CVE-2021-21021
📋 TL;DR
CVE-2021-21021 is 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. This affects users of Adobe Acrobat Reader DC across multiple versions on various operating systems.
💻 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, potentially leading to data theft, ransomware deployment, or 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 proper application sandboxing, minimal user privileges, and network segmentation preventing lateral movement.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious PDF). The vulnerability is in the wild and actively exploited according to Adobe's advisory.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2020.013.20074 (and later), 2020.001.30018 (and later), 2017.011.30188 (and 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 exploitation by disabling JavaScript execution in PDF files
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpen all PDFs in Protected View mode to limit potential damage
File > Preferences > Security (Enhanced) > Enable Protected View for all files
🧯 If You Can't Patch
- Restrict PDF file handling to alternative PDF readers that are not vulnerable
- Implement application whitelisting to block unauthorized executables from running
🔍 How to Verify
Check if Vulnerable:
Check Adobe Reader version in Help > About Adobe Acrobat Reader DC and compare to affected versionsCheck Version:
On Windows: wmic product where name="Adobe Acrobat Reader DC" get versionVerify Fix Applied:
Verify version is updated to 2020.013.20074 or later, 2020.001.30018 or later, or 2017.011.30188 or later📡 Detection & Monitoring
Log Indicators:
- Adobe Reader crash logs with memory access violations
- Windows Event Logs showing unexpected process creation from AcroRd32.exe
Network Indicators:
- Outbound connections from Adobe Reader process to unknown IPs
- DNS requests for suspicious domains from user workstations
SIEM Query:
process_name:"AcroRd32.exe" AND (event_id:1 OR event_id:4688) AND parent_process_name NOT IN ("explorer.exe", "cmd.exe")