CVE-2021-40726 in Acrobat Reader
Summary
by MITRE • 10/07/2021
Acrobat Reader DC versions 2021.005.20060 (and earlier), 2020.004.30006 (and earlier) and 2017.011.30199 (and earlier) are affected by a use-after-free vulnerability when processing AcroForm field that could result in arbitrary code execution in the context of the current user. User interaction is required to exploit this vulnerability in that the target must visit a malicious page or open a malicious file.
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Analysis
by VulDB Data Team • 10/14/2021
The vulnerability identified as CVE-2021-40726 represents a critical use-after-free flaw in Adobe Acrobat Reader DC across multiple versions including 2021.005.20060 and earlier, 2020.004.30006 and earlier, and 2017.011.30199 and earlier. This vulnerability resides within the processing of AcroForm fields which are interactive elements found in pdf documents that allow users to enter data, select options, or make selections. The flaw manifests when the application handles malformed or specially crafted AcroForm field data, creating conditions where memory previously allocated to a field object is accessed after it has been freed, leading to unpredictable behavior and potential exploitation.
The technical nature of this vulnerability places it squarely within the CWE-416 category of use-after-free conditions, where software attempts to access memory after it has been freed by the system or application. This type of vulnerability is particularly dangerous because it can be exploited to execute arbitrary code with the privileges of the current user, effectively allowing attackers to gain unauthorized access to system resources. The exploitation requires user interaction through visiting a malicious webpage or opening a malicious pdf file, making it a client-side attack vector that leverages social engineering techniques to deliver the payload.
From an operational impact perspective, this vulnerability poses significant risks to organizations relying on Adobe Acrobat Reader for document processing, as it can be exploited in targeted attacks against specific users or widespread phishing campaigns. The attack surface is extensive given that pdf documents are commonly shared through email, web downloads, and document management systems, making it a prime target for threat actors seeking to compromise user systems. The privilege escalation potential means that successful exploitation could lead to full system compromise, data exfiltration, or deployment of additional malicious software.
Mitigation strategies for CVE-2021-40726 primarily focus on immediate remediation through software updates to the latest versions of Adobe Acrobat Reader DC, which contain patches addressing the use-after-free vulnerability. Organizations should implement comprehensive patch management procedures to ensure all systems are updated promptly. Additional protective measures include deploying web application firewalls, implementing strict email filtering to block suspicious pdf attachments, and educating users about the risks of opening unknown or untrusted pdf documents. Security teams should also consider network segmentation and monitoring for unusual pdf document access patterns. The ATT&CK framework categorizes this vulnerability under T1203 - Exploitation for Client Execution, highlighting the need for endpoint protection and user awareness training to prevent successful exploitation attempts.