CSP: base-uri

The HTTP Content-Security-Policy base-uri directive restricts the URLs which can be used in a document's <base> element. If this value is absent, then any URI is allowed. If this directive is absent, the user agent will use the value in the <base> element.

CSP version	2
Directive type	Document directive
default-src fallback	No. Not setting this allows anything.

Syntax

One or more sources can be allowed for the base-uri policy:

Content-Security-Policy: base-uri <source>;
Content-Security-Policy: base-uri <source> <source>;

Sources

<source> can be one of the following:

<host-source>

Internet hosts by name or IP address, as well as an optional URL scheme and/or port number. The site's address may include an optional leading wildcard (the asterisk character, '*'), and you may use a wildcard (again, '*') as the port number, indicating that all legal ports are valid for the source.
Examples:

• http://*.example.com: Matches all attempts to load from any subdomain of example.com using the http: URL scheme.
• mail.example.com:443: Matches all attempts to access port 443 on mail.example.com.
• https://store.example.com: Matches all attempts to access store.example.com using https:.

<scheme-source>

A schema such as 'http:' or 'https:'. The colon is required, single quotes shouldn't be used. You can also specify data schemas (not recommended).

• data: Allows data: URIs to be used as a content source. This is insecure; an attacker can also inject arbitrary data: URIs. Use this sparingly and definitely not for scripts.
• mediastream: Allows mediastream: URIs to be used as a content source.
• blob: Allows blob: URIs to be used as a content source.
• filesystem: Allows filesystem: URIs to be used as a content source.

'self'

Refers to the origin from which the protected document is being served, including the same URL scheme and port number. You must include the single quotes. Some browsers specifically exclude blob and filesystem from source directives. Sites needing to allow these content types can specify them using the Data attribute.

'unsafe-inline'

Allows the use of inline resources, such as inline <script> elements, javascript: URLs, inline event handlers, and inline <style> elements. You must include the single quotes.

'unsafe-eval'

Allows the use of eval() and similar methods for creating code from strings. You must include the single quotes.

'none'

Refers to the empty set; that is, no URLs match. The single quotes are required.

'nonce-<base64-value>'

A whitelist for specific inline scripts using a cryptographic nonce (number used once). The server must generate a unique nonce value each time it transmits a policy. It is critical to provide an unguessable nonce, as bypassing a resource’s policy is otherwise trivial. See unsafe inline script for an example.

<hash-source>

A sha256, sha384 or sha512 of inline scripts or styles. When generating the hash, don't include the <script> or <style> tags and note that capitalization and whitespace matter, including leading or trailing whitespace. See unsafe inline script for an example.

'strict-dynamic'

The strict-dynamic source expression specifies that the trust explicitly given to a script present in the markup, by accompanying it with a nonce or a hash, shall be propagated to all the scripts loaded by that root script. At the same time, any whitelist or source expressions such as 'self' or 'unsafe-inline' will be ignored. See script-src for an example.

Examples

Meta tag configuration

<meta http-equiv="Content-Security-Policy" content="base-uri 'self'">

Apache configuration

<IfModule mod_headers.c> 
Header set Content-Security-Policy "base-uri 'self';
</IfModule>

Nginx configuration

add_header Content-Security-Policy "base-uri 'self';"

Violation case

Given your domain isn't example.com, using a <base> element with an href set to example.com will result in a CSP violation.

<meta http-equiv="Content-Security-Policy" content="base-uri 'self'">
<base href="http://example.com/">

// Error: Refused to set the document's base URI to 'http://example.com/' 
// because it violates the following Content Security Policy 
// directive: "base-uri 'self'"

Specifications

Specification	Status	Comment
Content Security Policy Level 3 The definition of 'base-uri' in that specification.	Editor's Draft	No changes.
Content Security Policy Level 2 The definition of 'base-uri' in that specification.	Candidate Recommendation	Initial definition.

Browser compatibility

See also

• Content-Security-Policy
• <base>
• Node.baseURI