CVE-2024-56522: TCPDF Hash Comparison Vulnerability

CVE-2024-56522 Overview

CVE-2024-56522 is a timing attack vulnerability discovered in TCPDF, a popular PHP library for generating PDF documents. The vulnerability exists in the unserializeTCPDFtag function, which uses PHP's loose comparison operator (!=) instead of a strict, constant-time comparison function when validating TCPDF tag hashes. This implementation flaw allows attackers to potentially bypass hash validation through timing-based side-channel attacks, leading to information disclosure.

Critical Impact

Attackers can exploit the timing differences in hash comparisons to potentially extract sensitive hash values, enabling unauthorized access to serialized TCPDF tag data and bypassing security controls.

Affected Products

• TCPDF versions prior to 6.8.0
• Applications and CMS platforms using vulnerable TCPDF libraries
• Debian-based systems with unpatched TCPDF packages

Discovery Timeline

• 2024-12-23 - TCPDF Project releases version 6.8.0 with security patch
• 2024-12-27 - CVE-2024-56522 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2024-56522

Vulnerability Analysis

This vulnerability stems from two distinct but related security weaknesses in the TCPDF library's tag unserialization mechanism. The unserializeTCPDFtag function is responsible for validating and processing serialized TCPDF tag data by comparing computed hash values against stored hash values.

The first weakness involves the use of PHP's loose comparison operator (!=) instead of the strict comparison operator (!==). In PHP, loose comparisons can lead to unexpected type juggling behavior, where values of different types may be considered equal due to automatic type coercion. This is classified as CWE-843 (Type Confusion), where improper type handling can lead to security bypasses.

The second and more critical weakness is the absence of a constant-time comparison function. Standard string comparison operations in PHP return early when a mismatch is found, creating measurable timing differences. An attacker can exploit these timing variations to deduce information about the expected hash value character by character, effectively enabling a side-channel timing attack.

Root Cause

The root cause of this vulnerability lies in the implementation of hash comparison logic within the unserializeTCPDFtag function. The use of != for comparing cryptographic hash values introduces both type confusion risks and timing oracle vulnerabilities. Cryptographic operations require constant-time comparisons to prevent side-channel attacks, but the original implementation used standard PHP comparison operators that do not provide timing-safe guarantees.

Attack Vector

The attack is network-accessible and requires no authentication or user interaction. An attacker can send crafted requests to applications using vulnerable TCPDF versions and measure response times to infer information about valid hash values. By systematically varying input and analyzing timing patterns, attackers can potentially reconstruct valid TCPDF tag hashes and bypass integrity checks on serialized data.

The vulnerability primarily impacts confidentiality, as successful exploitation could reveal sensitive hash values or enable manipulation of TCPDF tag data that would otherwise be protected by hash validation.

// Vulnerable code (before patch)
$hlen = intval(substr($data, 0, $hpos));
$hash = substr($data, $hpos + 1, $hlen);
$encoded = substr($data, $hpos + 2 + $hlen);
if ($hash != $this->hashTCPDFtag($encoded)) {
    $this->Error('Invalid parameters');
}
return json_decode(urldecode($encoded), true);

// Patched code (version 6.8.0)
$hlen = intval(substr($data, 0, $hpos));
$hash = substr($data, $hpos + 1, $hlen);
$encoded = substr($data, $hpos + 2 + $hlen);
if (!hash_equals( $this->hashTCPDFtag($encoded), $hash)) {
    $this->Error('Invalid parameters');
}
return json_decode(urldecode($encoded), true);

Source: GitHub Commit Reference

Detection Methods for CVE-2024-56522

Indicators of Compromise

• Unusual patterns of requests with varying TCPDF tag data targeting PDF generation endpoints
• High-frequency requests with incrementally modified hash values suggesting timing analysis
• Error logs showing repeated "Invalid parameters" messages from TCPDF tag processing
• Anomalous request timing patterns indicating potential timing-based reconnaissance

Detection Strategies

• Implement request rate limiting on PDF generation endpoints to detect and block timing attack attempts
• Deploy application-level monitoring to track TCPDF-related error frequencies and patterns
• Use Web Application Firewalls (WAF) configured to detect suspicious request patterns targeting PDF functionality
• Enable detailed logging for TCPDF operations to identify potential exploitation attempts

Monitoring Recommendations

• Monitor for unusual response time variations in PDF generation functionality
• Track error rates in TCPDF tag processing across application logs
• Implement alerting for high-volume requests targeting document generation endpoints
• Conduct periodic vulnerability scanning to identify applications using outdated TCPDF versions

How to Mitigate CVE-2024-56522

Immediate Actions Required

• Upgrade TCPDF to version 6.8.0 or later immediately across all environments
• Audit all applications and CMS platforms for embedded or bundled TCPDF dependencies
• Review access controls on PDF generation functionality to limit exposure
• Implement additional request validation and rate limiting as defense-in-depth measures

Patch Information

The TCPDF project has addressed this vulnerability in version 6.8.0, released on 2024-12-23. The fix replaces the insecure loose comparison with PHP's hash_equals() function, which provides constant-time string comparison specifically designed for cryptographic hash validation. This eliminates both the type confusion risk and the timing side-channel vulnerability.

The patch can be reviewed in the official commit. For a complete overview of changes between versions, see the version comparison on GitHub.

Debian users should refer to the Debian LTS Security Announcement for distribution-specific update instructions.

Workarounds

• If immediate patching is not possible, consider temporarily disabling PDF generation features that rely on TCPDF tag serialization
• Implement network-level controls to restrict access to PDF generation endpoints from untrusted sources
• Deploy additional rate limiting specifically on endpoints utilizing TCPDF functionality
• Monitor for and block requests exhibiting timing attack patterns through WAF rules

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

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