CVE-2023-33876: Foxit PDF Reader Use-After-Free Flaw

CVE-2023-33876 Overview

A use-after-free vulnerability exists in Foxit Reader 12.1.2.15332 that affects the way the application handles destroying annotations within PDF documents. When a specially crafted PDF file containing malicious JavaScript code is opened, the vulnerability can trigger reuse of a previously freed object, leading to memory corruption and potentially arbitrary code execution. This vulnerability requires user interaction—an attacker must trick the user into opening a malicious PDF file or visiting a specially crafted website if the Foxit Reader browser plugin extension is enabled.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code on the target system with the privileges of the current user, potentially leading to complete system compromise.

Affected Products

• Foxit PDF Reader version 12.1.2.15332

Discovery Timeline

• 2023-07-19 - CVE-2023-33876 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2023-33876

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) exists in the annotation destruction handling code within Foxit Reader. The flaw occurs when specially crafted JavaScript embedded in a malicious PDF document manipulates the annotation object lifecycle. During the destruction of annotation objects, the application fails to properly manage memory references, allowing an attacker to trigger the reuse of previously freed memory.

The vulnerability presents a significant security risk as it can be exploited through two attack vectors: direct file opening where a user is tricked into opening a malicious PDF document, or through the browser plugin extension when a user visits a specially crafted malicious website. In both scenarios, successful exploitation leads to memory corruption that can be leveraged to achieve arbitrary code execution within the context of the affected application.

Root Cause

The root cause of this vulnerability lies in improper memory management during the annotation destruction process in Foxit Reader. When annotations within a PDF document are destroyed, the application fails to properly invalidate or nullify references to the freed memory objects. This allows JavaScript code within the PDF to maintain references to deallocated memory regions and subsequently trigger operations on these freed objects, causing a use-after-free condition.

Attack Vector

The attack requires network-based delivery of a malicious PDF file or malicious web page to a victim. The attacker must craft a PDF document containing specially designed JavaScript that manipulates annotation objects in a way that triggers the use-after-free condition. When the victim opens the malicious PDF in Foxit Reader or encounters it through the browser plugin, the JavaScript code executes and exploits the memory management flaw.

The exploitation mechanism involves:

1. Creating annotation objects within the PDF document
2. Using JavaScript to manipulate the lifecycle of these annotations
3. Triggering the destruction of annotations while maintaining stale references
4. Reusing the freed memory through the stale references to achieve memory corruption
5. Leveraging the corruption to gain arbitrary code execution

For detailed technical analysis of this vulnerability, see the Talos Intelligence Vulnerability Report.

Detection Methods for CVE-2023-33876

Indicators of Compromise

• Unexpected crashes or hangs in Foxit Reader when opening PDF documents
• Suspicious JavaScript execution within PDF files attempting to create and destroy annotations rapidly
• PDF files containing obfuscated JavaScript code with annotation manipulation functions
• Network traffic patterns indicating download of suspicious PDF files from untrusted sources

Detection Strategies

• Monitor for PDF files containing JavaScript that manipulates annotation objects with unusual patterns
• Implement endpoint detection rules for Foxit Reader process anomalies such as unexpected memory access violations
• Deploy network-level inspection to identify and block known malicious PDF signatures
• Use behavioral analysis to detect exploitation attempts through unusual Foxit Reader child process creation

Monitoring Recommendations

• Enable process monitoring for FoxitReader.exe to detect abnormal behavior or unexpected child processes
• Configure logging for PDF file access events, particularly from email attachments and web downloads
• Implement memory protection monitoring to detect heap corruption attempts in Foxit Reader
• Monitor browser plugin activity for suspicious PDF rendering requests from untrusted websites

How to Mitigate CVE-2023-33876

Immediate Actions Required

• Update Foxit PDF Reader to the latest available version that addresses this vulnerability
• Disable the Foxit Reader browser plugin extension if not required for business operations
• Implement application whitelisting to prevent execution of untrusted PDF files
• Configure email security gateways to scan and quarantine suspicious PDF attachments
• Educate users about the risks of opening PDF documents from untrusted sources

Patch Information

Foxit has addressed this vulnerability in newer releases of Foxit PDF Reader. Organizations should upgrade from the affected version 12.1.2.15332 to the latest stable release available from Foxit's official website. For detailed patch information and the latest security updates, consult Foxit's security bulletins and the Talos Intelligence Vulnerability Report.

Workarounds

• Disable JavaScript execution in Foxit Reader by navigating to Preferences > JavaScript and unchecking "Enable JavaScript Actions"
• Remove or disable the Foxit Reader browser plugin to prevent web-based exploitation
• Use Protected View or Safe Reading Mode when opening PDF files from untrusted sources
• Implement network segmentation to limit the impact of potential code execution
• Configure PDF handling policies to open untrusted documents in sandboxed environments

To disable JavaScript in Foxit Reader via preferences:

Edit > Preferences > JavaScript > Uncheck "Enable JavaScript Actions"