CVE-2016-3366 in Office
Summary
by MITRE
Microsoft Outlook 2007 SP3, Outlook 2010 SP2, Outlook 2013 SP1, Outlook 2013 RT SP1, Outlook 2016, and Outlook 2016 for Mac do not properly implement RFC 2046, which allows remote attackers to bypass virus or spam detection via crafted MIME data in an e-mail attachment, aka "Microsoft Office Spoofing Vulnerability."
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Analysis
by VulDB Data Team • 09/16/2022
The Microsoft Outlook spoofing vulnerability identified as CVE-2016-3366 represents a critical implementation flaw in how the email client processes MIME (Multipurpose Internet Mail Extensions) data according to RFC 2046 standards. This vulnerability affects multiple versions of Microsoft Outlook spanning from 2007 through 2016 for both Windows and Mac platforms, creating a widespread attack surface that adversaries can exploit to circumvent security controls. The flaw specifically manifests when Outlook fails to properly validate the content type and structure of email attachments, allowing malicious actors to craft specially formatted email messages that appear legitimate while concealing harmful payloads.
The technical implementation issue stems from Outlook's insufficient validation of MIME content type headers and boundary parameters that define how email attachments should be parsed and displayed. When an email contains a crafted MIME structure with conflicting or malformed content type declarations, the vulnerable Outlook versions may misinterpret the attachment's actual nature, leading to bypass of virus and spam detection mechanisms. This occurs because the email client's parser does not rigorously enforce RFC 2046 compliance, particularly in handling multipart messages where the boundary parameter may be manipulated to obscure the true nature of the attachment. The vulnerability operates at the application layer of the network stack, specifically within the email parsing and rendering components that process incoming messages.
The operational impact of this vulnerability extends beyond simple email spoofing, as it provides attackers with a method to evade security controls that are fundamental to enterprise email protection strategies. Attackers can craft emails that appear to contain benign attachments such as text documents or images while actually delivering malicious payloads that bypass antivirus scanning and spam filtering. This creates a significant risk for organizations that rely on traditional signature-based detection methods, as the malicious content remains undetected during the initial filtering stages. The vulnerability is particularly dangerous in enterprise environments where email is the primary vector for malware delivery and phishing attacks, potentially allowing adversaries to establish initial footholds or escalate privileges through targeted email campaigns.
Organizations affected by this vulnerability should implement immediate mitigations including updating to patched versions of Microsoft Outlook, configuring additional email security layers such as advanced threat protection, and implementing stricter email content filtering rules that validate MIME structures more rigorously. The vulnerability aligns with CWE-20, which describes improper input validation, and maps to ATT&CK technique T1192, "Spearphishing Attachments," where adversaries use malicious attachments to compromise systems. Network administrators should also consider implementing email header analysis tools that can detect malformed MIME structures and deploy sandboxing solutions that analyze suspicious attachments in isolated environments before they reach end users. Regular security awareness training for employees remains crucial as this vulnerability demonstrates the importance of not only technical controls but also human factors in email security.