Shadow Attacks Let Attackers Replace Content in Digitally Signed PDFs
February 23, 2021Ravie Lakshmanan
Researchers have demonstrated a novel class of attacks that could allow a bad actor to potentially circumvent existing countermeasures and break the integrity protection of digitally signed PDF documents. Called " Shadow attacks " by academics from Ruhr-University Bochum, the technique uses the "enormous flexibility provided by the PDF specification so that shadow documents remain standard-compliant." The findings were presented yesterday at the Network and Distributed System Security Symposium (NDSS), with 16 of the 29 PDF viewers tested — including Adobe Acrobat, Foxit Reader, Perfect PDF, and Okular — found vulnerable to shadow attacks. To carry out the attack, a malicious actor creates a PDF document with two different contents: one which is the content that's expected by the party signing the document, and the other, a piece of hidden content that gets displayed once the PDF is signed. "The signers of the PDF receive the document, review it, and s
