CVE-2021-35981
📋 TL;DR
CVE-2021-35981 is a use-after-free vulnerability in Adobe Acrobat Reader DC that allows arbitrary code execution when a user opens a malicious PDF file. Attackers can exploit this to run code with the victim's privileges, potentially compromising the system. Users of affected Adobe Acrobat Reader DC versions are at risk.
💻 Affected Systems
- Adobe Acrobat Reader DC
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control of the victim's machine, data theft, ransomware deployment, and lateral movement within the network.
Likely Case
Malware installation leading to data exfiltration, credential theft, or system disruption for individual users who open malicious PDFs.
If Mitigated
Limited impact with only isolated user account compromise if proper application sandboxing and least privilege principles are enforced.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file) but no authentication. Use-after-free vulnerabilities typically require specific memory manipulation techniques.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2021.005.20055, 2020.004.30006, 2017.011.30198 or later
Vendor Advisory: https://helpx.adobe.com/security/products/acrobat/apsb21-51.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 JavaScript execution which may be used in exploitation chains
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpen PDFs in Protected View mode to limit potential damage
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 in Help > About Adobe Acrobat Reader DCCheck Version:
On Windows: wmic product where name="Adobe Acrobat Reader DC" get versionVerify Fix Applied:
Verify version is 2021.005.20055 or later, 2020.004.30006 or later, or 2017.011.30198 or later📡 Detection & Monitoring
Log Indicators:
- Unexpected process crashes of AcroRd32.exe
- Suspicious child processes spawned from AcroRd32.exe
Network Indicators:
- Outbound connections from AcroRd32.exe to unexpected destinations
- DNS requests for known malicious domains after PDF opening
SIEM Query:
process_name:"AcroRd32.exe" AND (event_id:1000 OR parent_process_name:"AcroRd32.exe")