CVE-2021-31179: Microsoft 365 Apps RCE Vulnerability

CVE-2021-31179 Overview

CVE-2021-31179 is a Remote Code Execution vulnerability affecting Microsoft Office products. This vulnerability allows attackers to execute arbitrary code on target systems when a user opens a specially crafted Office document. The attack requires user interaction, specifically opening a malicious file, but once triggered, it can lead to complete system compromise with the same privileges as the logged-in user.

Critical Impact
Successful exploitation enables attackers to execute arbitrary code with user-level privileges, potentially leading to data theft, malware installation, and lateral movement within enterprise environments.

Affected Products

Microsoft 365 Apps (Enterprise)
Microsoft Excel 2013 SP1, 2016
Microsoft Office 2013 SP1, 2016, 2019
Microsoft Office Online Server
Microsoft Office Web Apps Server 2013 SP1

Discovery Timeline

May 11, 2021 - CVE-2021-31179 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-31179

Vulnerability Analysis

This Remote Code Execution vulnerability exists within Microsoft Office's document processing functionality. The flaw allows attackers to craft malicious Office documents that, when opened by a victim, trigger code execution within the context of the current user. The vulnerability requires local access to the malicious file and user interaction to open it, but no prior authentication or privileges are required from the attacker's perspective.

The impact is significant as it affects the confidentiality, integrity, and availability of the target system. An attacker who successfully exploits this vulnerability could install programs, view, change, or delete data, or create new accounts with full user rights. Users whose accounts are configured with fewer user rights on the system could be less impacted than those who operate with administrative privileges.

Root Cause

Microsoft has not disclosed specific technical details about the root cause of this vulnerability. The vulnerability exists in the way Microsoft Office handles certain document elements during file parsing. When processing specially crafted content, the Office application fails to properly validate input, allowing malicious code to execute.

Attack Vector

The attack vector for CVE-2021-31179 is local, requiring an attacker to deliver a malicious Office document to the victim. Common delivery methods include:

Email attachments containing weaponized Office files
Malicious documents hosted on compromised or attacker-controlled websites
Documents shared through file-sharing platforms or collaboration tools
USB drives or other removable media containing malicious files

The victim must be convinced to open the malicious document, typically through social engineering tactics. Once the document is opened in a vulnerable version of Microsoft Office, the exploit executes with the permissions of the current user.

Detection Methods for CVE-2021-31179

Indicators of Compromise

Unusual child processes spawned by Microsoft Office applications (e.g., cmd.exe, powershell.exe, mshta.exe)
Office applications making unexpected network connections to unknown external IP addresses
Suspicious Office documents with embedded macros, OLE objects, or unusual file structures
Unexpected process behavior from WINWORD.EXE, EXCEL.EXE, or POWERPNT.EXE

Detection Strategies

Monitor for Office applications spawning child processes that are commonly used for exploitation (cmd.exe, powershell.exe, wscript.exe, cscript.exe)
Implement email attachment scanning to identify potentially malicious Office documents before delivery
Deploy endpoint detection rules to identify suspicious Office document behaviors during file opening
Enable Microsoft Office Protected View and Application Guard to sandbox untrusted documents

Monitoring Recommendations

Enable enhanced logging for Microsoft Office applications and Windows process creation events
Configure SIEM rules to alert on Office applications exhibiting suspicious process lineage
Monitor file system activity for temporary files created by Office applications in unusual locations
Track network connections initiated by Office processes, especially to external IP addresses

How to Mitigate CVE-2021-31179

Immediate Actions Required

Apply the latest Microsoft security updates for all affected Office products immediately
Enable Microsoft Office Protected View for documents originating from the internet or email
Configure Microsoft Office to block macros from running in files from the internet
Educate users about the risks of opening Office documents from unknown or untrusted sources
Consider deploying Microsoft Defender Application Guard for Office to isolate untrusted documents

Patch Information

Microsoft has released security updates to address this vulnerability as part of their May 2021 security updates. Administrators should apply the appropriate patches for their Office installations. For detailed patch information and download links, refer to the Microsoft Security Advisory for CVE-2021-31179.

Organizations using Microsoft 365 Apps should ensure automatic updates are enabled to receive the latest security fixes. For on-premises deployments of Office 2013, 2016, and 2019, manual deployment of security updates through WSUS or SCCM may be required.

Workarounds

Enable Protected View for all Office documents to prevent automatic execution of potentially malicious content
Configure Group Policy to disable Active Content (macros, ActiveX, DDE) in Office documents from untrusted sources
Use Microsoft Office Application Guard to open untrusted documents in an isolated container
Restrict Office file associations to open documents in read-only mode by default
Implement email filtering rules to quarantine or block Office attachments from external senders

bash

# Group Policy configuration to enable Protected View
# Navigate to: User Configuration > Administrative Templates > Microsoft Office > Security Settings
# Enable: "Protected View for files originating from the Internet"
# Enable: "Protected View for files located in potentially unsafe locations"

# Registry key to enable Protected View (per application)
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\Word\Security\ProtectedView" /v DisableInternetFilesInPV /t REG_DWORD /d 0 /f