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T1027.006 HTML Smuggling

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.31

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 text/plain and/or text/html. 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 msSaveBlob.3214

Item Value
ID T1027.006
Sub-techniques T1027.001, T1027.002, T1027.003, T1027.004, T1027.005, T1027.006, T1027.007, T1027.008, T1027.009, T1027.010, T1027.011
Tactics TA0005
Platforms Linux, Windows, macOS
Version 1.0
Created 20 May 2021
Last Modified 19 May 2022

Procedure Examples

ID Name Description
G0016 APT29 APT29 has embedded an ISO file within an HTML attachment that contained JavaScript code to initiate malware execution.8
S0634 EnvyScout EnvyScout contains JavaScript code that can extract an encoded blob from its HTML body and write it to disk.5
S0650 QakBot QakBot has been delivered in ZIP files via HTML smuggling.67


ID Data Source Data Component
DS0022 File File Creation