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Attack Patterns T1027.006 — HTML Smuggling
T1027.006

HTML Smuggling

Stealth
TLP:CLEAR

Description

Adversaries may smuggle data and files past content filters by hiding malicious payloads inside of seemingly benign HTML files. HTML documents can store large binary objects known as JavaScript Blobs (immutable data that represents raw bytes) that can later be constructed into file-like objects. Data may also be stored in Data URLs, which enable embedding media type or MIME files inline of HTML documents. HTML5 also introduced a download attribute that may be used to initiate file downloads.(Citation: HTML Smuggling Menlo Security 2020)(Citation: Outlflank HTML Smuggling 2018) Adversaries may deliver payloads to victims that bypass security controls through HTML Smuggling by abusing JavaScript Blobs and/or HTML5 download attributes. Security controls such as web content filters may not identify smuggled malicious files inside of HTML/JS files, as the content may be based on typically benign MIME types such as <code>text/plain</code> and/or <code>text/html</code>. Malicious files or data can be obfuscated and hidden inside of HTML files through Data URLs and/or JavaScript Blobs and can be deobfuscated when they reach the victim (i.e. Deobfuscate/Decode Files or Information), potentially bypassing content filters. For example, JavaScript Blobs can be abused to dynamically generate malicious files in the victim machine and may be dropped to disk by abusing JavaScript functions such as <code>msSaveBlob</code>.(Citation: HTML Smuggling Menlo Security 2020)(Citation: MSTIC NOBELIUM May 2021)(Citation: Outlflank HTML Smuggling 2018)(Citation: nccgroup Smuggling HTA 2017)

MITRE ATT&CK Detection Strategies
1

DET0313 Detection Strategy for HTML Smuggling via JavaScript Blob + Dynamic File Drop
AN0872 Windows

Detection of browser-based or email client-driven file creation (often from temp directories) following navigation to or execution of HTML files containing JavaScript Blob APIs or base64 Data URLs, with follow-on execution of the dropped payload. Leveraging Sysmon EventID 15 to inspect Zone.Identifier ADS for HostUrl/ReferrerUrl indicators (e.g., HostUrl=about:internet). Optional: absence of a large HTTP download record for the same URL/client in proxy logs (suggests local assembly)

WinEventLog:Sysmon WinEventLog:Sysmon EDR:detection WinEventLog:Sysmon Network Traffic
AN0873 Linux

Detection of browser-based downloads from HTML sources that trigger file creation in temp or user directories followed by execution of new files within short timeframes and suspicious parent-child lineage.

auditd:SYSCALL linux:osquery
AN0874 macOS

Detection of HTML-based downloads via Safari/Chrome that create obfuscated files (e.g., .zip, .app, .js) in user directories and are followed by suspicious executions from preview or launch services.

macos:unifiedlog macos:osquery gatekeeper/quarantine database

MITRE ATT&CK Mitigations
1

M1048

Application Isolation and Sandboxing

Application Isolation and Sandboxing refers to the technique of restricting the execution of code to a controlled and isolated environment (e.g., a virtual environment, container, or sandbox). This method prevents potentially malicious code from affecting the rest of the system or network by limiting access to sensitive resources and critical operations. The goal is to contain threats and minimize their impact. This mitigation can be implemented through the following measures: Browser Sandboxing: - Use Case: Implement browser sandboxing to isolate untrusted web content and prevent malicious web pages or scripts from accessing sensitive system resources or initiating unauthorized downloads. - Implementation: Use browsers with built-in sandboxing features (e.g., Google Chrome, Microsoft Edge) or deploy enhanced browser security frameworks that limit the execution scope of active content. Consider controls that monitor or restrict script-based file generation and downloads commonly abused in evasion techniques like HTML smuggling. Application Virtualization: - Use Case: Deploy critical or high-risk applications in a virtualized environment to ensure any compromise does not affect the host system. - Implementation: Use application virtualization platforms to run applications in isolated environments. Email Attachment Sandboxing: - Use Case: Route email attachments to a sandbox environment to detect and block malware before delivering emails to end-users. - Implementation: Integrate security solutions with sandbox capabilities to analyze email attachments. Endpoint Sandboxing: - Use Case: Run all downloaded files and applications in a restricted environment to monitor their behavior for malicious activity. - Implementation: Use endpoint protection tools for sandboxing at the endpoint level.

Details

Platforms
Linux
Macos
Windows
Added
May 2, 2026
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