CVE-2021-28449: Microsoft 365 Apps RCE Vulnerability

CVE-2021-28449 Overview

CVE-2021-28449 is a Remote Code Execution vulnerability affecting Microsoft Office products. This vulnerability allows an attacker to execute arbitrary code on a target system when a user opens a specially crafted Office document. The attack requires user interaction, as the victim must open a malicious file, but successful exploitation can lead to complete system compromise with the same privileges as the current user.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code with user privileges, potentially leading to full system compromise, data theft, or lateral movement within enterprise networks.

Affected Products

• Microsoft 365 Apps (Enterprise)
• Microsoft Excel 2010 SP2, 2013 SP1, 2016
• Microsoft Office 2010 SP2, 2013 SP1, 2016, 2019

Discovery Timeline

• April 13, 2021 - CVE-2021-28449 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-28449

Vulnerability Analysis

This Remote Code Execution vulnerability exists in the way Microsoft Office products handle certain objects in memory when processing Office documents. The vulnerability can be triggered when a user opens a specially crafted document that contains malicious content designed to exploit the flaw in Office's document parsing functionality.

The attack vector is local, meaning an attacker must convince a user to open a malicious file delivered through phishing emails, compromised websites, or file-sharing services. Once the document is opened, the exploit can execute arbitrary code in the context of the current user. If the user has administrative privileges, the attacker could gain complete control over the affected system.

The vulnerability affects multiple versions of Microsoft Office spanning from Office 2010 SP2 through Microsoft 365 Apps for Enterprise, representing a significant attack surface across enterprise environments that may still be running legacy Office versions.

Root Cause

The vulnerability stems from improper handling of objects in memory within Microsoft Office components. While specific technical details have not been publicly disclosed by Microsoft, this class of vulnerability typically involves memory corruption issues during document parsing operations. The flaw allows maliciously crafted content within Office documents to corrupt memory in a way that can be leveraged for code execution.

Attack Vector

The attack requires local interaction where a victim opens a malicious Office document. Attack scenarios include:

1. Phishing campaigns - Attackers send emails with malicious Office documents attached, using social engineering to convince recipients to open them
2. Watering hole attacks - Malicious documents hosted on compromised websites that users are likely to visit
3. Supply chain compromise - Malicious documents embedded in legitimate-looking business communications or shared through collaboration platforms

The attacker does not need any privileges on the target system to exploit this vulnerability, but user interaction is required. The vulnerability cannot be exploited remotely without user action.

Detection Methods for CVE-2021-28449

Indicators of Compromise

• Suspicious Office processes spawning unexpected child processes (e.g., cmd.exe, powershell.exe, mshta.exe)
• Office applications making unusual network connections immediately after opening documents
• Anomalous memory allocation patterns in Office process space
• Creation of unexpected files in temporary directories following document opening

Detection Strategies

• Monitor for Office applications spawning shell processes or script interpreters
• Implement behavioral analysis rules for Office process trees that deviate from normal patterns
• Deploy endpoint detection rules that alert on suspicious Office document execution chains
• Analyze Office documents in sandboxed environments before allowing user access

Monitoring Recommendations

• Enable detailed logging for Office application events and process creation
• Monitor network traffic from Office applications for unusual outbound connections
• Implement file integrity monitoring on critical system directories
• Configure security information and event management (SIEM) rules for Office-related exploitation patterns

How to Mitigate CVE-2021-28449

Immediate Actions Required

• Apply the Microsoft security update from the April 2021 Patch Tuesday release immediately
• Enable Protected View for Office documents from untrusted sources
• Configure attack surface reduction rules in Microsoft Defender for Endpoint
• Educate users about the risks of opening Office documents from unknown sources

Patch Information

Microsoft has released security updates to address this vulnerability as part of the April 2021 security updates. Patches are available for all affected products including Microsoft Office 2010 SP2, 2013 SP1, 2016, 2019, and Microsoft 365 Apps for Enterprise. For detailed patch information and download links, refer to the Microsoft Security Advisory for CVE-2021-28449.

Organizations should prioritize patching based on the criticality of systems and exposure to external document sources. Legacy Office versions (2010, 2013) should be considered for upgrade to supported versions to ensure continued security updates.

Workarounds

• Configure Microsoft Office to open documents in Protected View by default
• Implement Office Document Trusted Locations policies to restrict which locations can bypass security features
• Use Application Guard for Office to isolate potentially malicious documents
• Deploy email filtering solutions to block suspicious Office attachments before delivery to end users

# PowerShell: Enable Protected View via Registry for all Office applications
# Run in elevated PowerShell session
$OfficeVersions = @("16.0", "15.0", "14.0")
$OfficeApps = @("Excel", "Word", "PowerPoint")

foreach ($version in $OfficeVersions) {
    foreach ($app in $OfficeApps) {
        $regPath = "HKCU:\Software\Microsoft\Office\$version\$app\Security\ProtectedView"
        if (-not (Test-Path $regPath)) {
            New-Item -Path $regPath -Force | Out-Null
        }
        Set-ItemProperty -Path $regPath -Name "DisableInternetFilesInPV" -Value 0 -Type DWord
        Set-ItemProperty -Path $regPath -Name "DisableAttachmentsInPV" -Value 0 -Type DWord
        Set-ItemProperty -Path $regPath -Name "DisableUnsafeLocationsInPV" -Value 0 -Type DWord
    }
}
Write-Host "Protected View settings configured for all Office applications"