CVE-2026-3779: List Box Use-After-Free Vulnerability

CVE-2026-3779 Overview

CVE-2026-3779 is a use-after-free vulnerability affecting PDF document processing applications. The application's list box calculate array logic keeps stale references to page or form objects after they are deleted or re-created, which allows crafted documents to trigger a use-after-free when the calculation runs and can potentially lead to arbitrary code execution.

Critical Impact

This use-after-free vulnerability allows attackers to craft malicious PDF documents that, when opened by a victim, can execute arbitrary code on the target system. The vulnerability requires user interaction (opening a malicious document) but requires no special privileges to exploit.

Affected Products

• Foxit PDF Reader (specific versions not disclosed)
• Foxit PDF Editor (specific versions not disclosed)

Discovery Timeline

• 2026-04-01 - CVE-2026-3779 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-3779

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption vulnerability that occurs when a program continues to use a pointer after the memory it references has been freed. In this case, the list box calculate array logic within the PDF processing application maintains references to page or form objects even after those objects have been deleted or re-created during document processing.

When processing specially crafted PDF documents, the application fails to properly invalidate or update these references when the underlying objects are destroyed. Subsequent calculations that rely on these stale references will access freed memory, leading to undefined behavior that attackers can leverage to achieve code execution.

Root Cause

The root cause lies in improper memory management within the list box calculation routines. When page or form objects are deleted or re-created during document manipulation, the application does not properly clear or update the references stored in the calculate array. This creates dangling pointers that reference freed memory regions.

The vulnerability is triggered when:

1. A PDF document contains list box elements with calculation dependencies
2. Page or form objects referenced by the calculation logic are deleted or re-created
3. The calculate array retains stale references to the now-invalid objects
4. Subsequent calculation execution accesses the freed memory

Attack Vector

This is a local attack vector requiring user interaction. An attacker must craft a malicious PDF document designed to trigger the use-after-free condition and distribute it to potential victims. The attack chain typically involves:

1. Creating a specially crafted PDF document with list box elements and calculation logic
2. Structuring the document to trigger object deletion/re-creation during processing
3. Distributing the malicious document via email, web download, or other means
4. When a victim opens the document, the stale reference is accessed during calculation
5. The attacker achieves code execution in the context of the victim's user session

The vulnerability requires no authentication or special privileges, but does require convincing a user to open the malicious document. For detailed technical analysis, refer to the Talos Intelligence Vulnerability Report.

Detection Methods for CVE-2026-3779

Indicators of Compromise

• Unexpected PDF reader crashes or abnormal termination when opening PDF documents
• Process memory access violations in PDF processing components
• Suspicious PDF attachments from unknown or untrusted sources containing complex form elements
• Evidence of shellcode execution following PDF document opening

Detection Strategies

• Monitor for PDF reader processes exhibiting abnormal behavior such as crashes, high CPU usage, or unexpected child process creation
• Implement sandboxing for PDF document processing to isolate potential exploitation attempts
• Deploy endpoint detection and response (EDR) solutions capable of detecting use-after-free exploitation techniques
• Analyze PDF documents before user access using static and dynamic analysis tools

Monitoring Recommendations

• Enable crash dump collection for PDF reader applications to capture exploitation attempts
• Monitor for suspicious process spawning from PDF reader applications
• Implement email attachment scanning with PDF structure analysis capabilities
• Review application logs for PDF processing errors that may indicate exploitation attempts

How to Mitigate CVE-2026-3779

Immediate Actions Required

• Update Foxit PDF Reader and PDF Editor to the latest patched versions as specified in the Foxit Security Bulletins
• Avoid opening PDF documents from untrusted or unknown sources
• Enable Protected Mode or Safe Reading Mode in PDF applications where available
• Consider implementing application whitelisting to control PDF reader execution

Patch Information

Foxit has released security updates to address this vulnerability. Users should consult the Foxit Security Bulletins for specific patch information and download links. Organizations should prioritize deploying these updates across all affected systems.

Workarounds

• Enable Protected Mode or sandboxing features in PDF reader applications to limit the impact of potential exploitation
• Disable JavaScript execution in PDF documents to reduce the attack surface
• Use alternative PDF readers for processing documents from untrusted sources until patches can be applied
• Implement network-level filtering to scan PDF attachments before delivery to end users