urma / slack-image-metadata-poc

Transfer files embedded in image metadata over Slack

Slack File Transfer Over Image Metadata

Slack has a number of controls which can be used to prevent users from sharing potentially sensitive information over the platform. One of the most commonly deployed ones is disabling file uploads:

Image uploads are handled differently, though, and often sharing pictures is allowed even when arbitrary file uploads are not. Additionally, Slack preserves image metadata when those are uploaded, including EXIF data for JPEG files, and XMP frames in PNG files.

Because certain XMP frame types can be arbitrarily large, we can abuse the metadata preservation ability to embedded file contents in images share those over Slack.

Pre-Requisites

$ sudo apt install libexempi3      # or the equivalent in your OS/Linux distro
$ pip install -r requirements.txt

Usage

Embedding arbitrary files in image metadata is achieved via the obviously named embed.py script:

$ python embed.py --input some-png-file.png \
    --output file-to-send-to-slack.png \
    file_to_embed.txt another_file_to_embed.pdf

The script will simply create a ZIP file embedding all the listed files, encode it as base64 string and add it as a XMP frame to the input PNG file. Once that's done, you should be able to uploaed the file as an image to your Slack workspace.

Extracting the embedded content is just as simple: download the uploaded image from SLack and just use the extract.py script:

$ python extract.py file-to-send-to-slack.png

Improvements

The ZIP archive is embedded as-is, and thus anyone who has access to the image on Slack and knows how to extract the content from the metadata will have access to it. It also means that smarter DLP products might be able to pick up the ZIP signature embedded in the image and prevent it from being downloaded or uploaded.

Both of these issues could be solved by encrypting the generated archive. This can be easily achieved by using a module such as cryptography.