Detect Microsoft Office Word Reliance on Untrusted Inputs in Security Decision (CVE-2026-21514) in Splunk
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
SPL Detection Query
index=windows sourcetype=XmlWinEventLog:Microsoft-Windows-Sysmon/Operational EventCode=1
| where ParentImage LIKE "%WINWORD.EXE"
| where Image LIKE "%cmd.exe" OR Image LIKE "%powershell.exe" OR Image LIKE "%wscript.exe" OR Image LIKE "%cscript.exe" OR Image LIKE "%mshta.exe" OR Image LIKE "%rundll32.exe" OR Image LIKE "%regsvr32.exe" OR Image LIKE "%certutil.exe" OR Image LIKE "%bitsadmin.exe" OR Image LIKE "%msiexec.exe"
| eval risk_score=case(
Image LIKE "%mshta.exe", 90,
Image LIKE "%powershell.exe" AND (CommandLine LIKE "%-enc%" OR CommandLine LIKE "%-EncodedCommand%"), 95,
Image LIKE "%regsvr32.exe", 85,
Image LIKE "%certutil.exe", 80,
true(), 70
)
| table _time, host, user, ParentImage, ParentCommandLine, Image, CommandLine, risk_score
| sort -risk_score, -_timeDetects WINWORD.EXE spawning LOLBin or scripting engine child processes via Sysmon process creation events, with risk scoring based on child process type and command-line patterns.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate business macros that invoke PowerShell or cmd for document automation workflows
- IT administration tools embedded in Word templates that use scripting engines
- Document management systems that use Word COM automation and spawn helper processes
- Security scanning tools opening Word documents in instrumented environments
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.