CVE-2021-42292

What is Microsoft Excel?

Microsoft Excel is the world's most widely deployed spreadsheet application, part of Microsoft Office and Microsoft 365. Beyond standard spreadsheet functionality, Excel supports automation through macros: legacy XLM macros (Excel 4.0, from 1992) and modern VBA macros. XLM macros in particular are a well-documented malware delivery vector — threat actors embed malicious XLM macro code in .xls and .xlsm files and use phishing campaigns to distribute them. To protect users, Excel displays security warnings before opening files from the internet (Mark of the Web / Protected View) and before executing macro content (the "Enable Content" yellow bar). Security feature bypasses that circumvent these warnings without user awareness are high-value targets for attackers distributing malware via phishing.

Overview

CVE-2021-42292 is a security feature bypass vulnerability in Microsoft Excel that was exploited as a zero-day — actively exploited in the wild — at the time Microsoft patched it in November 2021 Patch Tuesday. The vulnerability allows a specially crafted Excel file to bypass Excel's security warning prompts that normally alert users before executing potentially malicious macro content. A victim who opens the malicious Excel file does not see the expected security prompt; content executes without explicit user acknowledgment of the risk. CISA added this to KEV one week after the patch, confirming active exploitation in targeted attacks.

The local attack vector (AV:L) reflects that the initial file delivery is local — the victim must open a file delivered via email, web download, or other channel. User interaction (UI:R) is required in the sense that the victim must open the file, but the bypass means they do not need to click "Enable Content" or acknowledge any security warning.

Affected Versions

Technical Details

• Root cause: A security feature bypass in Excel's macro/content execution security controls — a flaw in how Excel evaluates certain file format conditions allows macro content to execute without triggering the security warning bar
• Bypass target: Excel's "Enable Content" security warning, which is designed to require explicit user acknowledgment before executing XLM macros, VBA macros, or other potentially dangerous content in files from untrusted locations
• Attack vector: The attacker crafts a malicious Excel file (.xls, .xlsm, or similar) containing macro content designed to exploit the bypass condition. The file is delivered via phishing email, malicious link, or other document delivery mechanism
• User experience: The victim opens the file; instead of seeing a security warning bar requiring them to click "Enable Content," the content executes automatically — the bypass suppresses or circumvents the warning
• Impact: Full code execution in the context of the Excel process (CVSS shows C:H/I:H/A:H) — malicious macros can drop files, spawn processes, establish persistence, or install malware
• AV:L classification: The Local attack vector reflects the file-based delivery mechanism; the file must be present on the local system to execute, but it is typically delivered remotely via phishing

Discovery

Reported to Microsoft and patched as an actively exploited zero-day during November 2021 Patch Tuesday. The targeting and specific threat actors exploiting this zero-day were not publicly disclosed by Microsoft. CISA's KEV addition one week after the patch confirms confirmed active exploitation, likely in targeted phishing campaigns using malicious Excel files.

Exploitation Context

Microsoft Excel security feature bypasses are consistently valuable to phishing-based malware campaigns because they reduce the friction between file delivery and code execution. Normally, a user who receives a phishing email with an Excel attachment must click "Enable Content" to activate macros — this extra step causes some users to reconsider or triggers endpoint security alerts. A bypass that eliminates this step makes malware delivery significantly more reliable. The November 2021 zero-day window (between when attackers discovered the bypass and when Microsoft patched it) was exploited in targeted attacks. The combination of AV:L (file delivery) + UI:R (user must open the file) + security feature bypass represents the standard Excel-based phishing attack chain.

Remediation

1. Apply November 2021 Office/Microsoft 365 update — delivered via Microsoft Update or Office automatic updates
2. Enable Microsoft 365 Apps automatic updates: File → Account → Update Options → Enable Updates
3. Consider enabling the Attack Surface Reduction (ASR) rule that blocks Office applications from creating child processes: Block all Office applications from creating child processes (GUID: d4f940ab-401b-4efc-aadc-ad5f3c50688a)
4. Disable XLM (Excel 4.0) macros via Group Policy or Microsoft 365 admin center if your organization does not use legacy XLM macros — this eliminates an entire class of Excel-based malware delivery
5. Enable Protected View and ensure files from the internet open in Protected View before any content executes
6. Deploy Microsoft Defender for Office 365 Safe Attachments to sandbox Excel files before delivery to end users