CVE-2021-40725
📋 TL;DR
This CVE describes a use-after-free vulnerability in Adobe Acrobat Reader DC that could allow arbitrary code execution when processing a malicious AcroForm listbox. It affects users of specific older versions across multiple release tracks, requiring them to open a malicious PDF file or visit a malicious webpage. Successful exploitation runs code in the context of the current user, potentially compromising the system.
💻 Affected Systems
- Adobe Acrobat Reader DC
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise via arbitrary code execution, leading to data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Malware infection or data exfiltration after a user opens a malicious PDF, often delivered via phishing.
If Mitigated
No impact if patched or if users avoid opening untrusted PDFs; limited to user-level access if exploited.
🎯 Exploit Status
Exploitation requires user interaction (opening a malicious file) and may involve memory manipulation; weaponization is likely due to the high impact and common attack vector.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Update to versions after 2021.005.20060, 2020.004.30006, or 2017.011.30199, as specified in Adobe advisory.
Vendor Advisory: https://helpx.adobe.com/security/products/acrobat/apsb21-55.html
Restart Required: Yes
Instructions:
1. Open Adobe Acrobat Reader DC. 2. Go to Help > Check for Updates. 3. Follow prompts to install the latest version. 4. Restart the application and system if required.
🔧 Temporary Workarounds
Disable JavaScript in Acrobat Reader
allReduces attack surface by disabling JavaScript, which may be used in exploitation chains.
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View for Untrusted Files
allOpens PDFs in a sandboxed mode to limit potential damage from malicious content.
Ensure Protected View is enabled in Security settings
🧯 If You Can't Patch
- Restrict user permissions to limit impact of code execution (e.g., run with least privilege).
- Implement application whitelisting to block unauthorized executables from running.
🔍 How to Verify
Check if Vulnerable:
Check the version in Acrobat Reader under Help > About Adobe Acrobat Reader DC and compare to affected ranges.Check Version:
On Windows: wmic product where name="Adobe Acrobat Reader DC" get versionVerify Fix Applied:
Confirm the version is updated beyond the vulnerable versions listed in the advisory.📡 Detection & Monitoring
Log Indicators:
- Unusual process spawns from Acrobat Reader (e.g., cmd.exe, powershell.exe)
- Crash logs or error reports from Acrobat Reader
Network Indicators:
- Outbound connections to suspicious IPs after opening a PDF
- DNS requests for known malicious domains
SIEM Query:
EventID=4688 AND ParentProcessName="AcroRd32.exe" AND NewProcessName IN ("cmd.exe", "powershell.exe")