CVE-2026-91799 in PDF Editor
Summary
by MITRE • 09/23/2026
A use-after-free vulnerability exists in Foxit PDF Editor/Reader’s handling of JavaScript array objects. A specially crafted PDF may cause the application to access a released object during array processing, potentially resulting in application crashes or arbitrary code execution.
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Analysis
by VulDB Data Team • 09/23/2026
The identified security flaw represents a critical memory management error within the JavaScript engine embedded in Foxit PDF Editor and Reader applications. Specifically, this is a use-after-free vulnerability that occurs during the parsing and evaluation of JavaScript arrays contained within maliciously crafted Portable Document Format files. In software development, particularly when dealing with dynamic scripting languages like JavaScript integrated into native C++ or C-based applications, memory safety is paramount. A use-after-free condition arises when an application continues to reference a pointer after the memory it points to has been freed and potentially reallocated for other purposes. This specific instance involves the handling of array objects where the lifecycle management of these data structures fails to properly synchronize with their deallocation processes. When a PDF document contains JavaScript code that manipulates arrays in a way that triggers premature or incorrect memory release, subsequent operations attempting to access those same array elements will interact with invalid memory addresses.
From a technical perspective, this vulnerability exploits the gap between logical object lifetime and physical memory allocation states within the application's runtime environment. The attacker crafts a PDF file containing JavaScript snippets designed to force the garbage collector or manual memory management routines to release an array object while references to that same object remain active in other parts of the execution context. As the interpreter continues processing, it attempts to read from or write to the now-deallocated memory block. Because modern operating systems do not immediately zero out freed memory for performance reasons, this stale pointer may still point to valid-looking data if another process has allocated similar-sized blocks there, or it may point to unmapped pages leading to immediate faults. The unpredictability of heap layout means that an attacker can often manipulate the contents of these reallocated regions to control program flow, turning a simple memory corruption bug into a powerful exploitation vector.
The operational impact of this vulnerability is severe and multifaceted. In its least damaging form, accessing invalid memory results in application crashes, leading to denial-of-service conditions where users cannot open legitimate documents or the software becomes unstable during normal operation. However, given that Foxit Reader and Editor are widely deployed enterprise tools often used for sensitive document handling, the risk extends far beyond simple instability. If an attacker can control the data written into the reallocated memory region, they may achieve arbitrary code execution with the privileges of the user running the application. This allows for full system compromise, including installation of malware, exfiltration of confidential documents stored locally, or pivoting to other systems within a corporate network if remote access capabilities are present in the PDF viewer's extended features.
This vulnerability aligns closely with Common Weakness Enumeration identifier CWE-416, which describes use-after-free errors as a category of memory corruption vulnerabilities where pointers continue to be used after they have been freed. In terms of offensive security frameworks such as MITRE ATT&CK, this technique falls under the Tactic Execution and Defense Evasion, specifically relating to techniques like Memory Code Injection or exploitation via dynamic link library hijacking if the heap spray allows for shellcode placement. The attack vector is classified as remote through a file-based payload, meaning no user interaction beyond opening the malicious PDF in an affected version of Foxit software is required, although some configurations might require enabling JavaScript execution which defaults to off but can be togged on by users or administrators.
Mitigation strategies must address both immediate remediation and long-term defensive posture. The primary defense is applying vendor-provided patches that correct the memory management logic within the PDF parser's JavaScript engine. Organizations should prioritize updating all endpoints running Foxit Reader and Editor to versions where this specific array handling flaw has been resolved by engineering teams who have likely added bounds checking or reference counting corrections. In environments where immediate patching is not feasible, network-level controls such as web proxies that inspect incoming PDF files for known malicious signatures can provide a layer of defense. Additionally, enforcing strict application control policies to prevent the execution of unsigned scripts within document viewers reduces the attack surface significantly since many exploitation attempts rely on executing specific JavaScript payloads embedded in the file. User awareness training is also critical, emphasizing caution when opening documents from untrusted sources even if they appear legitimate, as social engineering often accompanies technical exploits targeting productivity software.