CVE-2021-26876 in Windows
Summary
by MITRE • 03/11/2021
OpenType Font Parsing Remote Code Execution Vulnerability
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Analysis
by VulDB Data Team • 03/30/2021
The CVE-2021-26876 vulnerability represents a critical remote code execution flaw in OpenType font parsing implementations across multiple Microsoft Windows operating systems. This vulnerability resides in the way Windows processes OpenType font files, specifically within the font rendering engine that handles font data structures and metadata. The flaw enables attackers to craft malicious font files that can trigger arbitrary code execution when the system attempts to render or process these fonts. The vulnerability affects Windows 10 versions 1909, 2004, and 20H2, as well as Windows Server 2019 and Windows Server 2016, making it particularly concerning given the widespread deployment of these operating systems across enterprise environments. The vulnerability is classified under CWE-125 as an out-of-bounds read condition, where the font parsing code fails to properly validate font data structures, leading to memory corruption that can be exploited by malicious actors.
The technical exploitation of this vulnerability involves crafting a specially malformed OpenType font file that triggers a buffer overflow or memory corruption during the parsing process. When a vulnerable system encounters such a font file, either through normal user interaction with documents containing embedded fonts or through automated exploitation via web browsers or email clients, the font rendering engine attempts to parse the malicious data structure. This parsing operation leads to memory corruption that allows attackers to execute arbitrary code with the privileges of the affected user. The vulnerability can be triggered through multiple attack vectors including web browsing, email attachments, document processing, or any scenario where OpenType fonts are rendered or processed by the Windows operating system. The exploitation requires no special privileges initially, as the vulnerability exists in the system-level font processing libraries that run with standard user permissions.
The operational impact of CVE-2021-26876 extends beyond simple remote code execution, as it represents a significant threat to enterprise security infrastructure and user workstations. Attackers can leverage this vulnerability to establish persistent access to compromised systems, deploy additional malware payloads, or escalate privileges to gain administrative control over affected machines. The vulnerability's presence in widely used font processing libraries means that even seemingly benign documents or web pages could serve as attack vectors, making it particularly dangerous in targeted campaigns. Organizations running affected Windows versions face significant risk of data breaches, system compromise, and potential lateral movement within their networks, as the vulnerability can be exploited through various attack surfaces including email systems, web applications, and document sharing platforms. This vulnerability directly aligns with attack techniques described in the MITRE ATT&CK framework under the T1059.007 sub-technique for "Command and Scripting Interpreter: PowerShell" and T1566 for "Phishing" as it can be delivered through email attachments and exploited through web-based attack vectors.
Mitigation strategies for CVE-2021-26876 require immediate implementation of Microsoft security patches and updates, as the vulnerability was addressed through the Microsoft Security Response Center's patch release in March 2021. Organizations should prioritize deployment of the relevant security updates across all affected systems, particularly those running the vulnerable Windows versions. Additional defensive measures include implementing strict font handling policies, disabling automatic font rendering in web browsers and email clients, and deploying network-based intrusion detection systems to monitor for exploitation attempts. Security teams should also consider implementing application whitelisting policies that restrict font processing to trusted applications and sources. The vulnerability highlights the importance of font security in modern computing environments and demonstrates the need for robust input validation in system libraries that handle untrusted data. Organizations should conduct thorough vulnerability assessments to identify all systems running affected Windows versions and ensure proper patch management processes are in place to prevent similar vulnerabilities from being exploited in the future. The remediation process should include comprehensive testing of patches in controlled environments before widespread deployment to avoid potential compatibility issues with legitimate font processing applications.