CVE-2021-21086 in Acrobat Reader
Summary
by MITRE • 09/02/2021
Acrobat Reader DC versions versions 2020.013.20074 (and earlier), 2020.001.30018 (and earlier) and 2017.011.30188 (and earlier) are affected by an Out-of-bounds Write vulnerability in the CoolType library. An unauthenticated attacker could leverage this vulnerability to achieve arbitrary code execution in the context of the current user. Exploitation of this issue requires user interaction in that a victim must open a malicious file.
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Analysis
by VulDB Data Team • 09/09/2021
The vulnerability identified as CVE-2021-21086 represents a critical out-of-bounds write flaw within Adobe Acrobat Reader DC's CoolType library implementation. This library is responsible for font rendering and processing within the PDF viewer application, making it a prime target for exploitation due to its frequent use in document processing. The affected versions span multiple release cycles including 2020.013.20074 and earlier, 2020.001.30018 and earlier, as well as 2017.011.30188 and earlier, indicating this vulnerability has persisted across several major releases and represents a significant security gap in Adobe's font processing subsystem. The CoolType library's role in handling font data makes it particularly susceptible to buffer overflow conditions when processing malformed font structures, as the library fails to properly validate input boundaries during font rendering operations.
This vulnerability falls under CWE-787, which specifically addresses out-of-bounds write conditions, and represents a classic example of how font processing libraries can become attack vectors in document viewers. The exploitability requires user interaction through opening a maliciously crafted PDF file containing malformed font data, which then triggers the buffer overflow when the CoolType library attempts to process the corrupted font structure. The vulnerability's design allows for arbitrary code execution in the context of the current user, meaning that successful exploitation could lead to complete system compromise without requiring administrative privileges. The attack surface is particularly concerning because PDF documents are widely distributed and frequently opened by users across various environments, making this vulnerability highly exploitable in real-world scenarios.
The operational impact of CVE-2021-21086 extends beyond simple privilege escalation, as it provides attackers with a reliable method to execute malicious code on target systems through social engineering campaigns targeting PDF document delivery. The vulnerability's exploitation path aligns with ATT&CK technique T1203, which describes the use of malicious documents to gain initial access, and T1059, which covers the execution of malicious code through legitimate system processes. Organizations using affected versions of Acrobat Reader DC face significant risk, as the vulnerability can be leveraged in targeted attacks, phishing campaigns, or supply chain compromises where attackers craft PDF documents specifically designed to trigger the buffer overflow condition. The fact that this vulnerability affects multiple versions of Acrobat Reader DC also means that many organizations may have been exposed for extended periods without proper mitigation.
Mitigation strategies should focus on immediate patching of all affected versions, as Adobe has released security updates addressing this specific vulnerability. Organizations should implement strict document filtering policies, particularly for PDF files from untrusted sources, and consider deploying sandboxing solutions for PDF processing to isolate potential exploitation attempts. Network-based defenses should include deep packet inspection for PDF files and monitoring for suspicious document behaviors. The vulnerability highlights the importance of keeping third-party libraries updated and maintaining robust security hygiene practices, as font processing libraries often receive less scrutiny than core application components. Additionally, user education regarding the risks of opening unknown PDF files remains crucial, as the attack requires user interaction and social engineering remains a primary exploitation vector for such vulnerabilities.