CVE-2021-28551 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 Out-of-bounds read 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
This vulnerability represents a critical out-of-bounds read flaw in Adobe Acrobat Reader DC across multiple version ranges including 2021.001.20155 and earlier, 2020.001.30025 and earlier, and 2017.011.30196 and earlier. The vulnerability falls under the CWE-129 weakness category, which specifically addresses insufficient validation of length parameters in buffer operations. The flaw occurs when the application processes maliciously crafted PDF files without proper bounds checking, allowing an attacker to manipulate memory access patterns that exceed allocated buffer boundaries. This type of vulnerability is particularly dangerous because it can be exploited through a simple file attachment in email or web-based attacks where users might inadvertently open malicious documents.
The technical implementation of this vulnerability involves the application's failure to validate the size or boundaries of data structures when parsing PDF content. When Acrobat Reader encounters malformed or specially crafted PDF elements, particularly within embedded objects or stream data, it attempts to read memory locations beyond the intended buffer limits. This behavior creates opportunities for attackers to manipulate the execution flow of the application, potentially leading to arbitrary code execution. The vulnerability requires user interaction through opening a malicious file, making it a classic example of a client-side exploit that leverages social engineering tactics to achieve its goals. The attack vector is consistent with the ATT&CK technique T1204.002 which describes user execution through malicious files.
The operational impact of this vulnerability extends beyond simple privilege escalation as it allows attackers to execute code with the privileges of the current user account. This means that if a user with administrative rights opens a malicious document, the attacker could potentially gain elevated system access. The vulnerability affects a wide range of Acrobat Reader installations, making it particularly attractive to threat actors who can target organizations with older software versions. The exploitation process requires minimal sophistication, as the attacker only needs to deliver a malicious PDF file to a target, making this vulnerability a significant concern for enterprise security teams. Organizations with outdated Acrobat Reader installations face increased risk due to the extended support windows for these older versions. The vulnerability's classification as a remote code execution flaw means that attackers can potentially compromise systems without requiring physical access or complex network penetration techniques.
Mitigation strategies should focus on immediate software updates to the latest Acrobat Reader DC versions that contain patches for this vulnerability. Organizations must implement comprehensive patch management processes to ensure all users have updated their applications. Additional protective measures include email filtering solutions that can detect and block malicious PDF attachments, application whitelisting to prevent unauthorized PDF readers from executing, and user education programs to raise awareness about suspicious file attachments. Security teams should also consider implementing network-based intrusion detection systems that can identify potential exploitation attempts through anomalous PDF parsing behavior. The vulnerability serves as a reminder of the importance of keeping software current and maintaining robust security hygiene practices to prevent exploitation of known vulnerabilities that can be leveraged for persistent access to target systems.