CVE-2026-91816 in PDF Editor
Summary
by MITRE • 09/23/2026
A use-after-free vulnerability exists in Foxit PDF Editor/Reader’s handling of PDF annotations. Reentrant annotation deletion triggered by embedded JavaScript can cause the application to access an annotation object after it has been released, resulting in a use-after-free condition and application crash.
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Analysis
by VulDB Data Team • 09/23/2026
The identified security flaw resides within the core processing logic of Foxit PDF Editor and Reader, specifically concerning the management of document annotations through embedded JavaScript execution. This vulnerability is classified as a Use-After-Free (CWE-416) error, which represents one of the most dangerous classes of memory corruption bugs in software engineering. The root cause lies in an improper handling of object lifecycles during reentrant operations. When a PDF document contains maliciously crafted JavaScript code designed to trigger annotation deletion, it initiates a sequence where resources are deallocated while still being referenced by other active processes or data structures within the application memory space.
The technical mechanism exploits the asynchronous nature of event handlers in modern PDF viewers. By embedding specific script commands that force the immediate removal of an annotation object, the attacker creates a race condition or logical error where the pointer to that annotation is not nullified or updated correctly after deallocation. Subsequent operations within the same execution context attempt to access this now-invalid memory address. Because the operating system may have reallocated that specific block of memory for other purposes, accessing it leads to unpredictable behavior. In many instances, as described in the initial report, this results in an immediate application crash due to a segmentation fault or general protection violation, effectively causing a denial-of-service against the user attempting to view the document.
From an operational impact perspective, while the primary manifestation is a crash, use-after-free vulnerabilities are frequently precursors to more severe security breaches if exploited with precise memory layout control. An attacker could potentially manipulate the contents of the freed memory block to achieve arbitrary code execution during the subsequent access phase. This would allow for remote code execution on the victim's machine simply by tricking them into opening a malicious PDF file, without any further interaction required from the user beyond viewing or rendering the document. The presence of embedded JavaScript significantly lowers the barrier for exploitation, as it allows attackers to automate complex memory manipulation techniques directly within the document itself, bypassing many traditional network-based defenses that do not inspect client-side application behavior.
This vulnerability aligns with several entries in the MITRE ATT&CK framework, particularly those related to Initial Exploitation and Defense Evasion via script interpretation. It highlights the risks associated with allowing untrusted content to execute code within a privileged desktop application environment. The reliance on JavaScript engines for document interactivity introduces attack surfaces that are often less rigorously audited than core rendering libraries. Security researchers emphasize that such flaws underscore the necessity of strict memory management practices, including proper pointer invalidation after deallocation and robust bounds checking before any object access occurs.
To mitigate this risk, users should ensure they are running the latest version of Foxit PDF Editor or Reader where patches for this specific use-after-free condition have been applied. Organizations relying on these tools for document processing should implement application whitelisting to prevent unauthorized scripts from executing within the viewer environment. Additionally, deploying endpoint detection and response solutions that can monitor for anomalous memory access patterns may help detect exploitation attempts in real-time. It is also advisable to disable JavaScript execution by default in PDF viewers unless explicitly required for trusted documents, thereby removing the primary vector through which this vulnerability is triggered. Regular security updates and adherence to secure coding standards such as those outlined in CWE-416 are critical for maintaining resilience against memory corruption attacks.