CVE-2018-18689 in Portable Document Format
Summary
by MITRE • 01/07/2021
The Portable Document Format (PDF) specification does not provide any information regarding the concrete procedure of how to validate signatures. Consequently, a Signature Wrapping vulnerability exists in multiple products. An attacker can use /ByteRange and xref manipulations that are not detected by the signature-validation logic. This affects Foxit Reader before 9.4 and PhantomPDF before 8.3.9 and 9.x before 9.4. It also affects eXpert PDF 12 Ultimate, Expert PDF Reader, Nitro Pro, Nitro Reader, PDF Architect 6, PDF Editor 6 Pro, PDF Experte 9 Ultimate, PDFelement6 Pro, PDF Studio Viewer 2018, PDF Studio Pro, PDF-XChange Editor and Viewer, Perfect PDF 10 Premium, Perfect PDF Reader, Soda PDF, and Soda PDF Desktop.
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Analysis
by VulDB Data Team • 05/04/2025
The vulnerability described in CVE-2018-18689 represents a critical signature validation weakness within PDF processing applications that stems from the inherent ambiguity in the PDF specification regarding signature validation procedures. This fundamental gap in the standard creates a Signature Wrapping vulnerability that allows attackers to manipulate PDF documents without detection by signature verification mechanisms. The issue manifests through improper handling of /ByteRange and xref table manipulations that bypass existing signature validation logic, effectively enabling attackers to alter document content while maintaining apparently valid digital signatures. This weakness affects a broad range of PDF processing software including Foxit Reader, PhantomPDF, and numerous other commercial and open-source PDF viewers and editors, creating widespread potential impact across enterprise and individual users.
The technical flaw exploits the lack of standardized validation procedures within the PDF specification, specifically targeting how applications interpret and verify signature data. Attackers can manipulate the /ByteRange parameter and manipulate cross-reference tables (xref) in ways that are not properly detected by the signature validation algorithms. This manipulation allows the attacker to insert malicious content into the document while keeping the digital signature intact, effectively creating a signature wrapping attack vector. The vulnerability exists because applications fail to properly validate the integrity of the entire document structure during signature verification, particularly when processing the byte ranges that define which portions of the document are covered by the signature. This issue falls under the CWE-353 weakness category related to inadequate input validation and represents a significant gap in cryptographic validation practices.
The operational impact of this vulnerability extends far beyond simple document manipulation, as it fundamentally undermines the security assurances provided by digital signatures in PDF documents. Organizations relying on PDF signatures for document integrity, authentication, and legal compliance face potential exposure to malicious document tampering that remains undetected by standard signature validation processes. This vulnerability affects not only the security of document verification but also impacts trust in electronic workflows, contract signing processes, and regulatory compliance scenarios where PDF signatures are critical. The widespread affected software ecosystem means that enterprises using multiple PDF processing applications across their organizations face coordinated risk exposure, potentially compromising sensitive business documents, legal agreements, and confidential communications that rely on digital signature validation for security.
Mitigation strategies for this vulnerability require immediate patching of affected software versions, with particular attention to Foxit Reader versions before 9.4 and PhantomPDF versions before 8.3.9 and 9.x before 9.4. Organizations should implement comprehensive software inventory management to identify all affected applications and ensure timely updates. Additional defensive measures include implementing strict document review processes for sensitive documents, deploying signature validation monitoring tools, and establishing network-level controls to detect and prevent the distribution of potentially compromised PDF files. Security teams should also consider implementing sandboxing techniques for PDF processing and establishing baseline signature validation procedures that go beyond standard application capabilities. The vulnerability demonstrates the importance of robust cryptographic validation standards and highlights the need for organizations to maintain updated security practices that account for specification ambiguities in widely used file formats. This issue also aligns with ATT&CK technique T1566 related to spearphishing attachments, as malicious actors could exploit this vulnerability to deliver compromised documents that appear legitimate through their valid signatures.