PDF is among the most widely used document formats worldwide. To ensure confidentiality, PDF supports document encryption. In this talk, we analyze PDF encryption and show two novel techniques for breaking the confidentiality of encrypted documents. First, we abuse the PDF feature of partially encrypted documents to wrap the encrypted part of the document within attacker-controlled content and therefore, exfiltrate the plaintext once the document is opened by a legitimate user. Second, we abuse a flaw in the PDF encryption specification to arbitrarily manipulate encrypted content. The only requirement is one single block of known plaintext, which we show is fulfilled by design. Our attacks allow the recovery of the entire plaintext of encrypted documents by using exfiltration channels, based on standard compliant PDF properties.
We evaluated our attacks on 27 widely used PDF viewers and found all of them to be vulnerable. We responsibly disclosed the vulnerabilities and supported the vendors mitigating the issues.