CVE-2021-21058
📋 TL;DR
CVE-2021-21058 is a memory corruption 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, requiring user interaction through opening a crafted file. This affects multiple versions of Acrobat Reader DC across Windows and macOS platforms.
💻 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, enabling data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Malware installation leading to credential theft, data exfiltration, or system disruption for individual users who open malicious PDFs.
If Mitigated
Limited impact with proper patching and security controls, potentially isolated to sandboxed environments or blocked by endpoint protection.
🎯 Exploit Status
Exploitation requires user interaction (opening malicious PDF) but is otherwise straightforward. Memory corruption vulnerabilities in PDF readers are commonly weaponized in phishing campaigns.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2020.013.20074 (for 2020 track), 2020.001.30018 (for 2020 track), 2017.011.30188 (for 2017 track) - update to versions after these
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 available updates. 4. Restart the application. Alternatively, download latest version from Adobe website.
🔧 Temporary Workarounds
Disable JavaScript in Adobe Reader
allDisabling JavaScript reduces attack surface as many PDF exploits rely on JavaScript execution
Edit > Preferences > JavaScript > Uncheck 'Enable Acrobat JavaScript'
Use Protected View
allEnable Protected View for files from potentially unsafe locations
Edit > Preferences > Security (Enhanced) > Enable Protected View for files originating from the Internet
🧯 If You Can't Patch
- Implement application whitelisting to block unauthorized PDF readers
- Deploy endpoint detection and response (EDR) to monitor for suspicious PDF execution behavior
🔍 How to Verify
Check if Vulnerable:
Check Adobe Reader version via Help > About Adobe Acrobat Reader DC and compare with affected versionsCheck Version:
On Windows: wmic product where name="Adobe Acrobat Reader DC" get version
On macOS: /Applications/Adobe\ Acrobat\ Reader\ DC.app/Contents/Info.plist | grep -A1 CFBundleShortVersionStringVerify Fix Applied:
Verify version is newer than affected versions: 2020.013.20074, 2020.001.30018, or 2017.011.30188 depending on your track📡 Detection & Monitoring
Log Indicators:
- Unusual process creation from AcroRd32.exe or Acrobat.exe
- Multiple crash reports from Adobe Reader
- Suspicious file downloads with .pdf extension
Network Indicators:
- Unexpected outbound connections from Adobe Reader process
- DNS requests for known malicious domains following PDF opening
SIEM Query:
source="*security*" (event_id=4688 OR process_name="AcroRd32.exe" OR process_name="Acrobat.exe") AND (parent_process!="explorer.exe" OR command_line="*http*")