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]
| ID | Name | Description |
|---|---|---|
| G0016 | APT29 |
APT29 has embedded an ISO file within an HTML attachment that contained JavaScript code to initiate malware execution.[5] |
| S0634 | EnvyScout |
EnvyScout contains JavaScript code that can extract an encoded blob from its HTML body and write it to disk.[6] |
| S0650 | QakBot |
QakBot has been delivered in ZIP files via HTML smuggling.[7][8] |
| ID | Mitigation | Description |
|---|---|---|
| M1048 | Application Isolation and Sandboxing |
Browser sandboxes can be used to mitigate some of the impact of exploitation, but sandbox escapes may still exist. |
| ID | Data Source | Data Component | Detects |
|---|---|---|---|
| DS0022 | File | File Creation |
Monitor for newly constructed files via JavaScript, developing rules for the different variants, with a combination of different encoding and/or encryption schemes, may be very challenging. Consider monitoring files downloaded from the Internet, possibly by HTML Smuggling, for suspicious activities. Data and events should not be viewed in isolation, but as part of a chain of behavior that could lead to other activities. |