CVE-2021-21822: Foxit Reader Use-After-Free Vulnerability

CVE-2021-21822 Overview

A use-after-free vulnerability exists in the JavaScript engine of Foxit Software's PDF Reader, version 10.1.3.37598. A specially crafted PDF document can trigger the reuse of previously freed memory, which can lead to arbitrary code execution. An attacker needs to trick the user into opening a malicious file or site to trigger this vulnerability if the browser plugin extension is enabled.

Critical Impact

This vulnerability enables remote attackers to execute arbitrary code on victim systems through maliciously crafted PDF documents, potentially leading to complete system compromise when users open untrusted PDF files or visit malicious websites with the Foxit browser plugin enabled.

Affected Products

• Foxit Reader version 10.1.3.37598
• Foxit PDF Reader with JavaScript engine enabled
• Foxit Reader browser plugin extension

Discovery Timeline

• 2021-05-10 - CVE CVE-2021-21822 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-21822

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 the context of Foxit PDF Reader's JavaScript engine, this flaw allows attackers to manipulate memory allocation patterns through carefully constructed PDF documents containing malicious JavaScript code.

The exploitation path requires user interaction—specifically, the victim must open a malicious PDF file or navigate to a compromised website while the Foxit browser plugin is enabled. Once triggered, the vulnerability allows the reuse of previously freed memory regions, which attackers can leverage to gain control over program execution flow and ultimately achieve arbitrary code execution with the privileges of the current user.

Root Cause

The root cause of this vulnerability lies in improper memory management within Foxit PDF Reader's JavaScript engine. When processing certain JavaScript operations embedded in PDF documents, the application fails to properly validate memory references before reuse. This occurs when memory is deallocated but the corresponding pointer is not nullified, leaving a dangling pointer that can be dereferenced during subsequent operations. The JavaScript engine's handling of object lifecycle management does not adequately protect against scenarios where freed memory blocks are reallocated and subsequently accessed through stale references.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker can exploit this vulnerability through multiple delivery mechanisms:

1. Malicious PDF Documents: Crafting a PDF file containing embedded JavaScript code designed to trigger the use-after-free condition, then distributing it via email attachments, file-sharing platforms, or download links.

2. Drive-by Download Attacks: If the Foxit browser plugin extension is enabled, attackers can host malicious PDF content on compromised or attacker-controlled websites, triggering the vulnerability when victims visit the page.

3. Social Engineering: Combining the technical exploit with social engineering tactics to convince targets to open the malicious PDF document.

The vulnerability exploits the trust relationship users have with PDF documents, which are commonly used for business communications and official documentation. For detailed technical analysis, refer to the Talos Intelligence Vulnerability Report.

Detection Methods for CVE-2021-21822

Indicators of Compromise

• Unusual JavaScript execution patterns within PDF documents processed by Foxit Reader
• Foxit Reader processes exhibiting abnormal memory allocation behavior or crashes
• Suspicious PDF files with heavily obfuscated or encoded JavaScript content
• Unexpected child processes spawned by FoxitReader.exe or FoxitPDFReader.exe

Detection Strategies

• Monitor for suspicious PDF files with embedded JavaScript that perform unusual memory operations
• Implement endpoint detection rules for Foxit Reader processes attempting to execute shell commands or spawn child processes
• Deploy behavioral analysis to detect memory corruption exploitation attempts targeting PDF readers
• Analyze network traffic for PDF downloads from untrusted or newly registered domains

Monitoring Recommendations

• Enable enhanced logging for Foxit Reader application events and crashes
• Configure SIEM rules to correlate PDF file access with subsequent suspicious process behavior
• Monitor endpoint telemetry for use-after-free exploitation indicators such as heap spray patterns
• Track browser plugin activity related to Foxit PDF Reader

How to Mitigate CVE-2021-21822

Immediate Actions Required

• Update Foxit PDF Reader to the latest available version that addresses this vulnerability
• Disable JavaScript execution in Foxit Reader via Edit → Preferences → JavaScript → uncheck "Enable JavaScript Actions"
• Disable or remove the Foxit browser plugin extension if not required for business operations
• Implement application whitelisting to prevent unauthorized code execution

Patch Information

Foxit Software has addressed this vulnerability in subsequent releases of Foxit PDF Reader. Organizations should upgrade to the latest version available from the official Foxit website. For detailed vulnerability information and patch guidance, review the Talos Intelligence Vulnerability Report.

Workarounds

• Disable JavaScript execution in Foxit Reader preferences as a primary mitigation
• Remove or disable the Foxit browser plugin to eliminate the drive-by attack vector
• Use alternative PDF readers that are not affected by this vulnerability until patching is complete
• Implement network-level filtering to scan and sanitize PDF attachments in email and web traffic

# Configuration example - Disable JavaScript in Foxit Reader via registry (Windows)
# Run as Administrator
reg add "HKCU\Software\Foxit Software\Foxit Reader\Preferences\Trust Manager" /v bEnableJS /t REG_DWORD /d 0 /f

# Verify the setting was applied
reg query "HKCU\Software\Foxit Software\Foxit Reader\Preferences\Trust Manager" /v bEnableJS