CVE-2026-91791 in PDF Editor
Summary
by MITRE • 09/23/2026
When processing a specially crafted PDF file, Foxit PDF Editor/Reader may encounter a reentrant execution condition involving JavaScript triggered by page-visibility events. This can cause the application to access a released page-view object while calculating annotation boundaries, resulting in an invalid memory read and application crash.
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Analysis
by VulDB Data Team • 09/23/2026
The vulnerability identified involves a critical flaw within Foxit PDF Editor and Reader related to the handling of JavaScript execution during specific user interface state changes. Specifically, when processing specially crafted Portable Document Format files that trigger page-visibility events, the application enters an unsafe reentrant execution condition. This scenario arises because the internal logic for managing document rendering does not adequately serialize or guard against recursive calls initiated by embedded scripts responding to visibility toggles such as minimizing, maximizing, or switching browser tabs while viewing a PDF. The core technical flaw lies in the lifecycle management of page-view objects within the application's memory space. When a visibility event occurs, the system attempts to recalculate annotation boundaries for visual consistency and layout accuracy. However, due to the reentrant nature of the JavaScript engine invocation triggered by these events, the underlying data structures representing the current page view may have already been deallocated or released from memory before the calculation routine completes its access cycle.
This race condition results in an invalid memory read operation where the application attempts to dereference a pointer that no longer points to valid heap memory. In practical terms, this manifests as an immediate application crash, which constitutes a denial of service against the user's workflow and system stability. While the primary impact observed is local instability, such memory corruption vulnerabilities often serve as precursors to more severe exploitation vectors if the attacker can control the content read from the freed memory region. The lack of proper synchronization mechanisms or reference counting checks allows the application state to become inconsistent between the UI thread handling visibility changes and the rendering engine processing annotation data. This type of flaw is characteristic of complex desktop applications that rely heavily on dynamic scripting engines for document interaction, where event handlers can fire unpredictably during rapid user interactions or automated testing scenarios designed to stress-test resource management.
From a classification perspective, this vulnerability aligns with CWE-416, Use After Free, as the application accesses memory after it has been freed. It also relates to CWE-362, Concurrent Execution using Shared Resource with Improper Synchronization, given that the reentrant execution creates a race condition between different parts of the software handling the same resource simultaneously without adequate locking or state verification. In terms of offensive security frameworks like MITRE ATT&CK, this behavior could be leveraged in techniques associated with Initial Exploitation for Denial of Service (T1499) if used to disrupt operations, though more critically it falls under potential code execution pathways via memory corruption vulnerabilities often categorized under Tactic 6: Defense Evasion or Tactic 7: Discovery depending on the ultimate payload intent. The vulnerability highlights the risks inherent in allowing untrusted content like PDFs to execute arbitrary JavaScript within a privileged desktop environment without strict sandboxing or state isolation boundaries.
Mitigation strategies for this issue primarily involve vendor-side patching that implements robust reference counting and null-check validation before accessing page-view objects during annotation boundary calculations. Developers should ensure that event handlers are properly debounced or throttled to prevent rapid reentrant calls from destabilizing the rendering engine. Additionally, implementing stricter memory safety checks within the JavaScript execution context can help detect and abort operations on invalid pointers before they cause crashes. For end-users, applying the latest security updates provided by Foxit is essential to resolve these synchronization flaws. Until patches are applied, restricting the ability of PDF files to execute embedded scripts through browser or application settings can reduce the attack surface significantly. Security administrators should also monitor for unusual process termination events associated with Foxit applications as an indicator of potential exploitation attempts targeting this specific memory corruption flaw.