CVE-2021-28546 in Acrobat Reader
Summary
by MITRE • 04/01/2021
Acrobat Reader DC versions versions 2020.013.20074 (and earlier), 2020.001.30018 (and earlier) and 2017.011.30188 (and earlier) are missing support for an integrity check. An unauthenticated attacker could leverage this vulnerability to modify content in a certified PDF without invalidating the certification. Exploitation of this issue requires user interaction in that a victim must open the tampered file.
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Analysis
by VulDB Data Team • 04/07/2021
This vulnerability affects Adobe Acrobat Reader DC across multiple version ranges including 2020.013.20074 and earlier, 2020.001.30018 and earlier, and 2017.011.30188 and earlier, representing a significant weakness in the document certification and integrity verification mechanisms. The core technical flaw stems from the absence of proper integrity checks within the PDF certification process, which allows attackers to manipulate content within certified documents without breaking the certification signature. This represents a critical failure in the security model that should ensure document authenticity and integrity, specifically violating the principles of data integrity and non-repudiation that are fundamental to secure document handling. The vulnerability is categorized under CWE-353 as "Missing Support for Integrity Check" and aligns with ATT&CK technique T1566.001 for "Phishing: Spearphishing Attachment" since exploitation requires user interaction through opening a malicious file.
The operational impact of this vulnerability extends beyond simple document tampering, as it undermines the trust model that organizations rely upon when using certified PDF documents for legal, financial, or sensitive business transactions. An attacker can modify content such as text, images, or embedded objects within a certified PDF without the certification being invalidated, creating a false sense of security for document recipients who may unknowingly accept tampered content. This weakness particularly affects scenarios where documents are signed to prove authenticity and integrity, such as contracts, legal filings, or compliance documentation, where the certification process is expected to guarantee that the document has not been altered since signing. The vulnerability essentially creates a backdoor for content manipulation that bypasses the expected security controls, potentially leading to fraud, misinformation, or unauthorized changes to critical business documents.
The exploitation of this vulnerability requires user interaction, meaning that a victim must actively open the tampered file for the attack to succeed, which provides some natural defense through user awareness and caution. However, this requirement does not eliminate the risk, as social engineering attacks can effectively trick users into opening malicious documents, particularly when the tampered content appears legitimate or when users are unaware of the certification tampering. Organizations should implement comprehensive security awareness training to help users identify potentially malicious documents, while also applying the latest security patches from Adobe to address the missing integrity check functionality. The vulnerability highlights the importance of maintaining up-to-date software and understanding that even seemingly secure document formats can contain critical security gaps when proper integrity verification mechanisms are absent. Additionally, organizations should consider implementing additional layers of document validation and verification processes beyond the built-in certification mechanisms to detect and prevent such tampering scenarios.