CVE-2021-21035
📋 TL;DR
CVE-2021-21035 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. Users of affected Adobe Acrobat Reader DC versions 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 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 proper application sandboxing, restricted user privileges, and network segmentation preventing lateral movement.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious PDF). No authentication needed for the vulnerability itself.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2020.013.20074 (Windows), 2020.001.30018 (macOS), 2017.011.30188 (Windows 7/8.1)
Vendor Advisory: https://helpx.adobe.com/security/products/acrobat/apsb21-09.html
Restart Required: No
Instructions:
1. Open Adobe Acrobat Reader DC. 2. Go to Help > Check for Updates. 3. Follow prompts to install latest version. 4. Alternatively, download and install from Adobe website.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allPrevents JavaScript-based exploitation vectors
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allOpens PDFs in sandboxed environment
File > Preferences > Security (Enhanced) > Enable Protected View for all files
🧯 If You Can't Patch
- Restrict PDF opening to trusted sources only via application control policies
- Implement network segmentation to limit lateral movement from compromised systems
🔍 How to Verify
Check if Vulnerable:
Check Adobe Reader version via Help > About Adobe Acrobat Reader DCCheck Version:
Windows: wmic product where name="Adobe Acrobat Reader DC" get versionVerify Fix Applied:
Verify version is newer than affected versions listed in advisory📡 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 unknown IPs
- DNS requests for suspicious domains from Adobe Reader
SIEM Query:
process_name="AcroRd32.exe" AND (event_id=1000 OR parent_process!="explorer.exe")