CVE-2021-24093: Windows 10 Graphics Component RCE Flaw

CVE-2021-24093 Overview

CVE-2021-24093 is a remote code execution vulnerability affecting the Windows Graphics Component, specifically within the DirectWrite fsg_ExecuteGlyph function. This vulnerability allows attackers to execute arbitrary code on vulnerable systems by convincing users to view specially crafted content that triggers a buffer overflow during font glyph processing. The attack requires user interaction, typically through visiting a malicious website or opening a malformed document containing embedded fonts.

Critical Impact

Successful exploitation enables remote code execution with the privileges of the current user, potentially leading to complete system compromise, data theft, or lateral movement within enterprise networks.

Affected Products

• Microsoft Windows 10 (versions 1607, 1803, 1809, 1909, 2004, 20H2)
• Microsoft Windows Server 2016 (including versions 1909, 2004, 20H2)
• Microsoft Windows Server 2019

Discovery Timeline

• February 25, 2021 - CVE-2021-24093 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-24093

Vulnerability Analysis

This vulnerability resides in the Windows Graphics Component, specifically within the DirectWrite subsystem responsible for font rendering. The flaw exists in the fsg_ExecuteGlyph function, which processes TrueType font glyph instructions. When parsing maliciously crafted font data, the function fails to properly validate boundary conditions, resulting in a buffer overflow condition.

The vulnerability can be triggered remotely through network-based attack vectors, though user interaction is required—typically by convincing a victim to visit a malicious webpage or open a crafted document. Due to its presence in a core graphics rendering component, exploitation can occur through multiple attack surfaces including web browsers, document viewers, and any application that renders fonts using DirectWrite.

Root Cause

The root cause is a buffer overflow in the fsg_ExecuteGlyph function within DirectWrite. The function processes TrueType font glyph instructions without adequate bounds checking, allowing specially crafted font data to write beyond allocated buffer boundaries. This memory corruption can be leveraged to overwrite critical data structures or redirect code execution flow.

Attack Vector

The attack vector is network-based, requiring an attacker to deliver malicious content to the victim. Common attack scenarios include:

• Web-based exploitation: Attacker hosts a webpage containing a malicious embedded font; when a user visits the page with a vulnerable browser, the font rendering triggers the overflow
• Document-based attacks: Malicious fonts embedded in Office documents, PDFs, or other document formats that support custom fonts
• Email campaigns: Phishing emails with attachments or links to malicious content

The vulnerability exploits the font rendering pipeline, which processes glyph instructions to render text. By crafting specific glyph instruction sequences, an attacker can trigger the buffer overflow condition in fsg_ExecuteGlyph, potentially achieving arbitrary code execution in the context of the rendering application.

Technical details regarding the buffer overflow in DirectWrite's fsg_ExecuteGlyph function are documented in the Packet Storm security report.

Detection Methods for CVE-2021-24093

Indicators of Compromise

• Unexpected crashes in applications performing font rendering, particularly with stack traces involving DirectWrite or fsg_ExecuteGlyph
• Anomalous network traffic fetching font files from untrusted or suspicious domains
• Presence of unusual or malformed font files (.ttf, .otf) in temporary directories or browser caches
• Process spawning from graphics or font rendering contexts with unexpected parent-child relationships

Detection Strategies

• Monitor for crashes and exceptions in processes utilizing DirectWrite, specifically those involving font glyph processing
• Implement endpoint detection rules to flag suspicious font file downloads from untrusted sources
• Deploy memory protection technologies to detect heap and buffer overflow attempts
• Enable Windows Defender Exploit Guard to monitor for exploitation attempts against graphics components

Monitoring Recommendations

• Configure Windows Event Log monitoring for Application crashes involving dwm.exe, browser processes, or Office applications with DirectWrite-related faults
• Implement network monitoring for downloads of font files from non-standard or newly registered domains
• Enable SentinelOne Singularity platform behavioral monitoring for anomalous process execution following font rendering activities

How to Mitigate CVE-2021-24093

Immediate Actions Required

• Apply the Microsoft security update from the February 2021 Patch Tuesday release immediately on all affected Windows systems
• Prioritize patching for systems with high user interaction exposure such as workstations and terminal servers
• Implement network segmentation to limit exposure of unpatched systems
• Consider restricting access to untrusted websites and documents until patches are deployed

Patch Information

Microsoft has released security updates addressing this vulnerability as part of the February 2021 security updates. The official security advisory and patch information is available from the Microsoft Security Response Center. Organizations should apply the relevant cumulative updates for their specific Windows versions through Windows Update, WSUS, or SCCM.

Workarounds

• Restrict web browsing to trusted sites only using browser policies or web filtering solutions
• Disable font downloading in browser settings where feasible (Block untrusted fonts policy)
• Deploy application whitelisting to prevent execution of potentially exploited processes
• Implement network-level controls to block font files from untrusted external sources

# Disable untrusted font loading via Group Policy (Windows 10+)
# Navigate to: Computer Configuration > Administrative Templates > System > Mitigation Options
# Enable "Untrusted Font Blocking" and set to "Block untrusted fonts"

# Alternative: Registry-based configuration
reg add "HKLM\SOFTWARE\Policies\Microsoft\Windows NT\MitigationOptions" /v MitigationOptions_FontBlocking /t REG_SZ /d "1000000000000" /f