Detect CVE-2026-21513 — Microsoft MSHTML Framework Protection Mechanism Failure in Elastic Security
Detects exploitation of CVE-2026-21513, a protection mechanism failure (CWE-693) in the Microsoft MSHTML framework on Windows. This KEV-listed vulnerability allows attackers to bypass security controls implemented in MSHTML, potentially enabling code execution via crafted web content processed by Internet Explorer compatibility components, Microsoft Office documents embedding web content, or applications using the WebBrowser control. Active exploitation has been confirmed by CISA.
MITRE ATT&CK
Elastic Detection Query
sequence by host.name, user.name with maxspan=5m
[process where event.action == "start" and
process.parent.name : ("winword.exe", "excel.exe", "powerpnt.exe", "outlook.exe", "mspub.exe", "msaccess.exe") and
process.name : ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe", "regsvr32.exe")]
[any where event.category : ("library", "network") and
(dll.name : ("mshtml.dll", "ieframe.dll", "jscript.dll", "jscript9.dll", "vbscript.dll") or
(network.direction == "outbound" and not destination.ip : ("10.0.0.0/8", "172.16.0.0/12", "192.168.0.0/16", "127.0.0.0/8")))]EQL sequence rule correlating Office application child process spawning with subsequent MSHTML DLL loads or external network activity within a 5-minute window on the same host and user, indicating CVE-2026-21513 exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Automated document processing pipelines that legitimately spawn scripting engines
- Office templates with embedded macros performing web service calls
- Security awareness tools that simulate phishing via Office documents
- Corporate intranet tools using MSHTML rendering within Office plugins
Other platforms for CVE-2026-21513
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 1MSHTML MkNode Method Invocation via mshta.exe
Expected signal: Sysmon Event ID 1 showing mshta.exe spawning cmd.exe; Sysmon Event ID 7 showing mshtml.dll and vbscript.dll loaded by mshta.exe; Process creation events in Windows Security Log (Event ID 4688)
- Test 2Office Document Spawning Scripting Engine via MSHTML
Expected signal: Process creation events showing WINWORD.EXE as parent of cmd.exe or powershell.exe; MSHTML DLL load events in Sysmon EventID 7 for the Office process; Network events if payload attempts callback
- Test 3rundll32.exe MSHTML Script Execution via ieframe.dll
Expected signal: Sysmon Event ID 1 for rundll32.exe execution; Event ID 7 showing ieframe.dll and mshtml.dll loaded by rundll32.exe; Any network connections initiated by the rundll32.exe process to external hosts
- Test 4PowerShell WebBrowser COM Object MSHTML Load
Expected signal: PowerShell process creation event; mshtml.dll loaded into powershell.exe process space (Sysmon Event ID 7); Potential network connection if URL is replaced with live payload URL
Unlock Pro Content
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