Is it safe to render user-created Django templates?

Question

Is it safe to let users make their own Django templates with a set of pre-defined variables, and then render this template on the server? I would only pass a very limited set of parameters to render, all of which are strings. Templates would be something like:

hey, my name is {{name}}.

So, the question is, are there any django template tags that can be abused to get information that users are not supposed to get? I'm most worried about the {% url %} tag.

P.S.

I noticed this question after filling out the title, however, my question is slightly different. I will probably allow no HTML/javascript at all, use Textile/Markdown, or find a way to restrict HTML to a very basic set of tags.

Answer 1

There're three main risks:

1. Users modifying the data. For example, rendering {{ request.user.kill }} will trigger kill() call during value lookup. To prevent this, you should set kill.alters_data = True in your model code. All built-in model methods that modify data are already marked, so the risk is only associated with your own methods or ones provided by poorly-written 3rd party apps.

2. Users directly accessing data they should not see. When RequestContext is used (which is most of the time), there're many variables added to template rendering context. Add user-defined templates and you're getting quite dangerous mix, because user can view anything added by any context processor.

3. Users accessing data they should not see through relations. When you pass model instance to template, its relations could be travesred futher than you could expect: {{ current_user.corporate_account.owner.ssn }} Oops... A good preventive measure would be carefully reviewing your model relations to make sure you're not exposing something sensitive.

Overall, I'd say it is safe as long as you are aware of risks above and render user-supplied strings separately from regular templates. And make sure you eplicitly forbid {% debug %}, {% include %}. {% ssi %} template tags, as they can give away quite sensitive information. Maybe you can play it safe and only allow variables and filters and forbid control tags altogether.

Answer 2

include and ssi looks too dangerous for my taste, especially ssi which uses absolute paths. My opinion is that this is too risky business.

Answer 3

Well, from a server-side perspective it's safe (probably, no one has ever audited it), however the users could obviously generate any Javascript they wanted to perform XSS attacks.

Answer 4

It's not safe, the templates documentation says:

The template system isn’t safe against untrusted template authors. For example, a site shouldn’t allow its users to provide their own templates, since template authors can do things like perform XSS attacks and access properties of template variables that may contain sensitive information.

It runs deeper than just XSS attacks, or accessing user's data, if you try to do it, you will most probably have some vulnerabilities and you won't be aware of them.