CVE-2021-39842
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC 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. Successful exploitation requires user interaction to open a specially crafted file.
💻 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 with attacker gaining full control of the victim's computer in the context of the current user, potentially leading to data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Malicious actors send phishing emails with malicious PDF attachments that, when opened, execute malware on the victim's system, leading to credential theft, data exfiltration, or persistence establishment.
If Mitigated
With proper security controls like application whitelisting, least privilege accounts, and email filtering, the impact is limited to potential denial of service if the application crashes.
🎯 Exploit Status
Exploitation requires user interaction to open a malicious file. The vulnerability is in memory management, making reliable exploitation moderately complex but feasible for skilled attackers.
🛠️ 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. 5. Verify version is updated to patched version.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allDisabling JavaScript reduces attack surface as many PDF exploits rely on JavaScript execution
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allEnable Protected View for files from potentially unsafe locations
Edit > Preferences > Security (Enhanced) > Enable Protected View at startup
🧯 If You Can't Patch
- Implement application control/whitelisting to block unauthorized PDF readers
- Use alternative PDF viewers that are not affected by this vulnerability
🔍 How to Verify
Check if Vulnerable:
Check Adobe Acrobat Reader DC version via Help > About Adobe Acrobat Reader DCCheck Version:
On 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:
- Application crashes of acrobat.exe or AcroRd32.exe
- Unusual process creation from Adobe Reader processes
- Security event logs showing PDF file execution from untrusted sources
Network Indicators:
- Outbound connections from Adobe Reader processes to suspicious IPs
- DNS queries for known malicious domains following PDF file opens
SIEM Query:
source="*security*" AND (process_name="acrobat.exe" OR process_name="AcroRd32.exe") AND (event_id="4688" OR event_id="1") AND parent_process_name="explorer.exe"