Detect HTML Smuggling in CrowdStrike LogScale
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 that can later be constructed into file-like objects. Data may also be stored in Data URLs, enabling embedding media type or MIME files inline of HTML documents. HTML5 introduced a download attribute that may be used to initiate file downloads. Adversaries deliver payloads that bypass security controls through HTML Smuggling by abusing JavaScript Blobs and/or HTML5 download attributes. APT29 (NOBELIUM) used HTML smuggling to deliver ISO files embedded in HTML attachments (EnvyScout). QakBot was delivered in ZIP files via HTML smuggling. This technique bypasses web content filters because the HTML file itself contains only text/html MIME content.
MITRE ATT&CK
- Tactic
- Defense Evasion
- Technique
- T1027 Obfuscated Files or Information
- Sub-technique
- T1027.006 HTML Smuggling
- Canonical reference
- https://attack.mitre.org/techniques/T1027/006/
LogScale Detection Query
#event_simpleName=NewExecutableWritten
| ImageFileName = /(?i)(msedge|chrome|firefox|iexplore|brave|opera)\.exe$/
| TargetFileName = /(?i)\.(iso|img|hta|js|vbs|lnk|bat|exe)$/
| FilePath = /(?i)(Downloads|Temp|AppData)/
| table(
[
@timestamp,
ComputerName,
UserName,
TargetFileName,
FilePath,
ImageFileName,
SHA256HashData,
TargetProcessId
]
)
| sort(field=@timestamp, order=desc)Detects HTML Smuggling (T1027.006) using CrowdStrike Falcon sensor telemetry in LogScale. The NewExecutableWritten event captures file write operations for executable-class files; ImageFileName identifies the responsible process (browser binary), and TargetFileName with FilePath scope the written artifact to suspicious extensions in user-accessible directories. SHA256HashData enables immediate threat intelligence lookup. For ISO/IMG disk image files not classified as executables by Falcon, supplement with a parallel query on ZipFileWritten or PeFileWritten event names depending on your sensor policy configuration.
Data Sources
Required Tables
False Positives & Tuning
- Chrome or Edge browser auto-update processes that write new installer EXE versions into AppData\Local\Google\Update or AppData\Local\Microsoft\EdgeUpdate directories as part of the browser self-update cycle
- Electron-based desktop applications (Slack, VS Code, Microsoft Teams) that use embedded Chromium to download and self-install updates, writing EXE or NUPKG files to Temp or AppData
- Enterprise web-based software delivery systems (PDQ Deploy web console, ManageEngine Desktop Central) that push EXE packages via browser session on managed endpoints
Other platforms for T1027.006
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Create HTML Smuggling Page with JavaScript Blob
Expected signal: Sysmon Event ID 11: smuggle_test.html created in %TEMP%, html_smuggled_payload.txt created in Downloads by msedge.exe. Sysmon Event ID 1: msedge.exe launched with HTML file argument. Browser file creation in Downloads directory.
- Test 2Simulate EnvyScout HTML Smuggling ISO Drop
Expected signal: Sysmon Event ID 11: envyscout_sim.html created in %TEMP%. The HTML file contains Base64-encoded data and Blob/download attribute JavaScript patterns. Opening in a browser would trigger creation of update.iso in Downloads.
- Test 3Download and Execute Payload from HTA File via Browser
Expected signal: Sysmon Event ID 11: smuggled.hta created in %TEMP%. Sysmon Event ID 1: mshta.exe launching the HTA file. The VBScript MessageBox dialog appears, confirming execution.
- Test 4Identify HTML Smuggling Pattern in HTML File
Expected signal: PowerShell process execution. ScriptBlock Log Event ID 4104 with the analysis code. Output identifies specific HTML smuggling constructs present in the file.
References (6)
- https://attack.mitre.org/techniques/T1027/006/
- https://outflank.nl/blog/2018/08/14/html-smuggling-explained/
- https://www.microsoft.com/security/blog/2021/05/27/new-sophisticated-email-based-attack-from-nobelium/
- https://www.menlosecurity.com/blog/new-attack-alert-duri
- https://www.nccgroup.com/us/research-blog/smuggling-hta-files-in-internet-exploreredge/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1027.006/T1027.006.md
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