CVE-2021-21028
📋 TL;DR
CVE-2021-21028 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 entire system. 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, data theft, ransomware deployment, and lateral movement within the network.
Likely Case
Malware installation leading to data exfiltration, credential theft, or system disruption.
If Mitigated
Limited impact with proper sandboxing and application controls preventing successful exploitation.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file). No authentication needed for initial access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2020.013.20075, 2020.001.30019, 2017.011.30189 or later
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 latest version. 4. Restart computer after installation.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allPrevents JavaScript-based exploitation vectors
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpen PDFs in sandboxed Protected View mode
File > Open > Select 'Protected View' option
🧯 If You Can't Patch
- Restrict PDF file handling to alternative PDF readers
- Implement application whitelisting to block Acrobat Reader 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 2020.013.20075+, 2020.001.30019+, or 2017.011.30189+📡 Detection & Monitoring
Log Indicators:
- Acrobat crash logs with memory access violations
- Unexpected child processes spawned from Acrobat.exe
Network Indicators:
- Outbound connections from Acrobat.exe to unknown IPs
- DNS requests for suspicious domains after PDF opening
SIEM Query:
process_name:"Acrobat.exe" AND (event_id:1000 OR parent_process_name:"Acrobat.exe")