Detect Microsoft Office Word Reliance on Untrusted Inputs in Security Decision (CVE-2026-21514) in Google Chronicle
Detects exploitation of CVE-2026-21514, a Microsoft Office Word vulnerability classified as CWE-807 (Reliance on Untrusted Inputs in a Security Decision). This flaw allows attackers to manipulate security-relevant decisions in Word by supplying crafted untrusted input, potentially bypassing security controls such as Protected View, macro policy enforcement, or document trust decisions. This CVE is listed on the CISA KEV catalog, indicating active exploitation in the wild.
MITRE ATT&CK
YARA-L Detection Query
rule cve_2026_21514_word_security_bypass {
meta:
author = "df00tech Detection Engineering"
description = "Detects Word spawning suspicious processes consistent with CVE-2026-21514 exploitation"
severity = "HIGH"
priority = "HIGH"
cve = "CVE-2026-21514"
reference = "https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21514"
events:
$proc.metadata.event_type = "PROCESS_LAUNCH"
$proc.principal.process.file.full_path = /(?i)WINWORD\.EXE$/
$proc.target.process.file.full_path = /(?i)(cmd|powershell|wscript|cscript|mshta|rundll32|regsvr32|certutil|bitsadmin|msiexec)\.exe$/
$proc.principal.hostname = $host
condition:
$proc
}Chronicle YARA-L rule detecting WINWORD.EXE launching LOLBins or scripting engines, consistent with security decision bypass exploitation of CVE-2026-21514.
Data Sources
Required Tables
False Positives & Tuning
- Approved enterprise macro-enabled documents that legitimately spawn system utilities
- Security operations tooling that instruments Word for behavioral analysis
- Document processing pipelines in controlled environments using Word automation
- Internal IT tools using Word COM objects with scripting engine dependencies
Other platforms for CVE-2026-21514
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 1Word Spawns PowerShell via Macro — Security Bypass Simulation
Expected signal: Sysmon Event ID 1 showing WINWORD.EXE as ParentImage with powershell.exe as child Image; PowerShell ScriptBlock logging Event ID 4104; network telemetry if payload attempts outbound connection
- Test 2Word Drops Executable to Temp via Macro
Expected signal: Sysmon Event ID 11 (FileCreate) with Image=WINWORD.EXE and TargetFilename ending in .exe under %TEMP%; DeviceFileEvents in MDE showing InitiatingProcessFileName=WINWORD.EXE
- Test 3Word Initiates External Network Connection via mshta Child Process
Expected signal: Sysmon Event ID 1 with ParentImage=WINWORD.EXE and Image=mshta.exe; subsequent cmd.exe spawn from mshta.exe visible in process tree; file creation in TEMP
- Test 4Mark-of-the-Web Bypass Check — Simulated Internet-Origin Document Open
Expected signal: Sysmon Event ID 15 (FileCreateStreamHash) capturing Zone.Identifier ADS creation and deletion; Windows Defender SmartScreen or Office MOTW telemetry in event logs
Unlock Pro Content
Get the full detection package for CVE-2026-21514 including response playbook, investigation guide, and atomic red team tests.