CVE-2021-42292

What it actually does

CVE-2021-42292 is a Microsoft Excel security feature bypass vulnerability that can be triggered when a user opens a specially crafted Excel document. The flaw allows malicious workbook content to evade intended Office protections and can enable attacker-controlled code or payload execution in the context of the signed-in user. Because the CVSS vector requires user interaction but no prior privileges, exploitation typically depends on convincing a user to open a malicious attachment or downloaded spreadsheet. Successful exploitation can affect confidentiality, integrity, and availability by allowing theft of local data accessible to the user, modification of files, execution of follow-on malware, or establishment of persistence through additional tooling.

Why it matters to the business

Organizations that use Microsoft Excel, Microsoft Office, Microsoft 365 Apps, or Office LTSC are exposed if endpoints have not received the relevant Microsoft security updates. In practical terms, a single opened spreadsheet can become an entry point for malware, credential theft, data exfiltration, lateral movement, or business disruption. This vulnerability is listed in CISA Known Exploited Vulnerabilities and has been observed as actively exploited in the wild, with a required federal remediation date of 2021-12-01, so unpatched systems should be treated as high-priority legacy exposure even though the issue is older. The EPSS score indicates meaningful likelihood of exploitation relative to many vulnerabilities, and document-based delivery remains common in phishing and intrusion campaigns.

How an attack unfolds

An attacker prepares a malicious Excel workbook designed to trigger the security feature bypass when opened in a vulnerable version of Excel. The attacker delivers the file through phishing email, malicious links, file-sharing services, compromised websites, or other social engineering channels. When the recipient opens the spreadsheet, Excel processes the crafted content and fails to enforce the intended security boundary. The attacker’s embedded logic or chained payload can then execute with the privileges of the current user or enable additional malicious activity. After initial execution, the attacker commonly downloads secondary malware, runs script interpreters or living-off-the-land binaries, steals credentials and documents, establishes persistence, and attempts lateral movement. Because the vulnerability is in a client application, exploitation artifacts are usually found on endpoints rather than on perimeter servers.

How to fix it

Apply the relevant Microsoft security updates for Microsoft Excel, Microsoft Office, Microsoft 365 Apps, and Office Long Term Servicing Channel. Where Microsoft 365 Apps are used, ensure clients are on a current supported update channel build that includes the November 2021 security fixes or later. Where perpetual Office or Office LTSC products are used, deploy the applicable Microsoft Office security update through Microsoft Update, WSUS, Configuration Manager, Intune, or the organization’s patch management platform. If immediate patching is not possible, reduce exposure by blocking or quarantining Excel attachments from untrusted sources, enforcing Protected View for internet-originated files, blocking macros from the internet, disabling unnecessary ActiveX/OLE behavior, using Attack Surface Reduction rules for Office child processes and executable content, and restricting script interpreters from being launched by Office applications. Because this is a KEV-listed vulnerability, any remaining vulnerable installations should be tracked as an exception with compensating controls and a near-term remediation deadline.

How to confirm the fix worked

Confirm that all affected Office and Excel installations have received Microsoft security updates released for this issue or later. Validate Microsoft 365 Apps update channel and build information from endpoint management inventory, Office account update status, registry/software inventory, or vulnerability scanner results. Run authenticated vulnerability scans capable of detecting missing Microsoft Office security updates and verify that CVE-2021-42292 is no longer reported. Confirm that endpoints enforce Office hardening controls, including Protected View, blocking macros from internet-originated files, and Attack Surface Reduction rules that prevent Office applications from creating child processes or launching executable content. For high-risk systems, test with benign Office security configuration checks rather than live exploit files, and verify telemetry shows Excel child-process creation is blocked or alerted.

Typical attack path

1. Attacker identifies users likely to open business-themed Excel files, such as invoices, reports, schedules, or financial documents.
2. Attacker crafts an Excel document that abuses the vulnerable security feature handling in unpatched Microsoft Excel or Office installations.
3. Attacker sends the file through phishing, shared drive links, collaboration platforms, or malicious web downloads.
4. User opens the spreadsheet on a vulnerable endpoint.
5. Excel processes the malicious file and bypasses the intended Office security protection.
6. Payload activity runs in the user context or enables a chained payload to execute.
7. Attacker launches follow-on commands using tools such as PowerShell, Windows Script Host, mshta, rundll32, regsvr32, or cmd.
8. Attacker retrieves secondary malware, steals data or credentials, and attempts persistence or lateral movement.

How to detect it

To determine whether this vulnerability was already exploited, focus on endpoint, email, web proxy, EDR, and Windows event telemetry around the time users opened Excel files from external or untrusted sources. Review email security logs for delivered or clicked Excel attachments and links, especially files with .xls, .xlsx, .xlsm, .xlam, .xla, .xlsb, or similar Office extensions. On endpoints, hunt for EXCEL.EXE spawning child processes that are unusual for normal spreadsheet use, including powershell.exe, cmd.exe, wscript.exe, cscript.exe, mshta.exe, rundll32.exe, regsvr32.exe, certutil.exe, bitsadmin.exe, curl.exe, wget.exe, or schtasks.exe. Correlate Excel process start events with network connections to unfamiliar domains or IPs, creation of executables or scripts in user-writable directories, suspicious scheduled tasks, Run key persistence, startup folder items, and credential-access activity. Useful Windows sources include Sysmon Event ID 1 for process creation, Event ID 3 for network connections, Event ID 11 for file creation, Windows Security Event ID 4688 for process creation if enabled, PowerShell logs including Event IDs 4103 and 4104, Defender/EDR alerts, Office telemetry, email gateway logs, and proxy/DNS logs. There are no universal public file hashes or network indicators for all exploitation of this vulnerability; behavioral detections are more reliable than static indicators.

• EXCEL.EXE
• excel.exe
• EXCEL.EXE powershell.exe
• EXCEL.EXE cmd.exe
• EXCEL.EXE wscript.exe
• EXCEL.EXE cscript.exe
• EXCEL.EXE mshta.exe
• EXCEL.EXE rundll32.exe
• EXCEL.EXE regsvr32.exe
• EXCEL.EXE certutil.exe
• EXCEL.EXE bitsadmin.exe
• EXCEL.EXE schtasks.exe
• powershell.exe -enc
• powershell.exe -EncodedCommand
• AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup
• Software\Microsoft\Windows\CurrentVersion\Run
• Zone.Identifier
• Mark-of-the-Web
• .xlsm
• .xlam
• .xlsb

What your security team should do

• Prioritize remediation on endpoints with Microsoft Excel or Office installed, especially systems used by finance, HR, legal, executive support, and other teams that commonly receive external spreadsheets.
• Treat any unpatched instance as urgent because the vulnerability is in the CISA KEV catalog and has active in-the-wild exploitation history.
• Deploy or verify Microsoft Office security updates across Microsoft 365 Apps, Microsoft Excel, Microsoft Office, and Office LTSC installations.
• Enable Microsoft Defender Attack Surface Reduction rules that block Office from creating child processes, creating executable content, and injecting into other processes where operationally feasible.
• Hunt for Excel spawning script interpreters, command shells, archive tools, download utilities, or LOLBins after users opened externally sourced spreadsheets.
• Block or detonate suspicious Excel attachments and downloads at email, web, and collaboration ingress points.
• Review endpoint detections for post-exploitation behaviors such as PowerShell encoded commands, mshta/rundll32/regsvr32 execution, unusual persistence creation, and outbound connections shortly after Excel execution.
• Educate users to report unexpected spreadsheets, especially invoice, payroll, delivery, tax, or financial-themed files that prompt unusual interaction.
• If evidence of exploitation is found, isolate affected endpoints, collect forensic images or triage data, reset potentially exposed credentials, and scope for lateral movement.