CVE-2021-28640
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC that allows authenticated attackers to execute arbitrary code when a victim opens a malicious PDF file. The vulnerability affects multiple versions across different release tracks. Successful exploitation requires user interaction through opening a malicious document.
💻 Affected Systems
- Adobe Acrobat Reader DC
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining the same privileges as the current user, potentially leading to data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Malicious actors sending phishing emails with weaponized PDF attachments that execute malware when opened by users, leading to credential theft or initial network access.
If Mitigated
With proper patching and security controls, impact is limited to isolated incidents that can be contained through endpoint detection and response systems.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious file) and authenticated access to the target system. The use-after-free vulnerability type is commonly exploited in PDF readers.
🛠️ 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 the update prompts. 4. Restart the application when complete. Alternatively, download the latest version from Adobe's website.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allPrevents JavaScript-based exploitation vectors that might be used in conjunction with this vulnerability
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allForce all PDFs to open in Protected View mode to limit potential damage
Edit > Preferences > Security (Enhanced) > Check 'Enable Protected View at startup'
🧯 If You Can't Patch
- Implement application whitelisting to block unauthorized PDF readers
- Deploy email filtering to block PDF attachments and use web proxies to block PDF downloads
🔍 How to Verify
Check if Vulnerable:
Check Adobe Reader version via Help > About Adobe Acrobat Reader DC and compare against affected versionsCheck Version:
On Windows: "C:\Program Files\Adobe\Acrobat Reader DC\Reader\AcroRd32.exe" /? | findstr /i versionVerify Fix Applied:
Verify version is 2021.005.20055 or higher, 2020.004.30006 or higher, or 2017.011.30198 or higher📡 Detection & Monitoring
Log Indicators:
- Adobe Reader crash logs with memory access violations
- Windows Event Logs showing application crashes (Event ID 1000)
Network Indicators:
- Unusual outbound connections from Adobe Reader process
- PDF file downloads from suspicious sources
SIEM Query:
source="*acrobat*" AND (event_id=1000 OR "access violation" OR "use-after-free")