CVE-2021-21870
📋 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. This affects users of Foxit PDF Reader version 10.1.4.37651, particularly when the browser plugin extension is enabled. Attackers can exploit this by tricking users into opening specially crafted PDF documents.
💻 Affected Systems
- Foxit PDF Reader
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining the same privileges as the user, potentially leading to data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Arbitrary code execution in the context of the current user, allowing malware installation, credential harvesting, and lateral movement within the network.
If Mitigated
Limited impact if user has minimal privileges, application sandboxing is effective, and network segmentation prevents lateral movement.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious PDF). Technical details and proof-of-concept are publicly available in the Talos Intelligence report.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 10.1.5 or later
Vendor Advisory: https://www.foxit.com/support/security-bulletins.html
Restart Required: Yes
Instructions:
1. Download latest version from Foxit website. 2. Run installer. 3. Restart system. 4. Verify update in Help > About.
🔧 Temporary Workarounds
Disable JavaScript in Foxit Reader
allPrevents exploitation by disabling JavaScript execution in PDF files
Open Foxit Reader > File > Preferences > JavaScript > Uncheck 'Enable JavaScript'
Disable Browser Plugin
allPrevents exploitation via malicious websites
Browser settings > Extensions/Add-ons > Disable Foxit PDF Reader plugin
🧯 If You Can't Patch
- Use alternative PDF readers that are not vulnerable
- Block PDF files from untrusted sources at email/web gateways
- Implement application whitelisting to prevent unauthorized executables
🔍 How to Verify
Check if Vulnerable:
Check Foxit Reader version in Help > About. If version is 10.1.4.37651 or earlier, system is vulnerable.Check Version:
On Windows: wmic product where name="Foxit Reader" get versionVerify Fix Applied:
Verify version is 10.1.5 or later in Help > About. Test with known safe PDF containing JavaScript.📡 Detection & Monitoring
Log Indicators:
- Foxit Reader crash logs with memory access violations
- Unexpected child processes spawned from Foxit Reader
- Network connections initiated by Foxit Reader process
Network Indicators:
- Unexpected outbound connections from user workstations after PDF opening
- DNS requests to suspicious domains following PDF access
SIEM Query:
process_name="FoxitReader.exe" AND (event_id=1000 OR child_process_creation=true)