[Bug]: indicate modifications after digital signature

[ousia]

Attach (recommended) or Link to PDF file

• signed.pdf: signed PDF document with testing certificate.

• modified-signed.pdf: modified PDF document after signature.

Web browser and its version

Firefox Nigthly 155a

Operating system and its version

Windows 11

PDF.js version

the one that comes with that nightly.

Is the bug present in the latest PDF.js version?

Yes

Is a browser extension

No

Steps to reproduce the problem

1. Digitally sign a document with a testing certificate ( signed.pdf was signed with pdfsig from poppler).¹

   Digital signature is considered valid (as verified).

   

2. Modify the text field with Acrobat Reader and save the results (modifications are saved as incremental updates).

   Digital signature is considered valid, with no further indication that the document has been modified after the signature:

   

What is the expected behavior?

Signature verification should indicate that the document contains unsigned modifications.

This is essential with fill-in forms, since giving no indication that the document has been modified after being digitally signed may be give the misleading information that the modifications have been accepted by the original signer.

What went wrong?

Valid signature with an added incremental update neither renders the signature invalid or

Compare the original signature info (using pdfsig from poppler-26.08.0):

$ pdfcurrentsig signed.pdf 
Digital Signature Info of: signed.pdf
Signature #1:
  - Signature Field Name: x
  - Signer Certificate Common Name: Testing Certificate [RSA 4096])
  - Signer full Distinguished Name: CN=Testing Certificate [RSA 4096])
  - Signing Time: Aug 03 2026 13:37:10
  - Signing Hash Algorithm: SHA-256
  - Signature Type: adbe.pkcs7.detached
  - Signed Ranges: [0 - 5646], [25648 - 25993]
  - Total document signed
  - Signature Validation: Signature is Valid.
  - Certificate Validation: Unknown issue with Certificate or corrupted data.

With the modified document after the signature:

$ pdfcurrentsig modified-signed.pdf
Digital Signature Info of: modified-signed.pdf
Signature #1:
  - Signature Field Name: x
  - Signer Certificate Common Name: Testing Certificate [RSA 4096])
  - Signer full Distinguished Name: CN=Testing Certificate [RSA 4096])
  - Signing Time: Aug 03 2026 13:37:10
  - Signing Hash Algorithm: SHA-256
  - Signature Type: adbe.pkcs7.detached
  - Signed Ranges: [0 - 5646], [25648 - 25993]
  - Not total document signed
  - Signature Validation: Signature is Valid.
  - Certificate Validation: Unknown issue with Certificate or corrupted data.

Link to a viewer

No response

Additional context

If electronic signatures are legally valid in EU/EEA Member States (article 25.2 of Regulation EU/910/2014), it is essential to know whether an electronically signed PDF document has been modified after being signed: signed contents are legally binding, but modifications after signing are ways to forge electronic documents and forgeries aren’t legally binding (when properly detected).

Again, first of all, users should be given hints that signed documents have been modified after being signed.

@beurdouche, congratulations for the implementation and many thanks for your excellent work.

Footnotes

1. pdfsig always signs PDF documents adding an incremental update (this is why modified-signed.pdf contains three %%EOF strings). ↩

[beurdouche]

Thanks for the report !
That is very helpful. There are indeed many issues that need to be handled before we can enable the feature.
Revocation status is another one that's highly non-trivial.

With respect to this specific issue, I like the idea of keeping things simple for the users, so my take would be to warn with "the document contains unsigned modification" instead of trying to detail what is signed and what is not. For a distant future, a super bonus might be to have a button to strip the unsigned content so that the user can manually inspect the signed version but that doesn't seem trivial.

[ousia]

Many thanks for your reply, @beurdouche.

Your reply reminded me of a passage from the PDF-1.7 specification:

If a signed document is modified and saved by incremental update (see 7.5.6, “Incremental Updates”), the data corresponding to the byte range of the original signature is preserved. Therefore, if the signature is valid, it is possible to recreate the state of the document as it existed at the time of signing.

Actually, current Acrobat Reader displays the modified PDF document this way:

When you click on Click to view this version (with darker gray background), the signed version is opened in a new tab.

For a distant future, a super bonus might be to have a button to strip the unsigned content so that the user can manually inspect the signed version but that doesn't seem trivial.

Well, considering - Signed Ranges: [0 - 5646], [25648 - 25993] (or more properly, /ByteRange [0 5646 25648 345 ]), I can binary edit modified-signed.pdf removing the incremental update.¹ I have discovered that myself (having no coding knowledge) to get a signed only document with no incremental updates.

Since /ByteRange only accepts arrays and the last signed byte is always the sum of their two last integers (l₁ + l₂), I didn’t know it was so difficult to automatically get the signed document without further incremental updates.

When this feature is added, please also allow the option to save the resulting signed document.

Besides certificate revocation, I think there is other essential issue with certificates for end users, would you like it to discuss it here or do you prefer to have a new issue or even a discussion?

Many thanks for your help again.

Footnotes

1. Invoking nvim -b modified-signed.pdf, typing 25993go↵ goes to the last signed byte, typing :+,$d↵ removes the incremental update and :wq↵ saves the signed only document. ↩