CVE-2024-4976

What it actually does

Missing object type validation in AcroForm field handling allows an out-of-bounds array write in memory during PDF parsing.

Why it matters to the business

Opening a crafted PDF document locally can crash the PDF viewer, interrupting document review workflows and requiring restarts or alternative tools.

How an attack unfolds

An attacker creates a malformed PDF containing an AcroForm field reference with an unexpected object type. When the victim opens the file with Xpdf on their local system, the missing type check triggers the out-of-bounds write, leading to a crash.

How to fix it

Upgrade Xpdf to the latest version that resolves the missing type check. Until a fix is available, avoid opening untrusted PDF files with affected versions and consider alternative PDF readers with stricter parsing.

How to confirm the fix worked

Confirm the installed Xpdf version exceeds 4.05 and test that sample PDFs containing invalid AcroForm references no longer cause crashes.

Typical attack path

1. Craft malicious PDF with invalid AcroForm object reference
2. Deliver the PDF to the target via email, download, or removable media
3. Victim opens the PDF using Xpdf 4.05 or earlier
4. Parser encounters malformed field reference and performs out-of-bounds write
5. Application crashes, causing denial of service

How to detect it

Monitor application crash logs and system event logs for unexpected terminations of the Xpdf process when PDF files are accessed; correlate with file access timestamps of recently received documents.

What your security team should do

• Prioritize patching Xpdf installations as soon as an update is released
• Configure systems to open PDFs with alternative hardened viewers by default
• Educate users on risks of opening PDFs from untrusted sources