Avast uncovers new encrypted PDF attack vector

According to Jiri Sejtko, the trick allows the creators of malicious PDF files to slide them past almost all AV scanners.

Anyone, he says, can create valid PDF files where the data uses – for example – five different filters or five layers of the same filter.

"All of these features are based on extremely liberal specifications, a fact which allows bad guys to utilise malicious files in a way that does not allow antivirus scanners access to the real payload", he says in his latest security blog.

"The filter used to encrypt text data is meant to be used only for black and white images. And apart from Avast, probably no other AV scanner is currently able to decode the payload because no other AV can detect those PDF files", he adds.

The esoteric attack vector centres on the misuse of the JBIG2 encoding mechanism, which provides for both lossy and lossless compression, and is, Sejtko notes, useful only for monochrome images.

By manipulating the JBIG2 pixels within an image, which is itself with a PDF file, hackers appear to have discovered a new method of obfuscating (hiding) malware within a PDF file, but without triggering conventional IT security software.

This means, Infosecurity notes, that it is possible for a TTF font data set to be hidden under JBIG2 stream.

The good news, however, is that Sejtko has detailed the attack methodology in his security blog, including the necessary fingerprint code to allow most AV software to look for the relevant digital signatures.

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