CVE-2021-36493
📋 TL;DR
A buffer overflow vulnerability in the pdfimages utility of xpdf 4.03 allows attackers to crash the application by providing a specially crafted PDF file. This affects users who process untrusted PDF files with the pdfimages command-line tool. The vulnerability could potentially lead to denial of service or arbitrary code execution.
💻 Affected Systems
- xpdf
📦 What is this software?
Xpdf by Xpdfreader
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise if the attacker can control execution flow through the buffer overflow.
Likely Case
Application crash (denial of service) when processing malicious PDF files.
If Mitigated
Limited impact if proper input validation and sandboxing are implemented.
🎯 Exploit Status
Exploitation requires the victim to run pdfimages on a malicious PDF file. No authentication needed for local exploitation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: xpdf 4.04
Vendor Advisory: https://forum.xpdfreader.com/viewtopic.php?f=3&t=42160
Restart Required: No
Instructions:
1. Download xpdf 4.04 or later from https://www.xpdfreader.com/download.html
2. Uninstall current xpdf version
3. Install the updated version
4. Verify installation with 'pdfimages -v'
🔧 Temporary Workarounds
Disable pdfimages usage
linuxRemove or restrict execute permissions on the pdfimages binary
sudo chmod -x /usr/bin/pdfimages
sudo mv /usr/bin/pdfimages /usr/bin/pdfimages.disabled
Use alternative PDF tools
linuxReplace pdfimages with alternative PDF processing tools like poppler-utils
sudo apt-get install poppler-utils
sudo yum install poppler-utils
🧯 If You Can't Patch
- Implement strict input validation for PDF files before processing
- Run pdfimages in a sandboxed environment with limited privileges
🔍 How to Verify
Check if Vulnerable:
Run 'pdfimages -v' and check if version is 4.03Check Version:
pdfimages -vVerify Fix Applied:
Run 'pdfimages -v' and confirm version is 4.04 or later📡 Detection & Monitoring
Log Indicators:
- Segmentation fault errors in system logs
- Abnormal termination of pdfimages process
Network Indicators:
- Unusual PDF file downloads followed by pdfimages execution
SIEM Query:
process_name="pdfimages" AND (event_type="process_crash" OR exit_code="139")