HTTP Security Headers: Content-Security-Policy
Author: HollyGraceful Published: 19 October 2020
Content Security Policy (CSP) allows the application to restrict the location of resources to an allow-list of approved locations, including where scripts can be loaded from and when the application may be framed. This can therefore mitigate reflected and stored cross-site scripting attacks as well as issues such as Clickjacking.
ClickJacking may also be prevented using the X-Frame-Options security, however this is not fully implemented in some browsers, for example in Chrome the only available options are DENY and SAMEORIGIN.
Content Security Policy is enabled by supplying a Content-Security-Policy HTTP response header. The CSP header is made up of two parts, a list of directives and a list of approved locations.
It is recommended that at a minimum the following directives are supplied: script-src, object-src, style-src, and frame-ancestors. It is also recommended that no directives marked as unsafe are included, for example 'unsafe-inline' and 'unsafe-eval'.
The following are examples of available directives:
|default-src||A default allow-list for where more specific directives have not been supplied.|
|img-src||Specifies where images can be loaded from.|
|style-src||Specifies where CSS can be loaded from.|
|connect-src||Specifies where features such as XMLHttpRequest may connect to.|
|font-src||Specifies where fonts may be loaded from.|
|object-src||Specifies where object, embed, and applet elements may be loaded from.|
|media-src||Specifies where audio, video and track elements may be loaded from.|
|frame-src||Frame-src is for where frames can be loaded from, which was deprecated in CSP Level 2 and then undeprecated in CSP Level 3.|
|frame-ancestors||Specifies where this page may be framed from. Setting frame-ancestors to ‘none’ is the equivalent of setting the X-Frame-Options header to DENY.|
The following are examples of source locations that can be given to directives:
|*||A wildcard to allow any URL except data: and filesystem: schemes.|
|‘none’||Disables this resource type|
|‘self’||Allows resources from the same-origin|
|data:||Allows the loading of resources via the data scheme, such as base64 encoded images|
|domain.example.com||Allows loading resources from the specified domain|
|*.example.com||Allows loading resources from any subdomain of the specified domain|
|https://example.com||Allow loading resources from the specified domain, if supplied over HTTPS|
|https:||Allow loading resources from any domain, if supplied over HTTPS|
|‘sha256-‘||Allow scripts to execute if they match the supplied hash. sha256-, sha384-, or sha512- may be used.|
|‘nonce-‘||Allow scripts to be used if they include a nonce= attribute which matches the supplied none entry in the header. These numbers should not be used for more than one script, instead additional nonce- entries should be supplied.|
The source list can be used, with the directives to build a policy. For example:
Content-Security-Policy: default-src: 'none'; script-src: js.example.com; img-src: static.example.com; font-src: static.example.com;