Fossil: The Default Content Security Policy (CSP)

When Fossil’s web interface generates an HTML page, it normally includes a Content Security Policy (CSP) which applies to all content on that page. The CSP tells the browser which types of content (HTML, image, CSS, JavaScript...) the repository is expected to serve, and thus which types of content a compliant browser is allowed to pay attention to. All of the major browsers enforce CSP restrictions.

A CSP is an important security measure on any site where some of the content it serves is user-provided. It defines a “white list” of content types believed to be safe and desirable, so that if any content outside of that definition gets inserted into the repository later, the browser will treat that unwanted content as if it was not part of the document at all.

A CSP is a fairly blunt tool. It cannot tell good and visually appealing enhancements from vandalism or outright attacks. All it does is tell the browser what types and sources for page content were anticipated by the site’s creators. The browser treats everything else as “unwanted” even if that’s not actually true. When the CSP prevents the browser from displaying wanted content, you then have to understand the current rules and what they’re trying to accomplish to decide on an appropriate workaround.

The Default Restrictions

The Fossil default CSP declares the following content restrictions:

default-src 'self' data

This policy means mixed-origin content isn’t allowed, so you can’t refer to resources on other web domains, so the following Markdown for an inline image hosted on another site will cause a CSP error:

     ![fancy 3D Fossil logotype](https://i.imgur.com/HalpMgt.png)

This policy allows inline data: URIs, which means you could data-encode your image content and put it inline within the document:

     ![small inline image](data:image/gif;base64,R0lGODlh...)

That method is best used for fairly small resources. Large data: URIs are hard to read and edit. Keep in mind that if you put such a thing into a Fossil forum post, anyone subscribed to email alerts will get a copy of the raw URI text, which is really ugly.

For larger files, you could instead store the file in Fossil as:

versioned content retrieved via a /raw URL
unversioned content retrieved via a /uv URL

Another path around this restriction is to serve your repo behind an HTTP proxy server, allowing mixed-mode content serving, with static images and such served directly by the HTTP server and the dynamic content by Fossil. That allows a URI scheme that prevents the browser’s CSP enforcement from distinguishing content from Fossil proper and that from the front-end proxy.

style-src 'self' 'unsafe-inline'

This policy means CSS files can only come from the Fossil server or via a front-end proxy as in the inline image workarounds above. It also says that inline CSS is disallowed; this will give a CSP error:

    <p style="margin-left: 4em">Some bit of indented text</p>

In practice, this means you must put your CSS into the “CSS” section of a custom skin, not inline within Markdown, Wiki, or HTML tags. You can refer to specific tags in the document through “id” and “class” attributes.

The reason for this restriction might not be obvious, but the risks boil down to this: CSS is sufficiently powerful that if someone can apply their CSS to your site, they can make it say things you don’t want it to say, hide important information, and more. Thus, we restrict all CSS to come from trusted channels only.

We do currently trust CSS checked into the repository as a file, but that stance might be overly-trusting, so we might revoke it later, as we do for JavaScript:

script-src 'self' 'nonce-%s'

This policy means HTML <script> tags are only allowed to be emitted into the output HTML by Fossil C or TH1 code, because only code running in those contexts can correctly apply the random “nonce” attribute to the tag that matches the one declared in the CSP, which changes on each HTTP hit Fossil handles.

This means the workarounds given above will not work for JavaScript. In effect, the only JavaScript that Fossil can serve is that which it directly provided, such as that for the CSS section of the skin and that behind the default hamburger menu.

We’re so restrictive about how we treat JavaScript because it can lead to difficult-to-avoid cross-site scripting attacks.

Replacing the Default CSP

If you wish to relax the default CSP’s restrictions or to tighten them further, there are two ways to accomplish that:

TH1 Setup Hook

The stock CSP text is hard-coded in the Fossil C source code, but it’s only used to set the default value of one of the TH1 skinning variables, $default_csp. That means you can override the default CSP by giving this variable a value before Fossil sees that it’s undefined and uses this default.

The best place to do that is from the th1-setup script, which runs before TH1 processing happens during skin processing:

    $ fossil set th1-setup "set default_csp {default-src: 'self'}"

This is the cleanest method, allowing you to set a custom CSP without recompiling Fossil or providing a hand-written <head> section in the Header section of a custom skin.

You can’t remove the CSP entirely with this method, but you can get the same effect by telling the browser there are no content restrictions:

    $ fossil set th1-setup 'set default_csp {default-src: *}'

Custom Skin Header

Fossil only inserts a CSP into the HTML pages it generates when the skin’s Header section doesn’t contain a <head> tag. None of the stock skins include a <head> tag,¹ so if you haven’t created a custom skin, you should be getting Fossil’s default CSP.

We say “should” because long-time Fossil users may be hanging onto a legacy behavior from before Fossil 2.5, when Fossil added this automatic <head> insertion feature. Repositories created before that release where the admin either defined a custom skin or chose one of the stock skins (!) will effectively override this automatic HTML <head> insertion feature because the skins from before that time did include these elements. Unless the admin for such a repository updated the skin to track this switch to automatic <head> insertion, the default CSP added to the generated header text in Fossil 2.7 is probably being overridden by the skin.

If you want the protection of the default CSP in your custom skin, the simplest method is to leave the <html><head>... elements out of the skin’s Header section, starting it with the <div class="head"> element instead as described in the custom skinning guide. Alternately, you can make use of $default_csp.

This then tells you one way to override Fossil’s default CSP: provide your own HTML header in a custom skin.

A useful combination is to entirely override the default CSP in the skin but then provide a new CSP in the front-end proxy layer using any of the many reverse proxy servers that can define custom HTTP headers.

Asides and Digressions:

The stock Bootstrap skin does actually include a <head> tag, but from Fossil 2.7 through Fossil 2.9, it just repeated the same CSP text that Fossil’s C code inserts into the HTML header for all other stock skins. With Fossil 2.10, the stock Bootstrap skin uses $default_csp instead, so you can override it as above.