Fossil: The Default Content Security Policy (CSP)

When Fossil’s web interface generates an HTML page, it normally includes a Content Security Policy (CSP) in the <head>. The CSP defines a “white list” to tell the browser what types of content (HTML, images, CSS, JavaScript...) the document may reference and the sources the browser is allowed to pull such content from. The aim is to prevent certain classes of cross-site scripting (XSS) and code injection attacks. The browser will not pull content types disallowed by the CSP.

Fossil has built-in server-side content filtering logic. For example, it purposely breaks <script> tags when it finds them in Markdown and Fossil Wiki documents. (But not in HTML-formatted embedded docs!) We also back that with multiple levels of analysis and checks to find and fix content security problems: compile-time static analysis, run-time dynamic analysis, and manual code inspection. Fossil is open source software, so it benefits from the “many eyeballs,” limited by the size of its developer community.

However, there is a practical limit to the power of server-side filtering and code quality practices.

First, there is an endless battle between those looking for clever paths around such barriers and those erecting the barriers. The developers of Fossil are committed to holding up our end of that fight, but this is, to some extent, a reactive posture. It is cold comfort if Fossil’s developers react quickly to a report of code injection — as we do! — if the bad guys learn of it and start exploiting it first.

Second, Fossil has purposefully powerful features that are inherently difficult to police from the server side: HTML in wiki and in Markdown docs, TH1 docs, etc.

Fossil’s strong default CSP adds client-side filtering to backstop our server-side measures.

Fossil site administrators can modify the default CSP, perhaps to add trusted external sources for auxiliary content. But for maximum safety, site developers are encouraged to work within the restrictions imposed by the default CSP and avoid the temptation to relax the CSP unless they fully understand the security implications of what they are doing.

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. Browsers will ignore a link like the one in the following Markdown under our default CSP:

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

If you look in the browser’s developer console, you should see a CSP error when attempting to render such a page.

The default policy does 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. There are secondary problems as well: if you put a large image into a Fossil forum post this way, anyone subscribed to email alerts will get a copy of the raw URI text, which can amount to pages and pages of ugly Base64-encoded text.

Fossil offers several alternatives for serving large content resources from within the repository:

versioned content via /raw
unversioned content via /uv
relative links

Only the first two options work in wiki articles, tickets, forum posts, and tech notes. The last option is a much simpler alternative, but it only works within embedded documentation:

     ![large inline image](./inlineimage.jpg)

Because all of these methods pull content from within the Fossil repository, they all count as “self” for the purposes of the CSP.

This rule also works when the Fossil repository is but one path in a larger website. The browser can’t distinguish Fossil-served content from that served by the rest of the same web domain, so your repository can refer to other resources within that same web site, whether they are static files served by an HTTP proxy in front of Fossil, by another Fossil repository served under that same domain, or dynamic content served by, say, a PHP app on that same site.

Beware that there are a number of problems that come up with using such out-of-repository resources, which all stem from the fact that they aren’t included in a sync:

Relative links break in fossil ui when run on a clone.
Absolute links break under certain types of failover and load-balancing schemes.
Absolute links fail when one’s purpose in using a clone is to recover from the loss of a project web site by standing that clone up as a server elsewhere.

You can avoid all of these problems by referring to in-repo resources exclusively.

style-src 'self' 'unsafe-inline'

This policy allows CSS information to come from separate files in the same domain of the Fossil server, or for CSS to be embedded inline within the document text.

The 'unsafe-inline' element means that an injection vulnerability in Fossil would allow an attacker to modify the CSS for a Fossil-generated page. This is not ideal, but nor is it as dangerous as allowing injected javascript to run, and Fossil uses of in-line CSS for things like setting background colors in timelines and defining line widths in bar graphs on the Activity Reports page, so it seems like in-line CSS is a necessary compromise at this time.

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

This policy disables in-line javascript and only allows <script> elements if the <script> includes a nonce= attribute the matches the %s section of the CSP. Fossil provides a different random nonce for every page it generates, and since an attacker has no way of predicting what that nonce will be, the attacker is unable to inject working javascript.

For documents generated by the CGI extensions, the value of the nonce is accessible in the FOSSIL_NONCE environment variable. TH1 scripts that run while generating the header or footer can access the nonce in the $nonce variable. The JavaScript section of a custom skin automatically includes the appropriate nonce.

Cross-Site Scripting via Ordinary User Capabilities

We’re so restrictive about how we treat JavaScript because it can lead to difficult-to-avoid scripting attacks. If we used the same CSP for <script> tags as for <style> tags, anyone with check-in rights on your repository could add a JavaScript file to your repository and then refer to it from other content added to the site. Since JavaScript code can access any data from any URI served under its same Internet domain, and many Fossil users host multiple Fossil repositories under a single Internet domain, such a CSP would only be safe if all of those repositories are trusted equally.

Consider the Chisel hosting service, which offers free Fossil repository hosting to anyone on the Internet, all served under the same http://chiselapp.com/user/$NAME/$REPO URL scheme. Any one of those hundreds of repositories could trick you into visiting their repository home page, set to an HTML-formatted embedded doc page via Admin → Configuration → Index Page, with this content:

     <script src="/doc/trunk/bad.js"></script>

That script can then do anything allowed in JavaScript to any other Chisel repository your browser can access.The possibilities for mischief are vast. For just one example, if you have login cookies on four different Chisel repositories, your attacker could harvest the login cookies for all of them through this path if we allowed Fossil to serve JavaScript files under the same CSP policy as we do for CSS files.

This is why the default configuration of Fossil has no way for embedded docs, wiki articles, tickets, forum posts, or tech notes to automatically insert a nonce into the page content. This is all user-provided content, which could link to user-provided JavaScript via check-in rights, effectively giving all such users a capability that is usually reserved to the repository’s administrator.

The default-disabled TH1 documents feature is the only known path around this restriction. If you are serving a Fossil repository that has any user you do not implicitly trust to a level that you would willingly run any JavaScript code they’ve provided, blind, you must not give the --with-th1-docs option when configuring Fossil, because that allows substitution of the pre-defined $nonce TH1 variable into HTML-formatted embedded docs:

     <script src="/doc/trunk/bad.js" nonce="$nonce"></script>

Even with this feature enabled, you cannot put <script> tags into Fossil Wiki or Markdown-formatted content, because our HTML generators for those formats purposely strip or disable such tags in the output. Therefore, if you trust those users with check-in rights to provide JavaScript but not those allowed to file tickets, append to wiki articles, etc., you might justify enabling TH1 docs on your repository, since the only way to create or modify HTML-formatted embedded docs is through check-ins.

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,² 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:

There is actually a third context that can correctly insert this nonce attribute: a CGI server extension, by use of the FOSSIL_NONCE variable sent to the CGI by Fossil.
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.