pdfalyzer VS CyberChef

Direct link to the folder with 3 .yara files compiling a bunch of YARA rule sources. Looking for anything not represented here, or even ideas for such.

The Pdfalyzer is a command line tool (paralyze) as well as a library for working with, visualizing, and scanning the contents of a PDF. Motivation for the project was personal: I got hacked by a PDF that turned out to be hiding its maleficent instructions inside the font binary where it was missed by modern malware scanners (twitter thread) (more details)

A few weeks ago I made a post here about a PDF that evaded all malware detection and caused a security breach, almost certainly through PDF instructions hidden inside of an Adobe Type1 Font binary stream embedded within a PDF. At the time I posted a link to a tool I wrote called The Pdfalyzer that diagrams a PDF's internal and scans for various suspect content.

I dramatically scaled up the binary data scouring and visualization in the pdfalyzer... can rip through every backtick/frontslash/single or double quoted/etc etc set of bytes in the binaries and try a bunch of aggressive approaches to force decode them.

The tool is the the pdfalyzer; I just open sourced it. Meant to fill in some gaps around pdf-parser.py and the rest of Didier Stevens's malicious PDF toolkit. Makes pretty charts, previews binary data, and (most importantly) digs through PDF font binaries for potentially executable stuff. Example output can be seen at the GitHub link.

Then we take the encrypted text and use CyberChef to decrypt it.

Let's go to CyberChef and insert our pieces of evidence.

A parameter was changed from '2AMBCgIQBg' to 'CgIIAdgDAQ%3D%3D' which is just the correct base64 encoding they should have been using the entire time.

I don't think this was a hostile action by Google, I think someone just added better input validation for security reasons and it accidently broke the bad requests they were sending.

https://gchq.github.io/CyberChef/#recipe=URL_Decode()From_Ba...

❗This is indeed the flag, but the text is encrypted with Base64. Usually, the presence of padding character "=" indicates that's Base64 type of encoding (but that's only one of the hints). To decrypt it, we can use CyberChef. Copy-paste the text and we either:

It uses a combination of magic bytes (like the `file` command), entropy analysis and character frequency detection to determine whether an output is likely to be of interest to the user.

The file type mechanism is written here[0]. There's a list of all signatures we detect here[1].

[0] https://github.com/gchq/CyberChef/blob/master/src/core/lib/F...

I think this can be deciphered using CyberChef...