CVE-2021-24067: Microsoft Excel RCE Vulnerability

CVE-2021-24067 Overview

CVE-2021-24067 is a remote code execution vulnerability affecting Microsoft Excel and related Office products. This Use After Free (CWE-416) vulnerability allows attackers to execute arbitrary code on the target system when a user opens a specially crafted Excel file. The vulnerability requires user interaction, as the victim must open a malicious document for exploitation to succeed.

Critical Impact

Successful exploitation could allow an attacker to execute arbitrary code with the privileges of the current user, potentially leading to complete system compromise, data theft, or lateral movement within an organization's network.

Affected Products

• Microsoft 365 Apps (Enterprise)
• Microsoft Excel 2010 SP2, 2013 SP1, 2016
• Microsoft Office 2019 (Windows and macOS)
• Microsoft Office Online Server
• Microsoft Office Web Apps 2013 SP1

Discovery Timeline

• 2021-02-25 - CVE-2021-24067 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-24067

Vulnerability Analysis

This vulnerability is classified as a Use After Free (UAF) memory corruption issue within Microsoft Excel's document parsing functionality. A Use After Free condition occurs when a program continues to reference memory after it has been freed, potentially allowing an attacker to manipulate the program's execution flow.

In the context of CVE-2021-24067, the vulnerability exists in how Excel handles certain objects within specially crafted spreadsheet files. When a malicious document is processed, an object may be freed but a dangling pointer to that memory region is retained. If an attacker can control the contents of the freed memory through subsequent allocations (a technique known as heap spraying or controlled allocation), they can potentially redirect program execution to attacker-controlled code.

The attack requires local access and user interaction—specifically, the victim must open a malicious Excel file delivered via email attachment, download link, or file share.

Root Cause

The root cause of CVE-2021-24067 lies in improper memory management within Microsoft Excel's object handling code. Specifically, the application fails to properly validate or nullify object references after memory deallocation, creating a dangling pointer condition. When this freed memory is subsequently accessed during document processing, it can lead to exploitation if an attacker has manipulated the memory contents to point to malicious shellcode or ROP gadgets.

Attack Vector

The attack vector for this vulnerability requires local access with user interaction. An attacker would typically:

1. Craft a malicious Excel document (.xlsx, .xlsm, or related formats) containing specially constructed objects designed to trigger the Use After Free condition
2. Deliver the malicious document to the victim via phishing email, compromised website download, or shared network location
3. Wait for the victim to open the document in a vulnerable version of Microsoft Excel
4. Upon opening, the malicious document triggers the UAF condition, allowing the attacker to execute arbitrary code with the victim's privileges

The vulnerability can be exploited through various Office file formats processed by Excel and Office Web Apps, making it particularly dangerous in enterprise environments where document sharing is common.

Detection Methods for CVE-2021-24067

Indicators of Compromise

• Unusual Excel processes spawning child processes (e.g., cmd.exe, powershell.exe, wscript.exe)
• Excel crashes followed by suspicious process creation or network activity
• Malformed or obfuscated Excel documents with unusual embedded objects
• Excel files from untrusted sources with unexpected macro-like behavior despite being .xlsx format

Detection Strategies

• Monitor for Excel.exe spawning suspicious child processes such as command interpreters or scripting engines
• Implement file analysis solutions to scan Excel documents for exploit signatures or suspicious embedded content
• Deploy endpoint detection rules to identify memory corruption exploitation techniques targeting Office applications
• Enable Windows Event Log auditing for process creation events (Event ID 4688) to track unusual Excel behavior

Monitoring Recommendations

• Enable Microsoft Defender for Office 365 Safe Attachments scanning for all incoming email attachments
• Configure SentinelOne's behavioral AI to detect exploitation attempts targeting Office applications
• Monitor for document files downloaded from untrusted sources followed by suspicious Excel activity
• Implement logging and alerting for Office application crashes that may indicate exploitation attempts

How to Mitigate CVE-2021-24067

Immediate Actions Required

• Apply Microsoft's February 2021 security updates immediately across all affected Office installations
• Enable Protected View for Excel to open documents from untrusted sources in a sandboxed environment
• Restrict user ability to download or open Excel files from untrusted external sources
• Ensure SentinelOne endpoint protection is deployed and updated to detect exploitation attempts

Patch Information

Microsoft released security patches for this vulnerability as part of their February 2021 Patch Tuesday updates. Organizations should apply the relevant updates through Windows Update, Microsoft Update, or Microsoft Update Catalog based on their deployment infrastructure.

For detailed patch information and download links, refer to the Microsoft Security Advisory CVE-2021-24067.

Affected products requiring patches include:

• Microsoft Excel 2010 SP2 through Excel 2016
• Microsoft 365 Apps for Enterprise
• Microsoft Office 2019 for Windows and macOS
• Microsoft Office Web Apps 2013 SP1
• Microsoft Office Online Server

Workarounds

• Enable Protected View in Excel trust center settings to open all external documents in read-only sandbox mode
• Disable the opening of Excel documents received from the internet by configuring Windows Zone-based security policies
• Use Microsoft Office's Application Guard feature where available to isolate untrusted documents
• Block or quarantine Excel file attachments at the email gateway until they can be scanned and verified

# Configuration example - Enable Protected View via Registry
# Run in elevated PowerShell or Command Prompt
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableUnsafeLocationsInPV /t REG_DWORD /d 0 /f
reg add "HKCU\Software\Microsoft\Office\16.0\Excel\Security\ProtectedView" /v DisableAttachmentsInPV /t REG_DWORD /d 0 /f