CSP: script-src

The HTTP Content-Security-Policy (CSP) script-src directive specifies valid sources for sources for JavaScript. This includes not only URLs loaded directly into <script> elements, but also things like inline script event handlers (onclick) and XSLT stylesheets which can trigger script execution.

CSP version	1
Directive type	Fetch directive
default-src fallback	Yes. If this directive is absent, the user agent will look for the default-src directive.

Syntax

One or more sources can be allowed for the script-src policy:

Content-Security-Policy: script-src <source>;
Content-Security-Policy: script-src <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

Violation case

Given this CSP header:

Content-Security-Policy: script-src https://example.com/

the following script is blocked and won't be loaded or executed:

<script src="https//not-example.com/js/library.js"></script>

Note that inline event handlers are blocked as well:

<button id="btn" onclick="doSomething()">

You should replaced them with addEventListener calls:

document.getElementById("btn").addEventListener('click', doSomething);

Unsafe inline script

To allow inline scripts and inline event handlers, 'unsafe-inline', a nonce-source or a hash-source that matches the inline block can be specified.

Content-Security-Policy: script-src 'unsafe-inline';

The above Content Security Policy will allow inline <script> elements

<script type="text/javascript"> 
  var inline = 1; 
</script>

You can use a nonce-source to only allow specific inline script blocks:

Content-Security-Policy: script-src 'nonce-2726c7f26c'

You will have to set the same nonce on the <script> element:

<script type="text/javascript" nonce="2726c7f26c">
  var inline = 1;
</script>

Alternatively, you can create hashes from your inline scripts. CSP supports sha256, sha384 and sha512.

Content-Security-Policy: script-src 'sha256-076c8f1ca6979ef156b510a121b69b6265011597557ca2971db5ad5a2743545f'

When generating the hash, don't include the <script> tags and note that capitalization and whitespace matter, including leading or trailing whitespace.

<script type="text/javascript">var inline = 1;</script>

Unsafe eval expressions

The 'unsafe-eval' source expression controls several script execution methods that create code from strings. If 'unsafe-eval' isn't specified with the script-src directive, the following methods are blocked and won't have any effect:

• eval()
• Function()
• When passing a string literal like to methods like: window.setTimeout("alert(\"Hello World!\");", 500);
  • window.setTimeout
  • window.setInterval
  • window.setImmediate
• window.execScript (IE < 11 only)

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. For example, a policy such as script-src 'strict-dynamic' 'nonce-R4nd0m' https://whitelisted.com/ would allow loading of a root script with <script nonce="R4nd0m" src="https://example.com/loader.js"> and propogate that trust to any script loaded by loader.js, but disallow loading scripts from https://whitelisted.com/.

script-src 'strict-dynamic' 'nonce-someNonce'

Or

script-src 'strict-dynamic' 'sha256-hash'

It is possible to deploy strict-dynamic in a backwards compatible way, without requiring user-agent sniffing.
The policy:

script-src 'unsafe-inline' https: 'nonce-abcdefg' 'strict-dynamic'

will act like'unsafe-inline' https: in browsers that support CSP1, https: 'nonce-abcdefg' in browsers that support CSP2, and 'nonce-abcdefg' 'strict-dynamic' in browsers that support CSP3.

Specifications

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

Browser compatibility

See also

• Content-Security-Policy
• <script>