
  )
tags:
  - "Attack surface reduction"
attack_types:
  - "BEC/Fraud"
  - "Credential Phishing"
  - "Malware/Ransomware"
tactics_and_techniques:
  - "Evasion"
  - "Spoofing"
detection_methods:
  - "Header analysis"
id: "723efe9f-8e22-5e39-befb-67c1af1f5320"