Is a Signed PDF Really Safe? What the Green Tick Proves

You open a contract and your PDF reader shows a green tick: Signed and all signatures are valid. Most people read that as “this document is genuine and unchanged”. It’s a reasonable reading. It’s also not quite what the tick means.

The gap between those two things is small in theory and, as a team of researchers demonstrated, large enough in practice to drive a contract through.

What a digital signature actually promises

Start with the honest version, because it’s genuinely useful technology and nothing here is an argument against using it.

When someone signs a PDF, their software calculates a fingerprint of the document’s contents and encrypts that fingerprint with a private key only they hold. Your reader later recalculates the fingerprint, decrypts theirs, and compares. If they match, two things are established: the document hasn’t changed since signing, and it was signed by whoever holds that key.

That’s real and valuable. It’s also narrower than “this document is trustworthy”. The signature says nothing about whether the contents were true, whether the signer read them, or whether the certificate belongs to who you think it does.

The awkward feature at the centre of this

Here’s where PDF gets interesting. The format allows incremental updates — you can append changes to the end of a file without rewriting it. Add a comment, fill in a form field, and the original bytes stay exactly where they were, with the additions bolted on afterwards.

There’s a good reason for this. It’s what lets several people sign the same document in sequence: each signature covers everything before it, and adding a second signature must not invalidate the first.

The consequence is that a signed PDF can legitimately change after signing and still show a valid signature. Readers are supposed to notice and tell you. Whether they do is the whole question.

The Shadow Attack

In 2020, researchers Christian Mainka, Vladislav Mladenov and Simon Rohlmann at Ruhr University Bochum published work they called Shadow Attacks, presented at the NDSS symposium.

The idea is unnervingly simple. The attacker prepares a document containing two sets of content: the version the victim expects to see and sign, and a second, hidden version. The victim reviews the innocent version, signs it, and returns it. The attacker then uses a perfectly legitimate incremental update to bring the hidden content to the front.

The signature still validates. Nothing was broken. The attacker only used features the specification provides — hidden layers, form fields, and the append-only update mechanism.

The researchers tested 29 PDF viewers. 16 of them were vulnerable, including widely used products such as Adobe Acrobat and Foxit Reader. Vendors were notified and fixes followed, which is exactly how this is supposed to work — but the finding tells you something durable about how much weight a green tick can carry.

Four other things the tick doesn’t tell you

Who actually holds the certificate. Anyone can obtain a certificate and put a name on it. Unless it chains back to an authority your system trusts, “signed by Jan Kowalski” means someone typed that name. Readers distinguish between a valid signature and a trusted one, and the difference is easy to miss at a glance.

Whether the certificate was valid at the time. Certificates expire and get revoked. A signature made with a certificate that was later revoked may still display happily unless your reader checks revocation — which requires a network lookup that sometimes silently fails.

What was signed. A signature can cover part of a document rather than all of it. Content added afterwards sits outside the protected region entirely.

That the signer agreed with it. A signature is a cryptographic operation, not evidence of comprehension. This is obvious when stated and routinely forgotten in practice.

Three things that look like signatures and aren’t

Before the checking advice, a distinction that causes more confusion than the cryptography does.

An image of a handwritten signature pasted into a document proves nothing whatsoever. It’s a picture. Anyone who receives that document can copy the picture into another one. This is extremely common and widely mistaken for a signed document.

A typed name in a signature block is a statement of intent, not a cryptographic fact. It may well be legally meaningful depending on context and jurisdiction — but your reader can’t verify anything about it, and it offers no protection against alteration.

A signature applied by an e-signing platform sits in between and varies enormously. Some platforms apply a genuine digital signature to the finished PDF, which your reader can check. Others record the signing event in their own audit trail and produce a PDF that merely shows an image and a reference number. The second kind is evidence, but it lives on the platform’s servers rather than in the file, and it disappears if you only keep the PDF.

If it matters, the test is simple: does your PDF reader show a signature panel with a certificate in it? If not, whatever you’re looking at isn’t a digital signature, whatever it looks like.

How to check a signature properly

None of this requires special software — just looking at what your reader already tells you, rather than at the tick alone.

Open the signature panel. Don’t stop at the status bar. In Acrobat Reader, click the signature panel and read the detail. You want three things: the signature is valid, the identity is trusted rather than merely present, and the document hasn’t been modified since signing.

Look for post-signature changes. Readers report these in wording like “the document has been modified since it was signed” or by listing document versions. That notice is not a formality. If a contract shows changes after signing, ask what they were before doing anything else.

Check who issued the certificate. A self-signed certificate is not the same as one from a recognised authority. For anything with legal weight, that distinction is the point.

Keep your reader updated. The Shadow Attack fixes shipped in updates. So do the fixes for whatever comes next.

For anything that matters, confirm out of band. If a signed document specifies where to send money, phone the counterparty on a number you already had. No cryptography defeats a compromised email account, and this single habit stops most invoice fraud regardless of how clever the file is.

Where this leaves everyday work

Signatures are worth using. The lesson isn’t to distrust them — it’s to read what they actually claim, which is narrower than the interface implies.

One practical note relevant to this site: if you’re handling signed documents, be careful about what you run them through. Many tools that merge, split or convert PDFs will invalidate a signature, because the signature covers the exact bytes of the file and those operations rewrite them. That isn’t a defect; it’s the signature doing its job. But it does mean a signed contract should be archived in its original form before anything touches it, and it’s a good reason to keep such documents on your own machine rather than uploading them to a service whose processing you can’t inspect.

PDF Manipulator runs entirely locally, so signed documents you work with never leave your computer. It won’t make an invalid signature valid, and it won’t tell you whether a certificate deserves trust — that judgment stays with you.

The short version

A green tick means the file matches its signature. It doesn’t mean the signer is who the name says, that the certificate is still valid, or that nothing was added afterwards. Open the signature panel, read the identity, check for post-signing changes — and for anything involving money, pick up the phone.