CVE-2021-40420
📋 TL;DR
A use-after-free vulnerability in Foxit PDF Reader's JavaScript engine allows arbitrary code execution when a user opens a malicious PDF file or visits a malicious website with the browser plugin enabled. This affects users of Foxit PDF Reader version 11.1.0.52543 who open untrusted PDF documents.
💻 Affected Systems
- Foxit PDF Reader
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining complete control of the victim's computer, enabling data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Malware installation leading to credential theft, data exfiltration, or system disruption through common attack vectors like phishing emails with malicious attachments.
If Mitigated
No impact if users avoid opening untrusted PDFs, disable JavaScript in PDF reader, or have updated to patched version.
🎯 Exploit Status
Exploitation requires user interaction (opening file or visiting malicious site); technical details and proof-of-concept are publicly available in Talos reports.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 11.1.1 or later
Vendor Advisory: https://www.foxit.com/support/security-bulletins.html
Restart Required: Yes
Instructions:
1. Open Foxit PDF Reader. 2. Go to Help > Check for Updates. 3. Follow prompts to install latest version (11.1.1+). 4. Restart computer after installation.
🔧 Temporary Workarounds
Disable JavaScript in PDF Reader
allPrevents JavaScript execution in PDF files, blocking the primary exploitation vector.
Open Foxit Reader > File > Preferences > JavaScript > Uncheck 'Enable JavaScript'
Disable Browser Plugin
allPrevents web-based exploitation through malicious websites.
Browser settings > Extensions/Add-ons > Disable Foxit PDF Reader plugin
🧯 If You Can't Patch
- Restrict PDF file opening to trusted sources only through application whitelisting or policy controls.
- Use alternative PDF readers that are not vulnerable to this specific CVE.
🔍 How to Verify
Check if Vulnerable:
Check Foxit PDF Reader version in Help > About. If version is exactly 11.1.0.52543, system is vulnerable.Check Version:
On Windows: wmic product where name="Foxit Reader" get versionVerify Fix Applied:
Confirm version is 11.1.1 or higher in Help > About.📡 Detection & Monitoring
Log Indicators:
- Unexpected Foxit Reader crashes
- Process creation from Foxit Reader with suspicious command lines
Network Indicators:
- Outbound connections from Foxit Reader process to unknown IPs
- DNS requests for suspicious domains after PDF opening
SIEM Query:
process_name:"FoxitReader.exe" AND (event_id:1 OR parent_process_name:"FoxitReader.exe")