CVE-2024-56522

What it actually does

The vulnerability permits timing side-channel attacks against TCPDF tag hash verification because loose != comparison is used instead of a constant-time equality function.

Why it matters to the business

Attackers could forge valid tags, undermining the integrity of generated PDF documents and potentially bypassing content or access controls in document workflows.

How an attack unfolds

An attacker sends crafted TCPDF tags with varying hash values, measures response timing differences caused by early-exit loose comparison, recovers a valid hash, and then submits a malicious tag that passes verification.

How to fix it

Upgrade tecnickcom/tcpdf to the revision containing commit d54b97cec33f4f1a5ad81119a82085cad93cec89 or later, which replaces loose comparisons with constant-time functions.

How to confirm the fix worked

Confirm the deployed TCPDF source contains the referenced commit; run targeted unit tests that assert constant-time behavior on tag hash comparisons.

Typical attack path

1. Locate applications using vulnerable TCPDF versions for PDF generation
2. Send probe requests containing TCPDF tags with guessed or mutated hashes
3. Measure server response timing to perform statistical timing analysis
4. Recover a correct tag hash and craft a malicious but accepted tag
5. Inject the forged tag to alter document content or bypass validation

How to detect it

Inspect application and web server logs for repeated TCPDF tag processing with unusual timing patterns or high volumes of validation attempts; correlate with PDF output anomalies.

What your security team should do

• Immediately update all instances of tecnickcom/tcpdf to the fixed commit
• Audit codebases for direct use of unserializeTCPDFtag and similar tag handling
• Add runtime monitoring for anomalous timing or validation failures in PDF generation paths