CVE-2021-28631 in Acrobat Reader
Summary
by MITRE • 08/25/2021
Acrobat Reader DC versions versions 2021.001.20155 (and earlier), 2020.001.30025 (and earlier) and 2017.011.30196 (and earlier) are affected by an Use After Free vulnerability. 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 • 08/29/2021
The vulnerability identified as CVE-2021-28631 represents a critical use after free flaw in Adobe Acrobat Reader DC software across multiple version ranges including 2021.001.20155 and earlier, 2020.001.30025 and earlier, and 2017.011.30196 and earlier. This type of vulnerability falls under the CWE-416 category of Use After Free, which occurs when a program continues to reference memory after it has been freed, potentially allowing attackers to manipulate the freed memory location for malicious purposes. The flaw exists within the document parsing functionality of Acrobat Reader, specifically when processing maliciously crafted PDF files that trigger improper memory management during the rendering process. The vulnerability is particularly concerning because it requires no authentication from the attacker and can be exploited through a simple file delivery mechanism, making it highly accessible for widespread exploitation.
The technical exploitation of this vulnerability requires user interaction, meaning victims must actively open a malicious PDF file to trigger the exploit. This user interaction requirement aligns with the ATT&CK technique T1203 - Exploitation for Client Execution, where adversaries leverage vulnerabilities in applications to execute arbitrary code on targeted systems. When a victim opens the malicious file, the Acrobat Reader application processes the PDF content and encounters a memory management error that leads to the use after free condition. This condition can be leveraged by attackers to overwrite memory contents with malicious code, potentially leading to privilege escalation or complete system compromise depending on the execution context. The vulnerability demonstrates a classic memory corruption issue where freed memory blocks are not properly invalidated, allowing subsequent allocations to reuse the same memory addresses and potentially contain attacker-controlled data.
The operational impact of CVE-2021-28631 extends beyond individual user compromise to potentially affect enterprise environments where Acrobat Reader is widely deployed. Organizations using older versions of Acrobat Reader DC face significant risk as the vulnerability can be exploited through phishing campaigns, malicious email attachments, or compromised websites that serve malicious PDF files. The attack vector is particularly dangerous because PDF files are commonly used in business communications and document sharing, making them an ideal delivery mechanism for exploitation. Security teams must consider that successful exploitation could result in data exfiltration, lateral movement within networks, or establishment of persistent backdoors on compromised systems. The vulnerability also impacts the broader security posture of organizations as it demonstrates how legacy software versions can contain critical flaws that remain unpatched in production environments.
Mitigation strategies for CVE-2021-28631 primarily focus on immediate remediation through software updates and version management. Adobe has released patches for affected versions, and organizations should prioritize updating Acrobat Reader DC to the latest available versions that contain fixes for this vulnerability. System administrators should implement strict document filtering policies that prevent automatic execution of potentially malicious files, particularly PDFs from untrusted sources. Network-based mitigations can include implementing web proxies that scan and filter PDF content before delivery to end users, while endpoint protection solutions should be configured to monitor for suspicious file execution patterns. Additional defensive measures include disabling JavaScript execution in Acrobat Reader, implementing sandboxing techniques for PDF processing, and conducting regular vulnerability assessments to identify systems running outdated versions of Acrobat Reader. The vulnerability also underscores the importance of maintaining current software inventory and implementing automated patch management processes to prevent similar issues from affecting other software components within the organization's attack surface.