CVE-2021-39840
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC's AcroForms processing that could allow arbitrary code execution when a user opens a malicious PDF file. The vulnerability affects multiple versions of Acrobat Reader DC across different release tracks. Attackers could exploit this to run malicious code with the privileges of the current user.
💻 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 via remote code execution leading to data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Malicious code execution leading to credential theft, data exfiltration, or lateral movement within the network.
If Mitigated
Limited impact due to application sandboxing, but potential for sandbox escape depending on other vulnerabilities.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file). No public exploit code available at disclosure.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2021.005.20061, 2020.004.30007, 2017.011.30200 or later
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.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allPrevents JavaScript-based exploitation vectors
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpens untrusted documents in sandboxed mode
File > Preferences > Security (Enhanced) > Enable Protected View for all files
🧯 If You Can't Patch
- Block PDF files from untrusted sources at network perimeter
- Implement application whitelisting to prevent unauthorized code execution
🔍 How to Verify
Check if Vulnerable:
Check Help > About Adobe Acrobat Reader DC and compare version against affected rangesCheck Version:
On Windows: wmic product where name="Adobe Acrobat Reader DC" get versionVerify Fix Applied:
Verify version is 2021.005.20061+, 2020.004.30007+, or 2017.011.30200+📡 Detection & Monitoring
Log Indicators:
- Acrobat crash logs with memory access violations
- Unexpected child processes spawned from Acrobat
Network Indicators:
- Outbound connections from Acrobat to unknown IPs
- DNS requests for suspicious domains after PDF opening
SIEM Query:
process_name:"AcroRd32.exe" AND (event_id:1000 OR event_id:1001) AND exception_code:0xc0000005