CVE-2023-21716: Microsoft Word RCE Vulnerability

CVE-2023-21716 Overview

CVE-2023-21716 is a critical remote code execution vulnerability affecting Microsoft Word and related Office products. This vulnerability allows unauthenticated attackers to execute arbitrary code on target systems through specially crafted documents. The flaw stems from an integer overflow weakness (CWE-190) in the RTF (Rich Text Format) parser component of Microsoft Word.

Critical Impact

This vulnerability enables unauthenticated remote attackers to achieve complete system compromise through malicious document files, potentially leading to full control over affected systems without any user interaction required.

Affected Products

• Microsoft Office 2019 (macOS)
• Microsoft Office Long Term Servicing Channel 2021 (macOS)
• Microsoft Office Online Server 2016
• Microsoft Office Web Apps 2013 SP1
• Microsoft SharePoint Enterprise Server 2013 SP1 and 2016
• Microsoft SharePoint Foundation 2013 SP1
• Microsoft SharePoint Server (Subscription, 2019)
• Microsoft Word 2013 SP1

Discovery Timeline

• February 14, 2023 - CVE-2023-21716 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-21716

Vulnerability Analysis

CVE-2023-21716 represents a severe security flaw in Microsoft Word's RTF file parser. The vulnerability is classified as an Integer Overflow (CWE-190), which occurs when an arithmetic operation attempts to create a numeric value that exceeds the range that can be represented with a given number of bits. In the context of this vulnerability, the integer overflow occurs during the processing of RTF font table entries.

The attack can be executed remotely over the network without requiring authentication or user interaction, making it particularly dangerous in enterprise environments where document sharing is common. An attacker can exploit this vulnerability by crafting a malicious RTF document that, when parsed by the vulnerable Word RTF parser, triggers the integer overflow condition leading to heap corruption and subsequent code execution.

Root Cause

The root cause of CVE-2023-21716 lies in improper handling of font table entries within the RTF parser. When processing the \fonttbl control word in RTF documents, the parser fails to properly validate integer values, leading to an integer overflow condition. This overflow corrupts heap memory structures, which can then be leveraged by attackers to gain control of program execution flow.

The integer overflow vulnerability specifically affects how font indices are processed and stored, allowing attackers to manipulate memory allocation sizes and overwrite critical data structures in the application's heap space.

Attack Vector

The attack vector for CVE-2023-21716 is network-based and requires no privileges or user interaction. An attacker can deliver the malicious RTF document through various channels:

1. Email attachments - The most common delivery method where victims receive emails with malicious RTF attachments
2. Web-based delivery - Hosting malicious documents on compromised or attacker-controlled websites
3. File shares - Placing malicious documents on network shares accessible to target users
4. Document collaboration platforms - Uploading malicious files to SharePoint or similar platforms

The vulnerability can be triggered simply by having Microsoft Word parse the malicious RTF content, which can occur through the Preview Pane in Outlook or Windows Explorer without explicitly opening the document.

Detection Methods for CVE-2023-21716

Indicators of Compromise

• Malformed RTF files with abnormally large font table entries or suspicious \fonttbl structures
• Unexpected crashes of Microsoft Word or related Office applications when processing RTF documents
• Memory access violations or heap corruption events logged by endpoint protection systems
• Network traffic containing RTF documents from untrusted sources with unusual file characteristics

Detection Strategies

• Implement file inspection rules to detect RTF documents with malformed font table structures
• Monitor for Microsoft Word process crashes or abnormal termination events that may indicate exploitation attempts
• Deploy endpoint detection rules targeting heap spray patterns commonly associated with integer overflow exploitation
• Enable enhanced logging for Office applications to capture document parsing events and anomalies

Monitoring Recommendations

• Configure email gateways to scan and quarantine suspicious RTF attachments before delivery to end users
• Implement network-level monitoring for RTF file transfers and flag documents exceeding normal size parameters
• Enable Windows Event Log collection for Office application crashes and correlate with document access events

How to Mitigate CVE-2023-21716

Immediate Actions Required

• Apply Microsoft's February 2023 security updates immediately to all affected systems
• Disable the Preview Pane in Microsoft Outlook and Windows Explorer to prevent automatic parsing of malicious documents
• Block RTF documents at email gateways until patches can be deployed across the environment
• Educate users about the risks of opening documents from unknown or untrusted sources

Patch Information

Microsoft has released security updates addressing CVE-2023-21716 as part of their February 2023 Patch Tuesday release. Detailed patch information and download links are available through the Microsoft Security Update Guide. Organizations should prioritize deployment of these updates given the critical severity and network-based attack vector.

The patches address the integer overflow condition in the RTF parser by implementing proper bounds checking and input validation for font table entries.

Workarounds

• Configure Microsoft Word to block RTF file types using the File Block Settings in the Trust Center
• Implement registry-based mitigations to disable RTF parsing in Microsoft Word
• Use Protected View settings to open documents from the internet in a sandboxed environment
• Deploy application whitelisting to control which document types can be opened by Office applications

# Registry configuration to block RTF files in Microsoft Word 2013
# Navigate to: HKCU\Software\Microsoft\Office\15.0\Word\Security\FileBlock
# Set RtfFiles DWORD value to 2 (Open/Save blocked, use open policy)

reg add "HKCU\Software\Microsoft\Office\15.0\Word\Security\FileBlock" /v RtfFiles /t REG_DWORD /d 2 /f

# For Microsoft Word 2016/2019/365, use version 16.0:
reg add "HKCU\Software\Microsoft\Office\16.0\Word\Security\FileBlock" /v RtfFiles /t REG_DWORD /d 2 /f