CVE-2026-18622: Foxit PDF Editor Signature Tampering Flaw

CVE-2026-18622 Overview

CVE-2026-18622 affects Foxit PDF Editor and Foxit PDF Reader. The application inconsistently alerts users when signature fields in a PDF are abnormally modified. These modifications include changes to signature appearance, coordinates, or field duplication. The user interface does not accurately reflect the actual integrity status of the signature after tampering.

Attackers can leverage this gap to mislead users into trusting documents that have been altered after signing. This maps to [CWE-451] Improper UI Representation of Critical Information. The vulnerability requires local access and user interaction to exploit.

Critical Impact
Users may trust the validity of a digitally signed PDF whose signature fields were tampered with, undermining the integrity guarantees of PDF digital signatures.

Affected Products

Foxit PDF Editor
Foxit PDF Reader
Refer to the Foxit Security Bulletins for specific affected versions

Discovery Timeline

2026-08-13 - CVE-2026-18622 published to the National Vulnerability Database (NVD)
2026-08-13 - Last updated in NVD database

Technical Details for CVE-2026-18622

Vulnerability Analysis

The vulnerability resides in how Foxit PDF Editor and Reader validate and communicate signature integrity to the user. When a signed PDF is opened, the application should signal any post-signature modification to signature fields. In vulnerable builds, several classes of modification do not trigger a reliable warning.

Affected modifications include altering the visual appearance of the signature widget, changing the coordinates of the signature field on the page, and duplicating the signature field elsewhere in the document. The cryptographic signature may remain technically valid over the byte range it covers, while the visible representation is manipulated to deceive the reader.

This is a UI representation defect rather than a cryptographic flaw. The trust decision users make relies on the application's status indicators, and those indicators do not consistently reflect the underlying document state.

Root Cause

The root cause is inconsistent enforcement of signature integrity checks against the visual and structural layer of the PDF. The signature validation logic does not treat field-level modifications, such as appearance stream changes or widget duplication, as events that require an integrity warning.

Attack Vector

An attacker prepares a legitimately signed PDF, then modifies the signature field appearance, moves its coordinates, or duplicates the field to display trusted signing information over untrusted content. The victim opens the manipulated document in a vulnerable Foxit build. The UI reports the signature as valid or fails to raise a warning, leading the victim to trust altered content.

Exploitation requires local file access and user interaction. No verified public proof-of-concept code is available. See the Foxit Security Bulletins for vendor technical details.

Detection Methods for CVE-2026-18622

Indicators of Compromise

PDF documents containing multiple signature field objects referencing the same signature dictionary, which can indicate field duplication.
Signed PDFs where signature widget /Rect coordinates or /AP appearance streams have been modified after signing, detectable through incremental update analysis.
Emails or file shares delivering signed PDFs from external senders that request approval, wire transfer, or credential actions.

Detection Strategies

Perform out-of-band signature validation using a second PDF viewer or command-line tool that strictly enforces field integrity checks.
Inspect PDF revisions using tools that parse incremental updates to identify post-signature modifications to signature field objects.
Correlate document origin metadata with expected signing workflows to flag anomalies in signature widget structure.

Monitoring Recommendations

Log Foxit PDF Editor and Reader process activity on endpoints, including files opened from email attachments and shared folders.
Monitor for the installation or execution of outdated Foxit versions that predate the vendor's patch for CVE-2026-18622.
Alert on inbound PDFs with digital signatures from high-risk senders or business processes involving financial approval.

How to Mitigate CVE-2026-18622

Immediate Actions Required

Update Foxit PDF Editor and Foxit PDF Reader to the fixed version identified in the vendor security bulletin.
Inventory endpoints for installed Foxit versions and prioritize updates for users who routinely handle signed PDFs.
Instruct users to independently verify signed PDFs using a secondary validated tool before acting on document content.

Patch Information

Foxit has published fixes through its security advisory process. Consult the Foxit Security Bulletins for the specific patched versions of Foxit PDF Editor and Foxit PDF Reader that address CVE-2026-18622.

Workarounds

Restrict opening of externally sourced signed PDFs until the update is deployed.
Validate high-value signed documents using an alternative PDF validator that enforces strict field integrity checks.
Train users to inspect signature panel details, including signer identity and any reported modifications, rather than relying solely on visual signature widgets.