In May 2021, researchers at Ruhr University Bochum (RUB) identified issues with certification signatures in PDF, and their findings are available on the PDF Insecurity website. Since iText's products are widely used in digital signing workflows, including certification signature creation, the brand has provided insights and recommendations on addressing these new attacks. PDF certification signatures are unique in terms of storage and interpretation within a document.

A certification signature, often created by the document author, serves as the first signature and defines the permissible changes after signing. Additional regular signatures can be added if permitted by the certification signature. However, the PDF format allows incremental updates, enabling the addition of multiple digital signatures.

Validating these updates and determining legitimate changes is the responsibility of the validator, supported by the enforcement mechanism known as DocMDP permission levels. The attacks discussed in this blog primarily target levels 2 and 3 of the DocMDP permission levels, exploiting viewer behavior and assumptions about incremental update validation. It's important to note that these attacks do not directly attack PDF libraries like iText, but rather highlight the lack of precise validation requirements in the PDF standard.

By offering insights into these attacks and providing guidance for mitigating them, iText ensures that its customers can confidently utilize their products as part of secure and reliable digital signing workflows