CVE-2021-39836
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC's AcroForm buttonGetIcon action processing. When exploited, it allows arbitrary code execution in the context of the current user by tricking them into opening a malicious PDF file. Users of affected Adobe 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
Full system compromise with attacker gaining the same privileges as the logged-in user, potentially leading to data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Malicious code execution leading to credential theft, data exfiltration, or installation of persistent malware on the victim's system.
If Mitigated
Limited impact with proper application sandboxing, privilege separation, and endpoint protection preventing successful exploitation.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file). No public proof-of-concept has been disclosed as of the advisory date.
🛠️ 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 which may be required for the vulnerability.
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpen PDFs in Protected View mode to prevent automatic script execution.
File > Open > Check 'Open in Protected View' or use default Protected View settings
🧯 If You Can't Patch
- Restrict PDF file opening to trusted sources only using application control policies.
- Implement network segmentation to limit lateral movement if exploitation occurs.
🔍 How to Verify
Check if Vulnerable:
Check Adobe Acrobat Reader DC version via Help > About Adobe Acrobat Reader DC and compare against affected versions.Check Version:
On Windows: wmic product where name="Adobe Acrobat Reader DC" get versionVerify Fix Applied:
Verify version is 2021.005.20061 or higher, 2020.004.30007 or higher, or 2017.011.30200 or higher.📡 Detection & Monitoring
Log Indicators:
- Unexpected process crashes of AcroRd32.exe
- Suspicious child processes spawned from AcroRd32.exe
Network Indicators:
- Unusual outbound connections from Acrobat Reader process
- DNS requests to suspicious domains after PDF opening
SIEM Query:
process_name:"AcroRd32.exe" AND (event_id:1000 OR parent_process_name:"AcroRd32.exe")