CVE-2021-39841 in Acrobat Reader
Summary
by MITRE • 09/29/2021
Acrobat Reader DC versions 2021.005.20060 (and earlier), 2020.004.30006 (and earlier) and 2017.011.30199 (and earlier) are affected by a Type Confusion vulnerability. An attacker could leverage this vulnerability to execute arbitrary code 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 • 10/04/2021
The Type Confusion vulnerability identified in CVE-2021-39841 represents a critical security flaw affecting multiple versions of Adobe Acrobat Reader DC, specifically targeting versions up to 2021.005.20060, 2020.004.30006, and 2017.011.30199. This vulnerability stems from improper handling of data types within the application's memory management system, creating a condition where the software incorrectly interprets the type of data being processed. The flaw manifests when the application encounters objects that are expected to be of one data type but are actually of a different type, leading to unpredictable behavior and potential system compromise. The vulnerability is classified under CWE-466 as a situation where a program attempts to access a data structure using the wrong type, which is a well-documented weakness in software development practices and represents a fundamental flaw in type safety mechanisms.
The exploitation of this vulnerability requires user interaction through social engineering techniques, as victims must voluntarily open a maliciously crafted file to trigger the exploit. This user interaction requirement makes the attack vector more targeted but also more challenging to defend against, as it relies on human factors and trust in document sources. When a user opens the malicious file, the application's memory management routines encounter the improperly typed data structures and execute code that has been crafted to exploit this type confusion scenario. The attacker can leverage this to execute arbitrary code with the privileges of the current user, potentially leading to full system compromise and unauthorized access to sensitive data. This aligns with ATT&CK technique T1203, which describes the use of malicious documents to gain initial access and execute code within a target environment.
The operational impact of this vulnerability extends beyond simple code execution, as it represents a significant risk to enterprise security environments where Adobe Reader is widely deployed. Organizations that have not updated to patched versions of the software remain vulnerable to targeted attacks that could result in data breaches, lateral movement within networks, and establishment of persistent access points. The vulnerability's presence in multiple version lines indicates that the underlying type confusion issue has persisted across several releases, suggesting inadequate quality assurance or regression testing processes. The memory corruption aspects of this vulnerability can potentially lead to application crashes, but more critically, they enable attackers to bypass security controls and execute malicious payloads that could include malware installation, credential theft, or network reconnaissance activities.
Organizations should prioritize immediate remediation by updating to patched versions of Adobe Acrobat Reader DC, as the vulnerability represents a high-severity risk that requires no additional attack vectors beyond user interaction. Security teams should also implement network monitoring to detect potential exploitation attempts and consider deploying application control solutions to restrict execution of untrusted documents. The vulnerability demonstrates the importance of proper type checking and memory management in security-critical applications, as well as the necessity of maintaining up-to-date software versions to protect against known exploits. Regular vulnerability assessments and penetration testing should include evaluation of document processing applications to identify similar type confusion issues that could potentially affect other software components within the organization's attack surface.