Detect HTML Smuggling in Elastic Security
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/
Elastic Detection Query
file where event.action == "creation" and
process.name : ("msedge.exe", "chrome.exe", "firefox.exe", "iexplore.exe", "brave.exe", "opera.exe") and
file.extension : ("iso", "img", "hta", "js", "vbs", "lnk", "bat", "exe", "zip") and
file.path : ("*\\Downloads\\*", "*\\Temp\\*", "*\\AppData\\*")Detects HTML Smuggling (T1027.006) by identifying browser processes creating disk image files (ISO, IMG) or executable/script artifacts (HTA, JS, VBS, LNK, BAT, EXE) in standard user-writable directories. The ECS file event model with process.name as the creating process mirrors the Sysmon Image field used in the SPL baseline. Matches EnvyScout-style delivery where a JavaScript Blob reconstructs a payload and triggers download via the HTML5 download attribute, bypassing network content inspection.
Data Sources
Required Tables
False Positives & Tuning
- Enterprise software deployment portals where browsers download legitimate ISO or EXE packages from internal SCCM/Intune distribution points
- Developer workflows involving browser-based CI/CD artifact downloads from GitHub Releases, Jenkins, or Artifactory fetching ZIP or EXE assets to the Downloads directory
- IT administrators using browser-based hypervisor management consoles (VMware vSphere, Proxmox) that serve ISO images for OS provisioning
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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