CVE-2024-56521
📋 TL;DR
This vulnerability in TCPDF before version 6.8.0 disables SSL certificate verification when libcurl is used, allowing man-in-the-middle attacks. Any application using TCPDF with libcurl for HTTPS connections is affected, potentially exposing sensitive data transmitted via PDF generation.
💻 Affected Systems
- TCPDF
📦 What is this software?
Tcpdf by Tcpdf Project
⚠️ Risk & Real-World Impact
Worst Case
Attackers can intercept and manipulate all HTTPS traffic between the application and external servers, potentially stealing credentials, session tokens, or sensitive data being transmitted during PDF generation processes.
Likely Case
Man-in-the-middle attacks intercepting API calls, webhook notifications, or external resource fetches during PDF generation, leading to data leakage or injection of malicious content.
If Mitigated
With proper network segmentation and TLS inspection, risk is limited to specific PDF generation endpoints, though data integrity issues may still occur.
🎯 Exploit Status
Exploitation requires man-in-the-middle position on network path between vulnerable server and external HTTPS endpoints it connects to.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 6.8.0
Vendor Advisory: https://github.com/tecnickcom/TCPDF/commit/aab43ab0a824e956276141a28a24c7c0be20f554
Restart Required: No
Instructions:
1. Update TCPDF to version 6.8.0 or later. 2. Replace all TCPDF files with new version. 3. Test PDF generation functionality.
🔧 Temporary Workarounds
Disable libcurl usage
allConfigure TCPDF to use alternative HTTP clients instead of libcurl
Modify TCPDF configuration to use file_get_contents or other HTTP clients
Network segmentation
allIsolate PDF generation servers from untrusted networks
Implement firewall rules to restrict outbound HTTPS connections from PDF servers
🧯 If You Can't Patch
- Implement strict outbound firewall rules to limit which HTTPS endpoints the server can connect to
- Deploy TLS inspection/proxy between vulnerable servers and external HTTPS endpoints
🔍 How to Verify
Check if Vulnerable:
Check TCPDF version in codebase or via composer show tecnickcom/tcpdf. If version is below 6.8.0 and libcurl is used for HTTPS, system is vulnerable.Check Version:
composer show tecnickcom/tcpdf | grep version OR check TCPDF_VERSION constant in tcpdf.phpVerify Fix Applied:
Confirm TCPDF version is 6.8.0 or higher and test HTTPS connections during PDF generation with invalid certificates (should fail).📡 Detection & Monitoring
Log Indicators:
- Unexpected HTTPS connections during PDF generation
- Failed SSL certificate validations
- Unusual external resource fetches
Network Indicators:
- Man-in-the-middle activity between PDF server and external HTTPS endpoints
- SSL/TLS interception attempts
SIEM Query:
source="web_logs" AND (uri="*.pdf" OR user_agent="*TCPDF*") AND (status_code=5xx OR bytes_transferred>threshold)