CVE-2021-21039
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC that allows arbitrary code execution when a user opens a malicious PDF file. An unauthenticated attacker can exploit this to run code with the victim's privileges. Users of affected Acrobat Reader DC versions are at risk.
💻 Affected Systems
- Adobe Acrobat Reader DC
📦 What is this software?
Acrobat by Adobe
Acrobat by Adobe
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise via arbitrary code execution with current user privileges, potentially leading to data theft, ransomware deployment, or lateral movement.
Likely Case
Malware installation or data exfiltration when users open malicious PDFs from phishing emails or compromised websites.
If Mitigated
Limited impact if users avoid opening untrusted PDFs and have proper endpoint protection.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious PDF). No public proof-of-concept was identified at disclosure time.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2020.013.20074 (Continuous Track), 2020.001.30018 (Classic Track), 2017.011.30188 (Classic 2017 Track) and later
Vendor Advisory: https://helpx.adobe.com/security/products/acrobat/apsb21-09.html
Restart Required: Yes
Instructions:
1. Open Adobe Acrobat Reader DC. 2. Go to Help > Check for Updates. 3. Follow prompts to install available updates. 4. Restart the application.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allReduces attack surface by disabling JavaScript execution in PDFs
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpen untrusted PDFs in Protected View mode
File > Open > Select 'Protected View' option when opening untrusted files
🧯 If You Can't Patch
- Block PDF files from untrusted sources at email/web gateways
- Implement application whitelisting to prevent unauthorized executables
🔍 How to Verify
Check if Vulnerable:
Check Adobe Reader version via Help > About Adobe Acrobat Reader DCCheck Version:
Windows: wmic product where name="Adobe Acrobat Reader DC" get version
macOS: /Applications/Adobe\ Acrobat\ Reader\ DC.app/Contents/Info.plist | grep -A1 CFBundleShortVersionStringVerify Fix Applied:
Verify version is 2020.013.20074 or later (Continuous), 2020.001.30018 or later (Classic 2020), or 2017.011.30188 or later (Classic 2017)📡 Detection & Monitoring
Log Indicators:
- Adobe Reader crash logs with memory access violations
- Unexpected child processes spawned from AcroRd32.exe
Network Indicators:
- Outbound connections from Adobe Reader to suspicious IPs
- DNS requests for known malicious domains after PDF opening
SIEM Query:
process_name:"AcroRd32.exe" AND (event_id:1000 OR event_id:1001) OR parent_process:"AcroRd32.exe" AND process_creation