What is Content Security Policy (CSP)?

Understanding Content Security Policy

Content security policy (CSP) is a capability defined by the W3C to limit the threat of cross-site scripting (XSS) attacks. Implemented as part of the web standards, CSP directs the browser to enforce certain client-side policies and restrict what scripts and resources it can load for a given website. For example, the script-src directive in CSP can specify an allowlist of known domains from which inline scripts can be loaded. This can thwart an attack involving a malicious shadow code injection from an unauthorized domain.

CSP can also restrict network communication to specific domains using the connect-src directive. This can prevent XHR requests to malicious domains that might be used to exfiltrate stolen data. CSP can be sent to the browser either as an HTML meta tag or as a content-security-policy header. When there are multiple content security policies specified, for example using a meta tag and a content-security-policy header, the browser will use the most restrictive directives.

Developers can use CSP to protect their applications from shadow code injection vulnerabilities such as XSS and reduce the privilege with which their applications execute. CSP is not a substitute for careful input validation and output encoding. It is best used as part of a defense-in-depth strategy. It reduces the harm caused by a malicious shadow code injection attack that can trigger an XSS vulnerability in the JavaScript application.

CSP is typically configured on the web server or injected via a content delivery network (CDN). The website administrator must ensure that the CSP directives include all required domains and must update them regularly. Failure to do so may result in websites with broken functionality and a poor user experience.

How Does a Content Security Policy Work?

Content security policy (CSP) consists of a set of directives sent to the browser either as a content-security-policy header sent as part of the HTTP response header, or an HTML meta tag included inline on the page. When a browser receives these directives, it inspects every resource and script that the page requests and checks to ensure that the origin domain is part of the allowlist. If not, it blocks the resource from loading.

CSP also supports a report-only mode which uses the content-security-policy-report-only header. In this mode, the browser inspects resources and scripts against the allowlist but does not block them. Instead, policy violations are simply reported to a server specified in the report-uri directive. This mode is not supported using HTML meta tags and the directives must be sent in the HTTP header.

These are the directives supported as part of CSP:

default-src Specifies the default policy for fetching all types of resources. This serves as a fallback when there is no explicit directive for a resource type.
script-src Specifies allowed sources for JavaScript.
object-src Specifies allowed sources for plugins, e.g. , or .
img-src Specifies allowed sources for images.
media-src Specifies allowed sources for media.
style-src Specifies allowed sources for stylesheets or CSS.
frame-src Specifies allowed sources for loading frames.
font-src Specifies allowed sources for loading fonts.
connect-src Specifies allowed domains for XHR, WebSocket, fetch(), or EventSource.
report-uri Specifies a URI where the browser must post reports of content security policy violations. This can also be used in the content-security-policy-report-only header which reports violations but does not block the resources from loading.
sandbox (optional) Loads the page in a sandbox that applies the same-origin policy and prevents popups, plugins and script execution, similar to the iframe sandbox. This directive can include values that selectively allow certain exceptions.
report-uri Specifies a URI where reports of policy violations must be posted.

The directives that end in ‘-src’ support values known as the source list. These could be domain names, wildcards or specific values defined in the CSP specification such as ‘self’, ‘none’, ‘unsafe-inline’ or ‘unsafe-eval’.

CSP is supported by most common browsers including Google Chrome, Safari, Firefox and Edge.

What Are Some Examples of Content Security Policy Headers?

The content security policy header is included as part of the HTTP response header. It consists of a set of directives followed by source lists.

The simplest example of CSP is one that only allows first-party resources:
Content-Security-Policy: default-src 'self';

Here is another example of a longer, more complex CSP header:
Content-Security-Policy: default-src 'self'; img-src *; script-src ‘self’ report-uri

This CSP specifies domains for loading images and scripts, and a report-uri domain. All other resources must be loaded from ‘self’, which is the domain serving this page.

Here is another example of a report-only CSP header:
Content-Security-Policy-Report-Only: default-src 'self' report-uri

This CSP header will report all ‘violations’, i.e. any resources not loaded as first-party, since the default-src directive only allows ‘self’. However this policy will not prevent third-party resources from being loaded. This is a great way to test a CSP without risking website functionality.

Why Do Businesses Need Content Security Policy?

Modern websites are a complex mix of content, resources, and JavaScript that creates a rich, dynamic user experience. All of this content is loaded and executed by the browser on the user’s laptop or mobile device, which improves performance and responsiveness. However, this also creates an attack surface for hackers to inject and execute malicious scripts from alternate domains. These scripts can be used to execute cross-site scripting (XSS) attacks, or digital skimming attacks such as Magecart that result in the theft of personal data from your website.

In addition, content security policy is a useful tool in the fight against unauthorized shadow code that plagues web applications. Shadow code is typically introduced via scripts from open source libraries or third-parties that make their way onto a site without formal approvals or security validation.

Because shadow code often runs on the client-side browser, it cannot be seen or controlled using server-side security tools. This leaves an entry point for cybercriminals to inject malicious scripts that skim user data. Client-side attacks, such as digital skimming, formjacking and Magecart, can lead to data breaches or compliance penalties.

CSP script-src directive can be used to restrict the domains from where scripts can be loaded. This ensures that scripts are only loaded from known good domains, reducing the risk of shadow code injections on the website. Additionally, the connect-src directive can be used to restrict network communication with unknown domains, which can stop a malicious script from communicating with its command and control (C2) server and thus limit its ability to cause harm.

What Are the Limitations of Content Security Policy?

While content security policy is a useful tool in the fight against XSS, digital skimming and Magecart attacks, it is not without its limitations.

  1. Browser support: CSP is almost universally supported by most browsers, with one notable exception – Internet Explorer (IE) – which only partially supports CSP.
  2. Browser extensions: Some browser extensions may completely disable CSP in the browser settings. This would render all the CSP directives ineffective.
  3. First-party attacks: Hackers can gain access to one of the allowed hosts or ‘self’ in the CSP and host the malicious scripts there. CSP will not protect against first-party attacks.
  4. Version management: CSP directives require an up-to-date source list that includes all the domains required to load all the scripts that power the website. With third-party scripts, this is very difficult to manage since vendors might frequently change how the script is delivered. Additionally, with the high pace of innovation in some industries such as e-commerce, the website itself might change frequently, requiring frequent updates to the CSP. It can be difficult to manage CSP in an agile environment where more than 70% of the scripts are external.
  5. Limited directives: CSP directives are limited in what they can control. For example, they cannot prevent a script from modifying the document object model (DOM), which could be used to insert fake checkout buttons or hijack users to a malicious domain. Also, CSP cannot prevent a script from writing to local storage, which could be used in a multi-phase attack.

Effective web application security solutions should be:

  • Easy to integrate: Solutions should not require a major change to the website architecture or content delivery network.
  • Easy to manage: Security policies often fall behind if they are difficult to maintain. Eventually such policies become ineffective as they are forced into fail-open configurations.
  • Performant: Solutions should not impose performance penalties that run the risk of degrading user experience.
  • Universally supported: Solutions should be uniformly supported across a wide variety of browsers and platforms. Exceptions create holes in your security defenses that attackers can exploit.
Content security policy only satisfies some of these requirements.

How Can Businesses Effectively Use Content Security Policy?

Content security policy is an attractive solution since it is easy to integrate, almost universally supported and does not impose any performance penalties. However, due to the many limitations of CSP, website administrators should continue to follow security best practices to minimize the chance of JavaScript security vulnerabilities. Examples include:

  • Sandboxing: This approach involves running third-party services within controlled sandboxes in the web application. These vendor-specific sandbox technologies can control what third-party scripts are allowed to do, thus providing an enforcement point over external code. Sandboxing technologies do impose a performance penalty since they leverage a client-side or server-side proxy. Also, they require changes to the website architecture and require frequent policy updates to accommodate changes in third-party scripts.
  • JavaScript sensors: JavaScript-based sensors running on the page have the ability to collect behavior signals from all scripts. Combined with a cloud-based machine learning engine, this technology can accurately baseline script behavior and flag anomalies that could indicate malicious code injection. This type of solution requires minimal configuration and maintenance and does not impact site performance or user experience.
  • Granular blocking: Rather than using CSP to block an entire script, granular controls allow you to prevent client-side JavaScript from accessing specific information (such as credit card numbers or addresses). This enables sites to take advantage of third-party script while still shoring up access controls.

How Does HUMAN Code Defender Use Content Security Policy?

Content security policy remains a powerful defense against XSS and client-side script injections, but it is not sufficient to stop client-side attacks alone. HUMAN Compliance and Supply Chain Defense is a web application security solution that partners CSP with granular JavaScript blocking to mitigate malicious client-side code. This multi-layered approach allows you to block specific actions in a script without blocking the entire script, and to block unwanted scripts entirely. 

With Compliance and Supply Chain Defense, you gain full visibility into the scripts running on your site, including their interactions and any exposure details. You can identify high-risk incidents involving personally identifiable information (PII), payment card industry (PCI) data, and other sensitive information. The solution provides the real-time visibility and control needed to protect against client-side attacks.


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