CVE-2021-27059: Microsoft Office RCE Vulnerability

CVE-2021-27059 Overview

CVE-2021-27059 is a Remote Code Execution vulnerability affecting Microsoft Office. This vulnerability allows an attacker to execute arbitrary code on a target system through a specially crafted Office document. The attack requires local access and user interaction, meaning a victim must be convinced to open a malicious document for exploitation to occur.

Critical Impact

This vulnerability is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Organizations should prioritize patching as threat actors have demonstrated real-world exploitation of this flaw.

Affected Products

• Microsoft Office 2010 SP2
• Microsoft Office 2013 SP1 (including RT edition)
• Microsoft Office 2016

Discovery Timeline

• 2021-03-11 - CVE-2021-27059 published to NVD
• 2025-10-30 - Last updated in NVD database

Technical Details for CVE-2021-27059

Vulnerability Analysis

This Remote Code Execution vulnerability in Microsoft Office allows attackers to achieve complete system compromise through malicious document payloads. The vulnerability requires high privileges and user interaction for successful exploitation, where a victim must open a specially crafted Office document. Upon successful exploitation, an attacker can gain full control over the affected system with the ability to compromise confidentiality, integrity, and availability of data and resources.

The inclusion of this CVE in the CISA Known Exploited Vulnerabilities catalog underscores its severity in real-world attack scenarios. Organizations running legacy Office installations (2010 SP2, 2013 SP1, 2016) remain particularly vulnerable if patches have not been applied.

Root Cause

While Microsoft has not disclosed specific technical details about the root cause (classified as NVD-CWE-noinfo), the vulnerability resides within Office's document processing components. Remote code execution vulnerabilities in Office typically stem from improper handling of embedded objects, malformed document structures, or unsafe parsing of document elements that allow attackers to inject and execute arbitrary code.

Attack Vector

The attack vector for CVE-2021-27059 is local, requiring the attacker to deliver a malicious Office document to the victim through methods such as:

1. Email-based delivery: Attaching malicious Office documents to phishing emails
2. Web downloads: Hosting weaponized documents on compromised or attacker-controlled websites
3. File sharing platforms: Distributing malicious files through cloud storage or collaboration tools
4. Social engineering: Convincing users to open documents through pretexting or other manipulation techniques

Once the victim opens the malicious document, the embedded payload executes with the privileges of the current user, potentially leading to full system compromise.

Detection Methods for CVE-2021-27059

Indicators of Compromise

• Unusual Office processes spawning child processes (e.g., WINWORD.EXE or EXCEL.EXE launching cmd.exe, powershell.exe, or other interpreters)
• Suspicious network connections originating from Office applications
• Unexpected file modifications or creation in user temp directories following Office document access
• Anomalous registry modifications correlating with Office document open events

Detection Strategies

• Deploy endpoint detection rules to monitor for Office applications spawning unexpected child processes
• Implement email gateway scanning for malicious Office document attachments using sandbox analysis
• Enable Microsoft Defender for Office 365 or equivalent advanced threat protection solutions
• Configure YARA rules to detect known malicious document structures associated with Office RCE exploits

Monitoring Recommendations

• Enable Windows Event logging for process creation events (Event ID 4688) with command line auditing
• Monitor Sysmon Event ID 1 (Process Create) for Office process hierarchy anomalies
• Implement file integrity monitoring on critical system directories
• Review security logs for unusual Office application behavior, particularly network connections and file system access patterns

How to Mitigate CVE-2021-27059

Immediate Actions Required

• Apply Microsoft security updates addressing CVE-2021-27059 immediately across all affected Office installations
• Enable Protected View and Block macros from the Internet in Office Trust Center settings
• Implement application whitelisting to prevent unauthorized code execution from Office processes
• Educate users about the risks of opening Office documents from untrusted sources

Patch Information

Microsoft has released security patches addressing this vulnerability. Organizations should obtain and deploy the appropriate updates from the Microsoft Security Response Center advisory for CVE-2021-27059. Given the confirmed exploitation in the wild (CISA KEV listing), immediate patching is critical.

Affected versions requiring updates:

• Microsoft Office 2010 SP2
• Microsoft Office 2013 SP1 (all editions including RT)
• Microsoft Office 2016

Workarounds

• Configure Office applications to open documents in Protected View by default
• Disable ActiveX controls in Office documents via Group Policy
• Implement Attack Surface Reduction (ASR) rules to block Office applications from creating child processes
• Consider migrating from legacy Office versions (2010, 2013, 2016) to Microsoft 365 with enhanced security features

# Attack Surface Reduction rule to block Office child processes
# Enable via Group Policy or PowerShell
Add-MpPreference -AttackSurfaceReductionRules_Ids D4F940AB-401B-4EFC-AADC-AD5F3C50688A -AttackSurfaceReductionRules_Actions Enabled

# Block Office applications from creating executable content
Add-MpPreference -AttackSurfaceReductionRules_Ids 3B576869-A4EC-4529-8536-B80A7769E899 -AttackSurfaceReductionRules_Actions Enabled