Detect Microsoft Office Word Reliance on Untrusted Inputs in Security Decision (CVE-2026-21514) in Elastic Security
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
Elastic Detection Query
sequence by host.name, user.name with maxspan=2m
[process where process.name : "WINWORD.EXE" and event.type == "start"]
[process where process.parent.name : "WINWORD.EXE" and
process.name : ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe",
"mshta.exe", "rundll32.exe", "regsvr32.exe", "certutil.exe",
"bitsadmin.exe", "msiexec.exe", "curl.exe", "wget.exe")]EQL sequence rule correlating a WINWORD.EXE start event followed within 2 minutes by a suspicious child process, scoped per host and user to reduce cross-session noise.
Data Sources
Required Tables
False Positives & Tuning
- Macro-driven document automation that legitimately calls cmd or PowerShell on known endpoints
- Sandboxed document analysis environments that generate synthetic process chains
- Word add-ins using scripting runtimes that match the suspicious process list
- Help desk or training environments where Word documents open terminals for instructional purposes
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.