CVE-2021-21893 in Foxit
Summary
by MITRE • 08/06/2021
A use-after-free vulnerability exists in the JavaScript engine of Foxit Software’s PDF Reader, version 11.0.0.49893. A specially crafted PDF document can trigger the reuse of previously freed memory, which can lead to arbitrary code execution. An attacker needs to trick the user to open the malicious file to trigger this vulnerability. Exploitation is also possible if a user visits a specially crafted, malicious site if the browser plugin extension is enabled.
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Analysis
by VulDB Data Team • 05/04/2026
The vulnerability identified as CVE-2021-21893 represents a critical use-after-free flaw within the JavaScript engine of Foxit Software's PDF Reader application. This specific weakness affects version 11.0.0.49893 and demonstrates a classic memory safety issue that has long been recognized as a primary attack vector in software security. The vulnerability stems from improper memory management practices where the application attempts to access memory locations that have already been freed and potentially reallocated, creating opportunities for malicious exploitation. Such flaws typically arise when developers fail to properly track object lifecycles and memory allocation patterns within complex software systems, particularly in interpreted environments like JavaScript engines that handle dynamic memory operations.
The technical exploitation of this vulnerability requires careful crafting of a malicious PDF document that specifically triggers the memory corruption condition during normal PDF processing operations. When a user opens the specially crafted PDF file, the JavaScript engine executes code that causes a use-after-free condition, leading to unpredictable memory access patterns that can be leveraged by attackers to execute arbitrary code with the privileges of the affected application. This type of vulnerability aligns with CWE-416, which specifically addresses use-after-free conditions in software systems, and represents a common target for attackers seeking to escalate privileges or gain unauthorized system access. The attack vector is particularly concerning because it can be delivered through social engineering tactics that trick users into opening malicious files, or through web-based delivery methods when browser plugins are enabled.
The operational impact of CVE-2021-21893 extends beyond simple code execution capabilities to potentially compromise entire user systems and data integrity. Successful exploitation could allow attackers to install malware, steal sensitive information, modify system configurations, or establish persistent backdoors within affected environments. Organizations using Foxit PDF Reader in enterprise settings face significant risks as this vulnerability could serve as an initial access point for broader network infiltration attempts. The vulnerability's exploitation requires user interaction, which means traditional security measures like email filtering and web proxies may not prevent all attack scenarios, particularly when users visit compromised websites with enabled browser plugins. This characteristic makes the vulnerability particularly dangerous in targeted attack scenarios where attackers can leverage social engineering to increase successful exploitation rates.
Mitigation strategies for CVE-2021-21893 should prioritize immediate software updates from Foxit Software to address the identified memory management flaws. Organizations must implement comprehensive patch management procedures to ensure all affected systems receive updates promptly, as the vulnerability exists in a widely deployed PDF reading application. Additional protective measures include disabling browser plugin extensions that may enable web-based exploitation, implementing user education programs about suspicious PDF files, and deploying network monitoring solutions to detect anomalous PDF-related traffic patterns. Security teams should also consider implementing application whitelisting policies to restrict execution of potentially malicious PDF files and maintain detailed logging of PDF processing activities to aid in incident response efforts. The vulnerability's classification as a use-after-free issue also necessitates enhanced code review processes and automated memory safety testing in future software development cycles to prevent similar flaws from emerging in other applications.