CVE-2021-28545 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 show arbitrary 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
The vulnerability described in CVE-2021-28545 represents a critical weakness in Adobe Acrobat Reader DC's digital signature verification mechanisms, specifically affecting multiple versions of the software across different release cycles. This flaw stems from the absence of proper integrity checking capabilities within the PDF certification process, creating a significant security gap that undermines the fundamental purpose of digital signatures in document authentication. The vulnerability manifests when the application fails to properly validate the integrity of certified PDF documents, allowing malicious actors to manipulate content while maintaining the appearance of valid certification. This issue directly impacts the trust model that digital signatures are designed to establish between document creators and recipients, effectively weakening the security assurances that certified PDFs should provide.
The technical implementation flaw lies in the software's failure to perform comprehensive integrity checks during the certification validation process, which is classified under CWE-347 - Improper Verification of Cryptographic Signature. When a PDF document is certified, the system should verify that the document has not been tampered with since the certification was applied, but this verification mechanism is absent or insufficient in the affected versions. The vulnerability allows attackers to modify PDF content, including text, images, or embedded objects, without the certification status being invalidated, as the application does not properly detect or prevent such modifications. This weakness creates a scenario where a malicious actor can inject arbitrary content into a certified document while preserving the document's certification status, effectively bypassing the security controls that should prevent such tampering.
The operational impact of this vulnerability is particularly concerning because exploitation requires only user interaction through document opening, making it highly practical for social engineering attacks. An attacker could craft a malicious PDF that appears to be a legitimate certified document, such as a contract, invoice, or official form, while secretly embedding malicious content or altering existing information. The attack vector is relatively simple to execute, requiring only the ability to modify PDF files and distribute them to targets who would open them with the vulnerable Acrobat Reader versions. This vulnerability undermines the integrity of digital workflows that rely on certified PDFs for legal or business purposes, potentially leading to fraud, data manipulation, or unauthorized access to sensitive information. The lack of authentication requirements for exploitation means that attackers can leverage this vulnerability in broad-scale campaigns without needing to establish specific access to target systems.
Organizations and users should immediately update to patched versions of Adobe Acrobat Reader DC to address this vulnerability, as the affected versions span multiple release cycles indicating a prolonged period of exposure. The recommended mitigation strategy involves not only updating software but also implementing additional verification processes for certified documents, particularly those containing sensitive information or used in legal transactions. Security teams should conduct comprehensive assessments of their document handling processes to identify any reliance on potentially compromised certified PDFs and establish protocols for verifying document integrity through alternative means. This vulnerability aligns with ATT&CK technique T1566 - Phishing, as it enables more sophisticated social engineering attacks where the appearance of legitimacy is maintained through the forged certification status. Organizations should also consider implementing network-based controls to monitor for suspicious PDF file activities and establish incident response procedures specifically addressing potential exploitation of this vulnerability in their environments.