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Is there anything tricky someone could do with user-uploaded CSS to harm a site? If I wanted to allow users to upload/share their own CSS themes to a site, is there anything I should look for or disallow?
EDIT: Assume I know how to check if it’s a valid CSS file etc. I’m looking for CSS specific exploits I need to avoid.
Even if you parse the file for valid CSS a hacker could still be malicious by using something like :before and :after. To ensure security you will want to whitelist a subset of css properties & selectors in your validation.