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pythonxssmarkdownsanitization

Best practice for allowing Markdown in Python, while preventing XSS attacks?

I need to let users enter Markdown content to my web app, which has a Python back end. I don’t want to needlessly restrict their entries (e.g. by not allowing any HTML, which goes against the spirit and spec of Markdown), but obviously I need to prevent cross-site scripting (XSS) attacks.

I can’t be the first one with this problem, but didn’t see any SO questions with all the keywords “python,” “Markdown,” and “XSS”, so here goes.

What’s a best-practice way to process Markdown and prevent XSS attacks using Python libraries? (Bonus points for supporting PHP Markdown Extra syntax.)

Solution

  • I was unable to determine “best practice,” but generally you have three choices when accepting Markdown input:

    1. Allow HTML within Markdown content (this is how Markdown originally/officially works, but if treated naïvely, this can invite XSS attacks).

    2. Just treat any HTML as plain text, essentially letting your Markdown processor escape the user’s input. Thus <small>…</small> in input will not create small text but rather the literal text “<small>…</small>”.

    3. Throw out all HTML tags within Markdown. This is pretty user-hostile and may choke on text like <3 depending on implementation. This is the approach taken here on Stack Overflow.

    My question regards case #1, specifically.

    Given that, what worked well for me is sending user input through

    1. Markdown for Python, which optionally supports Extra syntax and then through
    2. html5lib’s sanitizer.

    I threw a bunch of XSS attack attempts at this combination, and all failed (hurray!); but using benign tags like <strong> worked flawlessly.

    This way, you are in effect going with option #1 (as desired) except for potentially dangerous or malformed HTML snippets, which are treated as in option #2.

    (Thanks to Y.H Wong for pointing me in the direction of that Markdown library!)