This rule detects potential HTML smuggling attacks through the analysis of inbound email attachments. It checks for HTML files (with extensions such as .html, .htm, .shtml, .dhtml, .xhtml) or attachments with an unknown file type that are categorized as application/octet-stream. The rule specifically looks for high entropy values of 5 or greater in the content, as high entropy can indicate obfuscation techniques often used in malicious payloads. It also triggers on certain strings, such as 'body onload', which are indicative of tactics to execute scripts when the HTML document is loaded. The detection logic employs regex matching to identify base64 encoded images and looks for keywords associated with phishing or malware, like 'document pass'. Hence, this rule is specifically aimed at identifying sophisticated phishing attempts and malware distribution mechanisms through HTML attachments, assisting organizations in mitigating threats related to credential theft and malware deployment.