Further granularity of unsafe-inline styles

[jonathanKingston]

Could we consider decoupling <style> and style="..." usage in style-src 'unsafe-inline' CSP setup?

The rationale is that as far as I am aware style="..." has no modern security issues in CSP supporting browsers; where as given selector support <style> could be manipulated to check for data on the page and load images.

For example there are various teams wanting to implement CSS parsers to get around the style="..." support issue, where as <style> seems mostly easy to move to a secure setup. So for example Ember team is willing to take the property in and set it as JavaScript attributes after parsing to mitigate the warning.

There could also be room to discuss if granularity for SVG <style> tags could be separate too I guess?

@mozfreddyb mentioned that he would like to see scoped styles granularity also which probably makes sense too.

/cc @mikewest

[jonathanKingston]

For example @marumari's new site pokeinthe.io she was required to use default-src 'none'; frame-ancestors 'none'; style-src 'self' 'unsafe-inline' on images like:

https://pokeinthe.io/theme/images/icons/linkedin.svg

Which uses these:

style="fill-rule:evenodd;clip-rule:evenodd;stroke-linejoin:round;stroke-miterlimit:1.41421;"

This sort of thing is common place in an SVG however all other browsers besides Firefox don't block this when loading an external document.

[april]

I will say that it's near impossible to find any site of a significant size that doesn't have style="..." at least somewhere in their codebase. Even on my lousy personal website where I went well out of my way to not use any style attributes (which is actually annoyingly painful), I still got caught flat-footed because my content creation tools (Affinity Designer) use style attributes in their SVG exports.

In my scanning of the top 100000 or so sites, of the sites that use CSP, very few of them don't have 'unsafe-inline' inside style-src. GitHub, Twitter, Facebook, AMO (addons.mozilla.org, which I personally helped with) — they all have 'unsafe-inline' inside style-src.

The fact that style attributes and style tags so are tightly coupled means that a lot of sites have actually made themselves vulnerable to cross site styling attacks completely unnecessarily, out of a need to support style attributes. If we're going to call it 'unsafe-inline', then it should actually be, you know, unsafe.

My messed up nginx config, just to make SVGs work:

add_header Content-Security-Policy "default-src 'none'; child-src https://air.mozilla.org; font-src 'self' https://fonts.gstatic.com; frame-ancestors 'none'; frame-src https://air.mozilla.org; img-src 'self'; media-src 'self'; script-src 'self'; style-src 'self' https://fonts.googleapis.com";

location ~ \.svg$ {
    add_header Content-Security-Policy "default-src 'none'; frame-ancestors 'none'; style-src 'self' 'unsafe-inline'";
}

[michaelficarra]

@marumari

then it should actually be, you know, unsafe

It is unsafe. An adversary who has control over a style attribute may manipulate the styling to mislead a user into taking actions they don't want to take or believing something that is not true.

I would support separating 'unsafe-inline' and 'unsafe-inline-attr' or something, but I would not support simply allowing style attributes by default or naming the keyword anything that doesn't include unsafe.

[mikewest]

@arturjanc, et al: Anyone interested in squeezing a proposal into CSP3?

[jonathanKingston]

Repinging here due to the panic around: https://news.ycombinator.com/item?id=16422696

I also want to highlight we could probably do something about the font range attack in CSS too (restricting unicode-range to bigger ranges), however @annevk thinks that might become a wackamole exercise.

[mikewest]

@jonathanKingston: It's not clear what that PoC has to do with inline styles. It relies on attribute selectors, which you can't put into style attributes. I think there's probably something we could do to mitigate that style of attack (either by banning attribute selectors entirely or in part on input field values, opting-in via something like writeonly, or something else I haven't thought of), but it's not clear what CSP should be doing differently. We give folks the ability to block CSS from untrusted sources. What more would you like to see?

[jonathanKingston]

@mikewest the proposal here is regarding the attack vector of <style> whilst style= is much more common and less of an attack vector.

If we added a new directive (or two) unsafe-inline-block and unsafe-inline-attr then pages could permit either/or and not be vulnerable to the other.

My issue here isn't with attributes and perhaps this thread has taken a turn in this direction. The original topic here and the attack vector described in the Hacker News thread is regarding <style> which has a much broader set of issues than the attribute.

[arturjanc]

I'm not sure why I never commented here (sorry!), but I like this approach, as evidenced in #202 which is duped to this issue.

+1 to what @jonathanKingston is saying: it would be valuable for developers to allow style= attributes while enforcing restrictions on stylesheets (i.e. <style> blocks or external CSS loaded via link#rel=stylesheet). In particular, this would be easy to deploy in applications using nonce-based policies because it would only require adding nonces to <style> / <link> elements without making any other application changes.

The main problem here is, as usual, backwards-compatibility with CSP2. If we add 'unsafe-inline-attr' as a new style-src keyword, the application would need to set a policy of:

style-src 'unsafe-inline' 'unsafe-inline-attr' 'nonce-foo'

The problem is that in CSP2 browsers, the nonce will cause the browser to ignore 'unsafe-inline', and style attributes will be blocked. One way to tackle this is similar to what we've considered for 'unsafe-hashed-attributes' and introduce a new spelling for nonces/hashes, e.g.

style-src 'unsafe-inline' 'unsafe-inline-attr' 'csp3-nonce-foo'

This would work properly in CSP<=2 browsers (only 'unsafe-inline' will be recognized), and in CSP3 browsers (the browser will enforce the nonce on style blocks, but will permit the use of attributes).

[jonathanKingston]

A different name might assist the issue @arturjanc speaks of. Perhaps block-style-tags which would be a noop if unsafe-inline wasn't present.
However this would start a precedent of naming which is just as bad (ignoring upgrade-insecure).

[vrastogi]

I would also like to second work on this issue. Many of the legacy websites we are working with have hundreds of style attributes. They are often prepared with automatic frameworks and it is going to be very difficult to make these websites CSP compatible.

The new spelling proposal of @arturjanc looks good.