Detect Microsoft Office Security Feature Bypass (CVE-2026-21509) in Google Chronicle
Detects exploitation of CVE-2026-21509, a security feature bypass vulnerability in Microsoft Office classified under CWE-807 (Reliance on Untrusted Inputs in a Security Decision). This vulnerability is actively exploited in the wild (CISA KEV) and allows attackers to bypass security controls within Office applications, potentially enabling malicious document execution without expected security warnings or Protected View enforcement.
MITRE ATT&CK
- Tactic
- Initial Access Defense Evasion
YARA-L Detection Query
rule cve_2026_21509_office_security_bypass {
meta:
author = "df00tech Detection Engineering"
description = "Detects Microsoft Office spawning suspicious child processes - CVE-2026-21509 Security Feature Bypass"
severity = "HIGH"
priority = "HIGH"
reference = "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509"
cve = "CVE-2026-21509"
events:
$proc.metadata.event_type = "PROCESS_LAUNCH"
$proc.principal.process.file.full_path = /(?i)(WINWORD|EXCEL|POWERPNT|OUTLOOK|MSPUB|ONENOTE)\.EXE$/
$proc.target.process.file.full_path = /(?i)(cmd|powershell|wscript|cscript|mshta|rundll32|regsvr32|certutil|bitsadmin)\.exe$/
$proc.principal.hostname = $hostname
condition:
$proc
}Chronicle YARA-L rule detecting Microsoft Office processes launching known suspicious child processes that are characteristic of CVE-2026-21509 security feature bypass exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate Office macro automation in enterprise environments
- Approved IT tooling using Office COM interfaces with command-line utilities
- Sanctioned document processing systems using scripting engines
- QA and development environments testing Office extensions
Other platforms for CVE-2026-21509
Testing Methodology
Validate this detection against 3 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 1Office Protected View Bypass via Registry Modification
Expected signal: Registry write events (Sysmon EventID 13) targeting HKCU\Software\Microsoft\Office\16.0\Word\Security with values DisableProtectedView and VBAWarnings modified
- Test 2Office Spawning PowerShell Child Process
Expected signal: Sysmon EventID 1 / Windows EventID 4688 showing WINWORD.EXE as ParentImage and powershell.exe as child Image with encoded or suspicious command-line arguments
- Test 3Office Dropping Executable to Temp Directory
Expected signal: Sysmon EventID 11 (FileCreate) showing an .exe file written to %TEMP% by an Office-related initiating process; DeviceFileEvents in MDE with InitiatingProcessFileName matching an Office executable
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