CVE-2024-56521: TCPDF SSL Verification Vulnerability

CVE-2024-56521 Overview

CVE-2024-56521 is a critical SSL/TLS certificate validation bypass vulnerability discovered in TCPDF, a popular open-source PHP library used for generating PDF documents. The vulnerability exists in versions prior to 6.8.0, where the library unsafely configures CURLOPT_SSL_VERIFYHOST and CURLOPT_SSL_VERIFYPEER options when using libcurl for network operations. This misconfiguration effectively disables SSL/TLS certificate verification, allowing attackers to perform man-in-the-middle (MITM) attacks against applications using TCPDF to fetch remote resources.

Critical Impact

Applications using vulnerable TCPDF versions may be susceptible to man-in-the-middle attacks, enabling interception and manipulation of data fetched over HTTPS connections, potentially leading to remote code execution or sensitive data exposure.

Affected Products

• TCPDF versions prior to 6.8.0
• Applications and CMS platforms integrating TCPDF for PDF generation
• PHP applications using TCPDF with libcurl for remote resource fetching

Discovery Timeline

• 2024-12-27 - CVE CVE-2024-56521 published to NVD
• 2025-04-21 - Last updated in NVD database

Technical Details for CVE-2024-56521

Vulnerability Analysis

This vulnerability falls under CWE-295 (Improper Certificate Validation), a class of cryptographic weaknesses where applications fail to properly verify the authenticity of SSL/TLS certificates during secure communications. In TCPDF versions before 6.8.0, when the library utilizes libcurl to fetch remote resources such as images or fonts for PDF generation, the cURL options responsible for SSL certificate verification are configured insecurely.

The improper configuration of CURLOPT_SSL_VERIFYHOST and CURLOPT_SSL_VERIFYPEER allows connections to proceed even when the remote server presents an invalid, expired, self-signed, or completely fraudulent SSL certificate. This fundamentally breaks the trust model of HTTPS communications and opens a significant attack surface for network-based adversaries.

Root Cause

The root cause of this vulnerability lies in the default cURL configuration within TCPDF's network handling code. Prior to version 6.8.0, the library either set CURLOPT_SSL_VERIFYPEER to false or CURLOPT_SSL_VERIFYHOST to 0 (or both), which instructs libcurl to skip SSL/TLS certificate chain validation and hostname verification respectively. This configuration was likely implemented to avoid SSL errors in development environments or misconfigured systems but creates severe security implications in production deployments.

Attack Vector

An attacker positioned in the network path between a TCPDF application and a remote resource server (e.g., through ARP spoofing, DNS hijacking, or compromised network infrastructure) can intercept HTTPS connections initiated by TCPDF. Since certificate validation is disabled, the attacker can present their own certificate and establish a successful TLS connection with the vulnerable application.

This enables several attack scenarios:

1. Data Interception: Capture sensitive data being transmitted to or from remote servers
2. Content Injection: Replace legitimate remote resources (images, fonts) with malicious payloads
3. Supply Chain Attacks: Inject malicious content into generated PDFs that could exploit downstream PDF readers

	/**
     * Array of default cURL options for curl_setopt_array.
     *
     * @var array<int, bool|int|string> cURL options.
     */
    protected const CURLOPT_DEFAULT = [
        CURLOPT_CONNECTTIMEOUT => 5,
        CURLOPT_MAXREDIRS => 5,
        CURLOPT_PROTOCOLS => CURLPROTO_HTTPS | CURLPROTO_HTTP | CURLPROTO_FTP | CURLPROTO_FTPS,
        CURLOPT_SSL_VERIFYHOST => 2,
        CURLOPT_SSL_VERIFYPEER => true,
        CURLOPT_TIMEOUT => 30,
        CURLOPT_USERAGENT => 'tcpdf',
    ];

    /**
     * Array of fixed cURL options for curl_setopt_array.
     *
     * @var array<int, bool|int|string> cURL options.
     */
    protected const CURLOPT_FIXED = [
        CURLOPT_FAILONERROR => true,
        CURLOPT_RETURNTRANSFER => true,
    ];

Source: GitHub TCPDF Commit

Detection Methods for CVE-2024-56521

Indicators of Compromise

• Unusual network traffic patterns from web servers to external resources without proper certificate validation
• PDF documents containing unexpected or modified remote resources
• Log entries indicating SSL/TLS certificate errors that were bypassed or ignored
• Network monitoring alerts for potential MITM activity targeting HTTPS connections from PHP applications

Detection Strategies

• Audit TCPDF installation version across all PHP applications using dependency scanning tools such as Composer audit
• Review application logs for SSL-related warnings or errors when TCPDF fetches remote resources
• Implement network monitoring to detect certificate validation anomalies on outbound HTTPS connections
• Use software composition analysis (SCA) tools to identify vulnerable TCPDF versions in codebases

Monitoring Recommendations

• Enable verbose logging for cURL operations in PHP applications to capture SSL verification events
• Monitor for outbound connections from web servers to unexpected external hosts
• Implement certificate pinning validation at the network layer for critical remote resources
• Deploy intrusion detection systems (IDS) to identify potential MITM attack patterns

How to Mitigate CVE-2024-56521

Immediate Actions Required

• Upgrade TCPDF to version 6.8.0 or later immediately across all affected applications
• Audit all PHP applications for TCPDF usage and verify installed versions using composer show tecnickcom/tcpdf
• Review and validate the K_CURLOPTS configuration array if using custom cURL options
• Implement network segmentation to limit exposure of vulnerable applications

Patch Information

The TCPDF development team addressed this vulnerability in version 6.8.0 by introducing secure default cURL options. The patch adds a new K_CURLOPTS configuration array that properly sets CURLOPT_SSL_VERIFYHOST to 2 and CURLOPT_SSL_VERIFYPEER to true, ensuring proper SSL/TLS certificate validation. The fix also includes additional security hardening such as protocol restrictions and connection timeout configurations.

For detailed changes, see the GitHub TCPDF Version Comparison and the specific security commit.

Workarounds

• If immediate upgrade is not possible, manually configure cURL options to enforce SSL verification before TCPDF operations
• Disable remote resource fetching in TCPDF by using only local resources for PDF generation
• Implement a proxy layer that enforces certificate validation for outbound HTTPS connections
• Use Web Application Firewall (WAF) rules to restrict outbound connections from affected applications

# Update TCPDF using Composer
composer require tecnickcom/tcpdf:^6.8.0

# Verify the installed version
composer show tecnickcom/tcpdf | grep versions

# Clear application cache after update
php artisan cache:clear  # For Laravel applications