CSP: default-src

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since August 2016.

The HTTP Content-Security-Policy (CSP) default-src directive serves as a fallback for the other CSP fetch directives. For each of the following directives that are absent, the user agent looks for the default-src directive and uses this value for it:

CSP version 1
Directive type Fetch directive

Syntax

http
Content-Security-Policy: default-src 'none';
Content-Security-Policy: default-src <source-expression-list>;

This directive may have one of the following values:

'none'

No resources may be loaded. The single quotes are mandatory.

<source-expression-list>

A space-separated list of source expression values. Resources may be loaded if they match any of the given source expressions. For this directive, any of the source expression values listed in Fetch directive syntax are applicable.

Examples

No inheritance with default-src

If there are other directives specified, default-src does not influence them. The following header:

http
Content-Security-Policy: default-src 'self'; script-src https://example.com

is the same as:

http
Content-Security-Policy: connect-src 'self';
                         font-src 'self';
                         frame-src 'self';
                         img-src 'self';
                         manifest-src 'self';
                         media-src 'self';
                         object-src 'self';
                         script-src https://example.com;
                         style-src 'self';
                         worker-src 'self'

Firefox default-src: none SVG sprite blocking issue

CSP guidelines often recommend starting with default-src 'none' to lock down all resource loading and then adding further directives to open up the policy, allowing you to load just the resources you need. For example, to allow same-origin loading of images only:

http
Content-Security-Policy: default-src 'none'; img-src 'self'

However, there is a problem here. If you are embedding SVG sprites defined in external files via the <use> element, for example:

svg
<svg>
  <use href="/images/icons.svg#icon"/>
</svg>

your SVG images will be blocked in Firefox if you have a default-src 'none' policy set. Firefox does not treat the SVG as an embedded image like other browsers do, therefore img-src 'self' will not allow them to be loaded. You need to use default-src 'self' if you want your external sprites to load in Firefox (see bug 1773976 and this CSP spec issue for more information).

Alternatively, if the default-src 'none' policy is a hard requirement, you can include the SVG sprites inline in the HTML page:

html
<body>
  <svg style="display: none">
    <symbol id="icon" viewBox="0 0 24 24">
      <path d="…" />
    </symbol>
  </svg>
  …
  <svg>
    <use href="#icon" />
  </svg>
</body>

Specifications

Specification
Content Security Policy Level 3
# directive-default-src

Browser compatibility

BCD tables only load in the browser

See also