CSP: base-uri

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.

Syntax

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

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

Sources

While this directive uses the same arguments as other CSP directives, some of them don’t make sense for `<base>`, such as the keywords 'unsafe-inline' and 'strict-dynamic'

<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:.
• *.example.com: Matches all attempts to load from any subdomain of example.com using the current protocol.

<scheme-source>

A scheme such as http: or https:. The colon is required. Unlike other values below, single quotes shouldn't be used. You can also specify data schemes (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-eval'

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

'unsafe-hashes'

Allows enabling specific inline event handlers. If you only need to allow inline event handlers and not inline <script> elements or javascript: URLs, this is a safer method than using the unsafe-inline expression.

'unsafe-inline'

Allows the use of inline resources, such as inline <script> elements, javascript: URLs, inline event handlers, and inline <style> elements. The single quotes are required.

'none'

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

'nonce-<base64-value>'

An allow-list 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. Specifying nonce makes a modern browser ignore 'unsafe-inline' which could still be set for older browsers without nonce support.

Note: The CSP nonce source can only be applied to nonceable elements (e.g., as the <img> element has no nonce attribute, there is no way to associate it with this CSP source).

'<hash-algorithm>-<base64-value>'

A sha256, sha384 or sha512 hash of scripts or styles. The use of this source consists of two portions separated by a dash: the encryption algorithm used to create the hash and the base64-encoded hash of the script or style. 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. In CSP 2.0, this is applied only to inline scripts. CSP 3.0 allows it in the case of script-src for external scripts.

'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 allow-list or source expressions such as 'self' or 'unsafe-inline' are ignored. See script-src for an example.

'report-sample'

Requires a sample of the violating code to be included in the violation report.

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

Since your domain isn't example.com, a <base> element with its href set to https://example.com will result in a CSP violation.

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

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

Specifications

Browser compatibility

See also

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