CVE-2018-18689: 
Foxit PDF Reader vulnerability analysis and mitigation

The Portable Document Format (PDF) specification vulnerability (CVE-2018-18689) is a Signature Wrapping vulnerability that affects multiple PDF processing applications. The vulnerability was discovered in 2018 and disclosed in February 2019 by researchers from Ruhr University Bochum and Hackmanit GmbH. The vulnerability exists because the PDF specification does not provide concrete procedures for signature validation, allowing attackers to use /ByteRange and xref manipulations that are not detected by signature-validation logic. This affects numerous PDF applications including Foxit Reader before 9.4, PhantomPDF before 8.3.9 and 9.x before 9.4, eXpert PDF 12 Ultimate, Expert PDF Reader, Nitro Pro, Nitro Reader, PDF Architect 6, and several others (PDF Insecurity, PDFA).

The vulnerability is classified as a signature validation bypass issue with a CVSS v3.1 Base Score of 5.3 (Medium) and vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N. The vulnerability stems from improper verification of cryptographic signatures (CWE-347). The attack involves manipulating the /ByteRange parameter and xref tables in PDF documents to circumvent signature validation checks, which can result in malicious changes being undetected by the signature-validation logic (NVD).

When exploited, this vulnerability allows attackers to modify signed PDF documents while maintaining an apparently valid digital signature. This could lead to unauthorized modifications of signed documents going undetected, potentially compromising the integrity and authenticity of digitally signed PDF documents (PDFA).

The primary mitigation is to update affected PDF applications to their latest versions. Foxit has released patches in version 9.4 and later for Foxit Reader, and version 8.3.9/9.4 and later for PhantomPDF. For proper mitigation, implementations should follow PAdES standards and avoid error-tolerant behavior when validating PDF signature objects (Foxit Security, PDFA).

The PDF Association acknowledged the significance of the vulnerability and emphasized the importance of following current standards and best practices in validating digital signatures. The research community praised the responsible disclosure process, as the researchers notified affected vendors before public disclosure, allowing patches to be implemented (PDFA).