Description

A signature validation vulnerability exists in Foxit PDF Editor/Reader’s handling of incrementally updated PDF documents. Changes to visible document content may not invalidate the existing signature, allowing attackers to alter signed content and potentially carry out content spoofing while the document continues to appear validly signed.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-09-23T08:17:13.673Z",
  "pubdate": "2026-09-23T08:17:13.673Z",
  "executiveSummary": "This vulnerability involves an improper signature validation mechanism within Foxit PDF Editor and Foxit PDF Reader when processing incrementally updated PDF documents.\nThe flaw allows the modification of visible document content without invalidating existing cryptographic signatures, effectively bypassing the integrity protection provided by digital signatures.\nThe impact is significant as it facilitates content spoofing, allowing an attacker to present a modified document that appears authentic and legitimately signed to the user.\nThe vulnerability affects Foxit PDF Editor and Foxit PDF Reader, posing a high risk to document integrity and trust-based workflows.\nAn attacker can exploit this by crafting a malicious PDF file containing an incremental update that alters the rendered content while maintaining the validity of the original signature object.\nNo specific user interaction or elevated privileges are strictly required beyond the victim opening the manipulated PDF file, making it a viable vector for social engineering and document tampering.",
  "technicalDetails": "The root cause of the vulnerability lies in the way Foxit PDF Editor and Reader handle incremental updates to signed PDF files. The PDF specification allows for incremental updates to append new data to a file without modifying existing objects, which is intended to support signing processes.\nWhen a PDF document is signed, the signature covers specific byte ranges of the file. A secure implementation must verify that the content rendered corresponds exactly to the content covered by the cryptographic signature. The vulnerable component fails to strictly validate the integrity of visual elements when incremental updates are introduced after the initial signing.\nThe exploitation flow begins with the attacker obtaining a legitimately signed PDF document. Leveraging the structure of the PDF incremental update (defined in the PDF specification via the '%%EOF' markers and cross-reference table updates), the attacker appends a new revision to the document.\nThis update introduces new objects or modifies existing ones to change the visual representation of the document, such as replacing text or images. Because the incremental update does not invalidate the original signature's byte range, the application's signature verification logic erroneously assumes that the changes are either non-intrusive or authorized by the original signature process.\nThe application renders the modified visual layer instead of the original, while the signature validation subsystem continues to report that the signature is 'Valid' because the underlying signature object and the covered byte range remain technically intact according to the flawed validation logic.\nThis results in a disparity between the signed cryptographic data and the presented document content, enabling the attacker to alter the document's intent or information while maintaining a false sense of security for the end-user.\nThe attack does not require network exposure or authentication, as the malicious payload is contained within the PDF file structure itself and is executed upon opening the file in the affected versions of Foxit PDF Editor or Reader."
}