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New Portable Document Format (PDF) attack on encryption features

    The Portable Document Format (PDF) standard has been able to provide many benefits that unify communications across many different software and hardware platforms. One of those elements is the encryption schemes that allow users to password protect their documents from view, edit, or saving permissions without the required password. Another encryption feature included with the PDF standard is the ability to sign documents with an electronic signature with the same legal standing as a handwritten signature, this may include digital signing which uses cryptographic measures to assure authenticity.

    Researchers from Ruhr University Bochum, FH Münster University of Applied Sciences, and Hackmanit GmbH have developed a two pronged attack on the security measures of PDFs and their encryption schemes. They have named their attack PDFex. In their research they developed methods for the exfiltration of the contents of the encrypted PDF with minimal prior knowledge of the contents of the PDF file. The methods studied can also modify the contents to change the plain text as well as add malicious functionality. The first prong of PDFex attack methods rely on how an encrypted PDF only encrypts portions of the PDF file leaving other portions unencrypted and unprotected. The attacker is then able to modify the contents of the unencrypted portions of the file. In this way they can plant data which submits a form including the contents of the PDF to an attacker controlled server granting the attacker access to the contents of the PDF. The attacker can edit an unencrypted field with a URL which will be sent encrypted and unencrypted strings from the document. The last method in this attack on the unencrypted portion of PDF files injects JavaScript code into the document which then ex filtrates the data within the file. This is the "Direct Exfiltration" method of the PDFex attack.

    The other prong of this attack uses CBC malleability gadgets, tools that are able to edit cipher texts encrypted with the cipher block chaining (CBC) encryption mode without integrity checks. It just so happens that the PDF standard does exactly that. This method can modify plain text as well as add in new encrypted content to the file. This technique can enact the PDF forms and hyperlink techniques as listed in the Direct Exfiltration method. The CBC Gadgets method can also edit PDF object streams such that they submit themselves to an attacker controlled server. Both attacks require the victim to open the tainted document so that the traps can deliver the finally decrypted information to the attacker. The researchers have tested their techniques on 27 PDF viewers and all were susceptible to at least one method of the PDFex attack.
    The attack requires that the attacker have access to the file to modify it, some of the attacks have other requirements such as the ability to trigger URL s from the viewer, or for the viewer to have permission to use JavaScript in the background. One of the researchers reported to Threatpost that "There are currently no effective countermeasures, as the weaknesses lie in the PDF encryption standard itself" and that the best mitigation is to use additional layers of encryption outside of the PDF standard to protect their data.

Sources:

https://www.pdf-insecurity.org/download/paper-pdf_encryptionccs2019.pdf

https://threatpost.com/hack-breakspdf-encryption/148834/

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