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CSP: base-uri

HTTP 协议 Content-Security-Policy 首部字段中的 base-uri 指令限制了可以应用于一个文档的 <base> 元素的 URL。假如指令值为空，那么任何 URL 都是允许的。如果指令不存在，那么用户代理会使用 <base> 元素中的值。 

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

语法

base-uri 安全策略可以设置一个或多个源：

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

源

<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.

示例

Meta tag 配置

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

Apache 配置

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

Nginx 配置

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

违犯策略的案例

假如你的域名不是 example.com，那么将 <base> 元素的 href 属性值设置成 example.com 会违犯 CSP 策略。

<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'"

规范

Specification Status Comment
Content Security Policy Level 3
base-uri		 Working Draft No changes.
Content Security Policy Level 2
base-uri		 Recommendation Initial definition.

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