CVE-2021-39838
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC's AcroForm buttonGetCaption action. If exploited, it allows arbitrary code execution in the context of the current user when a victim opens a malicious PDF file. Users of Adobe Acrobat Reader DC versions 2021.005.20060 and earlier, 2020.004.30006 and earlier, and 2017.011.30199 and earlier are affected.
💻 Affected Systems
- Adobe Acrobat Reader DC
📦 What is this software?
Acrobat by Adobe
Acrobat by Adobe
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining the same privileges as the logged-in user, potentially leading to data theft, ransomware deployment, or lateral movement.
Likely Case
Malicious code execution leading to malware installation, credential theft, or data exfiltration from the affected system.
If Mitigated
Limited impact with proper application sandboxing, least privilege user accounts, and network segmentation preventing lateral movement.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file). No public exploit code was available at disclosure time.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2021.005.20061, 2020.004.30007, 2017.011.30200
Vendor Advisory: https://helpx.adobe.com/security/products/acrobat/apsb21-55.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 when prompted.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allPrevents exploitation by disabling JavaScript execution in PDF files
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpen untrusted PDFs in Protected View mode to limit potential damage
File > Open > Select 'Protected View' option when opening files
🧯 If You Can't Patch
- Implement application whitelisting to block unauthorized PDF readers
- Deploy network segmentation to limit lateral movement from compromised systems
🔍 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 versionVerify Fix Applied:
Verify version is 2021.005.20061 or later, 2020.004.30007 or later, or 2017.011.30200 or later📡 Detection & Monitoring
Log Indicators:
- Adobe Reader crash logs with memory access violations
- Unexpected child processes spawned from Adobe Reader
Network Indicators:
- Outbound connections from Adobe Reader process to unknown IPs
- DNS requests for suspicious domains from Adobe Reader
SIEM Query:
process_name:"AcroRd32.exe" AND (event_id:1000 OR event_id:1001) AND memory_access_violation