Tuesday, July 21, 2020

Shadow Attacks: Hiding and Replacing Content in Signed PDFs

Last year we presented How to Spoof PDF Signatures. We showed three different attack classes. In cooperation with the CERT-Bund (BSI), we contacted the vendors of affected PDF applications to inform them about the vulnerabilities and to support them in developing countermeasures. Most vendors reacted promptly and closed the reported vulnerabilities promptly.
One of those three attack classes was Incremental Saving Attacks (ISA). The proposed countermeasures aimed to distinguish PDF objects appended to the file via updates into dangerous and non-dangerous. In other words, black and whitelisting approaches were used. 

Naturally, this countermeasure succeeds as long as the PDF update contains evil objects. So we came up with the idea to attack PDFs with only non-dangerous updates. We achieve this by adding invisible, malicious content when creating the PDF document (before it is signed) and to reveal them after signing.
Today, we present Shadow Attacks! Our evaluation of 28 PDF applications reveals that 15 of them, including Adobe Acrobat and Foxit Reader, are vulnerable.
We responsibly disclosed all affected vendors. Together with the CERT-Bund (BSI), we supported the vendors in developing suitable countermeasures. The attacks are documented in CVE-2020-9592 and CVE-2020-9596.
Full results are available in our vulnerability report and on our website.

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