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 and interpret 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; the CSP also adds restrictions on the types of inline content the browser is allowed to interpret.

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 tags in wiki and in Markdown docs, TH1 docs, the Admin → Wiki → “Use HTML as wiki markup language” mode, etc.

Fossil’s strong default CSP adds client-side filtering as a backstop for all of this.

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 allow 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.

For inline images within embedded documentation, it suffices to store the referred-to files in the repo and then refer to them using repo-relative URLs:

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

This avoids bloating the doc text with data: URI blobs:

There are many other cases, covered below.

style-src 'self' 'unsafe-inline'

This policy allows CSS information to come from separate files hosted under the Fossil repo server’s Internet domain. It also allows inline CSS <style> tags within the document text.

The 'unsafe-inline' declaration allows CSS within individual HTML elements:

    <p style="margin-left: 4em">Indented text.</p>

As the "unsafe-" prefix on the name implies, the 'unsafe-inline' feature is suboptimal for security. However, there are a few places in the Fossil-generated HTML that benefit from this flexibility and the work-arounds are verbose and difficult to maintain. Futhermore, the harm that can be done with style injections is far less than the harm possible with injected javascript. And so the 'unsafe-inline' compromise is accepted for now, though it might go away in some future release of Fossil.

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

This policy disables in-line JavaScript and only allows <script> elements if the <script> includes a nonce attribute that matches the one declared by the CSP. That nonce is a large random number, unique for each HTTP page generated by Fossil, so an attacker cannot guess the value, so the browser will ignore an attacker’s injected JavaScript.

That nonce can only come from one of three sources, all of which should be protected at the system administration level on the Fossil server:

Fossil server C code: All code paths in Fossil that emit <script> elements include the nonce attribute. There are several cases, such as the “JavaScript” section of a custom skin. That text is currently inserted into each HTML page generated by Fossil,¹ which means it needs to include a nonce attribute to allow it to run under this default CSP. We consider JavaScript emitted via these paths to be safe because it’s audited by the Fossil developers. We assume that you got your Fossil server’s code from a trustworthy source and that an attacker cannot replace your Fossil server binary.
TH1 code: The Fossil TH1 interpreter pre-defines the $nonce variable for use in custom skins. For example, some of the stock skins that ship with Fossil include a wall clock feature up in the corner that updates once a minute. These paths are safe in the default Fossil configuration because only the all-powerful Setup user can write TH1 code that executes in the server’s running context.

There is, however, a default-disabled path to beware of, covered in the next section.
CGI server extensions: Fossil exports the nonce to the CGI in the FOSSIL_NONCE environment variable, which it can then use in <script> elements it generates. Because these extensions can only be installed by the Fossil server’s system administrator, this path is also considered safe.

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.

Serving Files Within the Limits

There are several ways to serve files within the above restrictions, avoiding the need to override the default CSP. In decreasing order of simplicity and preference:

Within embedded documentation (only!) you can refer to files stored in the repo using document-relative file URLs:
```
 ![inline image](./inlineimage.jpg)
```
Relative file URLs don’t work from wiki articles, tickets, forum posts, or tech notes, but you can still refer to them inside the repo with /doc or /raw URLs:
```
 ![inline image](/doc/trunk/images/inlineimage.jpg)
 <img src="/raw/logo.png" style="float: right; margin-left: 2em">
```
Store the files as unversioned content, referred to using /uv URLs instead:
```
 ![logo](/uv/logo.png)
```
Use the optional CGI server extensions feature to serve such content via /ext URLs.
Put Fossil behind a front-end proxy server as a virtual subdirectory within the site, so that our default CSP’s “self” rules match static file routes on that same site. For instance, your repo might be at https://example.com/code, allowing documentes in that repo to refer to:
- images as /image/foo.png
- JavaScript files as /js/bar.js
- CSS style sheets as /style/qux.css
Although those files are all outside the Fossil repo at /code, keep in mind that it is the browser’s notion of “self” that matters here, not Fossil’s. All resources come from the same Internet domain, so the browser cannot distinguish Fossil-provided content from static content served directly by the proxy server.

This method opens up many other potential benefits, such as TLS encryption, high-performance tuning via custom HTTP headers, integration with other web technologies like PHP, etc.

You might wonder why we rank in-repo content as most preferred above. It is because the first two options are the only ones that cause such resources to be included in an initial clone or in subsequent repo syncs. The methods further down the list have a number of undesirable properties:

Relative links to out-of-repo files break in fossil ui when run on a clone.
Absolute links back to the public repo instance solve that:
```
![inline image](https://example.com/images/logo.png)
```
...but using them breaks some types of failover and load-balancing schemes, because it creates a single point of failure.
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 probably forgot to copy such external resources in the backup copies, so that when the main repo site disappears, so do those files.

Unversioned content is in the middle of the first list above — between fully-external content and fully in-repo content — because it isn’t included in a clone unless you give the --unversioned flag. If you then want updates to the unversioned content to be included in syncs, you have to give the same flag to a sync command. There is no equivalent with other commands such as up and pull, so you must then remember to give fossil uv commands when necessary to pull new unversioned content down.

Thus our recommendation that you refer to in-repo resources exclusively.

Overriding 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:

Fossil might someday switch to serving the “JavaScript” section of a custom skin as a virtual text file, allowing it to be cached by the browser, reducing page load times.
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.