Obfuscated Files or Information: HTML Smuggling
- ID
- T1027.006
- Tactics
- TA0005 Stealth
- Sub-technique of
- T1027 Obfuscated Files or Information
- Platforms
- Linux, macOS, Windows
- Data sources
- File Creation, Process Creation, File Metadata, Network Traffic Content
- Source
- attack.mitre.org
- MITRE last modified
- 12 May 2026
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.[1][2]
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.[1][3][2][4]
Detection
- DET0313 Detection Strategy for HTML Smuggling via JavaScript Blob + Dynamic File Drop
Data components: File Creation, Process Creation, File Metadata, Network Traffic Content
Mitigations (1)
- M1048 Application Isolation and Sandboxing
Use Browser Extensions or Built-in Security Tools that:
- Monitor JavaScript API calls such as
Blob,URL.createObjectURL,andmsSaveOrOpenBlob
- Intercept and analyze HTML5downloadattributes for suspicious payload generation
- Alert or block behaviors that match known HTML smuggling patterns (e.g., blob-to-disk payload construction)Apply Content Security Policy (CSP) headers to:
- Restrict inline JavaScript and dynamic script generation
- Disallow downloads from unauthorized sources or blob URIs
- Prevent cross-origin resource sharing (CORS) abuse commonly used in smuggling chainsEnable or enforce enterprise browser security controls, such as:
- Endpoint's Network Protection and Attack Surface Reduction (ASR) rules, which can block Office and browser processes from creating child processes or writing to disk in suspicious ways
- Google Chrome Enterprise Policies, which can control file download behavior, restrict extensions, and isolate risky browsing environmentsDeploy browser sandboxing solutions that can isolate JavaScript execution environments and enforce behavioral policy restrictions