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while mat2 has both a thread model (a thread pool that strips metadata in parallel) and a threat model (a list of malicious adversaries and their capabilities that we are trying to defeat), i think this paragraph is talking about the latter.
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Implementation notes
Lightweight cleaning mode
Due to popular request, MAT2 is providing a lightweight cleaning mode, that only cleans the superficial metadata of your file, but not the ones that might be in embedded resources. Like for example, images in a PDF or an office document.
Revisions handling
Revisions are handled according to the principle of least astonishment: they are entirely removed.
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Either the users aren't aware of the revisions, are thus they should be deleted. For example journalists that are editing a document to erase mentions sources mentions.
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Or they are aware of it, and will likely not expect MAT2 to be able to keep the revisions, that are basically traces about how, when and who edited the document.
Race conditions
MAT2 does its very best to avoid crashing at runtime. This is why it's checking if the file is valid at parser creation. MAT2 doesn't take any measure to ensure that the file is not changed between the time the parser is instantiated, and the call to clean or show the metadata.
Symlink attacks
MAT2 output predictable filenames (like yourfile.jpg.cleaned). This may lead to symlink attack. Please check if you OS prevent against them
Archives handling
MAT2 doesn't support archives yet, because we haven't found an usable way to ask the user what to do when a non-supported files are encountered.
PDF handling
MAT was doing some kind of rendering for PDF files, on a cairo surface, then printed it to a file. This kept the text selectable, but unfortunately, it didn't remove any deep metadata, like the ones in embedded pictures. This was on of the reason MAT was abandoned: the absence of satisfying solution to handle PDF. But apparently, people are ok with pdf redact tools, that simply transform the PDF into images. So this is what's MAT2 is doing too.
Of course, it would be possible to detect images in PDf file, and process them with MAT2, but since a PDF can contain a lot of things, like images, videos, javascript, pdf, blobs, … this is the easiest and safest way to clean them.
Images handling
When possible, images are handled like PDF: rendered on a surface, then saved to the filesystem. This ensures that every metadata is removed.
XML attacks
Since our threat model conveniently excludes files crafted to specifically bypass MAT2, fileformats containing harmful XML are out of our scope. But since MAT2 is using etree to process XML, it's "only" vulnerable to DoS, and not memory corruption: odds are that the user will notice that the cleaning didn't succeed.