CSP is a mechanism to improve web page security by limiting the source of resource loading. Its core is to set policies through HTTP headers or meta tags, and control the browser to load only scripts, styles and other resources from the specified source to prevent XSS attacks. To configure CSP, you need to set the Content-Security-Policy header, for example, default-src 'self' restricts the default resource source, script-src specifies the script source that is allowed to be loaded, and style-src controls stylesheet loading. Common problems after activation include resource interception, which can be solved through report-uri reporting, Report-Only mode testing, avoiding the use of 'unsafe-inline' and 'unsafe-eval', and debugging in combination with developer tools. In actual applications, you need to pay attention to the limitations of dynamically inserting inline scripts and styles, and third-party resources need to be added to the whitelist. Reasonable planning of strategies can reduce maintenance costs.

CSP (Content Security Policy) is a very practical technology in modern web security, especially in preventing XSS (cross-site scripting attacks). It greatly reduces the risk of malicious code injection by restricting the browser to only load resources from specified sources.

What is CSP?

CSP is an HTTP response header, which can also be set through the HTML <meta> tag. Its core function is to tell the browser which resources can be loaded and executed and which ones should be blocked. For example, you can set that only JavaScript and CSS files are allowed to be loaded from your own server or specific CDN, and other sources are prohibited.

This mechanism is particularly useful, especially on websites where users can submit content. Without CSP, an attacker may inject malicious scripts through comments, forms, etc. and execute them; while with CSP, even if the script is inserted into the page, it will not be executed, thus protecting user data.

How to configure basic CSP policies?

Configuring CSP is mainly done by setting Content-Security-Policy HTTP header. Here is a simple example:

 Content-Security-Policy: default-src 'self'; script-src 'self' https://cdn.example.com; style-src 'self' 'unsafe-inline'

The above strategy means:

• All resources can only be loaded from the current domain name by default ( default-src 'self' )
• The script can be loaded from the current domain name and https://cdn.example.com ( script-src )
• Stylesheets allow inline ( 'unsafe-inline' )

Of course, this is just the basic writing method. In actual use, more fine-grained rules can be defined as needed, such as limiting the source of resources such as images, fonts, and iframes.

FAQs and debugging tips

After enabling CSP, the most common situation is that some resources are accidentally intercepted, resulting in abnormal page functionality. At this time, you can check it through the following methods:

• Use report-uri or report-to field : Report policy violations for easy analysis and repair.
• First use Content-Security-Policy-Report-Only mode to test : the resource will not be really intercepted, it will only report violations.
• Note the risk of using 'unsafe-inline' : Although convenient, it will weaken the security of CSP.
• Avoid using eval() and new Function() : These dynamic code execution methods will also be blocked by CSP unless 'unsafe-eval' is used, but this is not recommended.

It is recommended to cooperate with browser developer tools to view the console error information during development, so as to locate problems faster.

Small details in practical applications of CSP

Sometimes, we may encounter some "seemingly reasonable" practices that do not actually comply with CSP specifications. For example:

• Loading scripts using the <script inline></script> tag may be blocked even if the script content is written by itself unless explicitly allowed 'unsafe-inline'
• Dynamically generated style strings are inserted into the DOM and may also be intercepted by the style-src policy
• When introducing third-party statistics scripts or ad codes, if they are not whitelisted, they will be rejected as illegal resources.

Therefore, before introducing any external resources, it is best to plan your CSP policy in advance and classify the resources.

Basically that's it. CSP is not complicated, but details are easily overlooked. As long as you consciously design your strategy from the beginning, it will not be too troublesome to maintain it later.

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