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ATT&CK Technique

HTML Smuggling

T1027.006 · stealth

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. 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.

LinuxmacOSWindows

Actors Using This

1
russiaAPT29

Atomic Tests

1
Executable Atomic Red Team test cases for exercising this technique in a lab. Copy a command, run it on the listed platform, confirm your detections fire.
powershellwindowsHTML Smuggling Remote Payload
The HTML file will download an ISO file from [T1553.005](https://github.com/redcanaryco/atomic-red-team/blob/d0dad62dbcae9c60c519368e82c196a3db577055/atomics/T1553.005/bin/FeelTheBurn.iso) without user interaction. The HTML file is based off of the work from [Stan Hegt](https://outflank.nl/blog/2018/08/14/html-smuggling-explained/)
& "PathToAtomicsFolder\T1027.006\bin\T1027_006_remote.html"

Mitigations

1
MITRE ATT&CK mitigations - vendor-agnostic guidance for reducing exposure to this technique.
M1048Application 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.

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.

Detection Coverage

0/6 layers
Coverage across standard detection surfaces. Rows marked none have no rule of that type mapped. Some are real blind spots worth closing; others are simply not applicable to this technique (e.g. YARA matches malware files, not network behaviour).
Behavioral / log (Sigma) none
Analytics (MITRE CAR) none
Runtime / container (Falco) none
File / malware (YARA) none
Network (Suricata/Snort) none
Vuln scan (Nuclei) none
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