CVE-2021-21088
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC that allows arbitrary code execution when a user opens a malicious PDF file. An unauthenticated attacker can exploit this to run code with the victim's privileges. All users running affected versions of Acrobat Reader DC are vulnerable.
💻 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 and potentially pivoting to other systems on the network.
Likely Case
Malware installation, data theft, or ransomware deployment on individual workstations where users open malicious PDFs.
If Mitigated
Limited impact with proper application sandboxing, endpoint protection, and user awareness training preventing successful exploitation.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file) but no authentication. Use-after-free vulnerabilities are commonly weaponized in PDF-based attacks.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2020.013.20074 (Windows), 2020.001.30018 (macOS), 2017.011.30188 (Windows 7/8.1) and later versions
Vendor Advisory: https://helpx.adobe.com/security/products/acrobat/apsb21-09.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-based exploitation vectors that might leverage this vulnerability
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpen PDFs in Protected View mode to limit potential damage
File > Properties > Security > Enable Protected View for all files
🧯 If You Can't Patch
- Block PDF files at network perimeter and email gateways
- Implement application whitelisting to prevent unauthorized executables from running
🔍 How to Verify
Check if Vulnerable:
Check Adobe Reader version via Help > About Adobe Acrobat Reader DC and compare to affected versionsCheck Version:
Windows: wmic product where name="Adobe Acrobat Reader DC" get version
macOS: /Applications/Adobe\ Acrobat\ Reader\ DC.app/Contents/Info.plist | grep -A1 CFBundleShortVersionStringVerify Fix Applied:
Verify version is newer than affected versions: Windows > 2020.013.20074, macOS > 2020.001.30018, Windows 7/8.1 > 2017.011.30188📡 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 suspicious IPs
- DNS requests for known malicious domains from Adobe Reader
SIEM Query:
process_name:"AcroRd32.exe" AND (event_id:1000 OR event_id:1001) AND exception_code:0xc0000005