What is a Content Security Policy?
A Content Security Policy (CSP) is an important aspect of securing websites and web applications. CSP is a resource security specification (W3C recommendation) for web browsers. It is configured by sending directives in an HTTP header. Enforcing the directives is the responsibility of web browsers. After years of battling XSS and various injection attacks in websites, the CSP specification was created to provide websites a way of controlling what resources can be loaded within the site’s pages. By restricting the loading and execution of resources to a trusted whitelist, a website’s security can be substantially improved.
How does CSP work?
A common attack approach is to find existing application code that allows execution of arbitrary script provided by the user, possibly in an input field or in the URL query string. Once such a vulnerability has been identified a compact script is supplied that then loads a more complicated script by making a remote request. With a sufficiently strict CSP configuration, the remote request would be blocked by the browser since the remote URL is not a trusted source for script execution. It is still important to avoid deploying application code that executes user-supplied script!
CSP resource configuration options
The basic CSP directive is
default-src. This specifies the default source whitelist for all resources. Any resource type that is unspecified will fully inherit from
default-src. However, if a resource type has any values specified then that resource type does not inherit at all from
default-src. Some of the more specific resource types:
image-src: for <img> elements
style-src: for CSS
font-src: for fonts
media-src: for <audio> and <video> elements
rame-src: for <iframe> elements
Specifying source values
There are some keywords for source values:
none: block all sources (generally only useful if you want to completely block a specific resource type)
self: allow same-origin requests
unsafe-inline: allow inline scripts or styles (avoid if possible)
Values can also be specified by host or scheme. Hostnames can be schemeless and include wildcards (*). For example:
default-src: 'self'; script-src: 'self' *.cdn.com; font-src: 'self' https://fonts.gstatic.com
The default-src value restricts resources to the same origin. The
script-src value additionally allows <script> elements to specify sources from any subdomain of cdn.com (http or https). The
font-src value allows the same origin and fonts.gstatic.com (https only, http will be blocked).
How to configure CSP
For a small website a strict policy of
default-src: 'self' can be added as the first meta tag in the document head and each page tested. The web browser will log CSP violations in the console and rules can be added as needed to resolve violations. Once a working policy has been built it can be deployed, ideally using the Content-Security-Policy header instead of the meta tag.
Larger and more complicated websites may require a more complicated security policy, possibly even with page-specific policies. CSP provides a report-only configuration by using the Content-Security-Policy-Report-Only header. With this header, violations will not be enforced, but they will be logged to the browser console, and if a reporting URI has been provided then a request with a JSON body will be sent to the reporting URI. If you don’t want to create your own server to receive reports there are reporting services like Report URI and URIports. Report URI even includes a wizard reporting URI that will help you build up your policy by starting with a strict policy and monitoring logged violations to build up your whitelist.
Not all violation reports are useful
When CSP is enabled for a website violations are reported the same regardless of whether the violations were caused by your site content or external manipulation like attempted attacks or, more commonly, browser extensions. Fortunately, logging services include filtering options. Dropbox shared their experience analyzing CSP logs and provided some good advice on filtering.
Building secure web applications
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