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Common Error Messages for Content Security Policy Headers

Last modified on Wednesday, May 8, 2024

5 minute read

Common Error Messages for Content Security Policy Headers

Common Error Messages for Content Security Policy Headers

A strong content security policy header is important, so important in fact, that it counts towards our Cyber Security Risk Score for assessing a company's cyber security level. Below are some common error messages in Content Security Policy Headers. It is recommended to use Content-Security-Policy-Report-Only while you are in the process of controling your site's access to scripts, frames, styles, and more. Stellastra can assist you with your Content Security Policy journey. Below are some common Content Security Policy Errors and ways to fix them.

Content Security Policy of your Site Blocks the Use of 'eval' in JavaScript.

By default, a content security policy will block eval, in fact blocking the use of 'eval' in JavaScript is one of the primary goals of Content Security Policy in the first place. 'Eval' in JavaScript executes string as code, which means malicious actors can feed code into your database, leading to all sorts of problems, such as data exfiltration, ransomware encryption, website compromise, card fraud, and theft of funds. You should avoid using eval() directly in your code, or indirectly, through a called 3rd, 4th, or 5th party function. Some use unsafe-eval as an alternative, but the hint is in the name, it should be avoided and only ever used with strong compensating controls.

Refused to Connect to 'URL' Because it Violates the Following Content Security Policy Directive: "connect-src 'self'"

Connect-src in CSP allow lists which URLs the client's browser is permitted to establish connections with, regulating all client URL calls including anchor hrefs, fetch(), AJAX requests and WebSocket connections. An example of connect-src would be "connect-src https://stellastra.com/". With connect-src, only Stellastra.com is valid as a link in anchor href tags (<a href="https://stellastra.com/">The Cyber Security Comparison Platform</a>). The connect-src also identifies other, less utilized forms of inter-webpage connecting such as other XML HTTPS requests, fetches, etc.

Content Security Policy: the Page’s Settings Blocked the Loading of a Resource

When you encounter a Content Security Policy (CSP) error message indicating that the page's settings blocked the loading of a resource, it means that the CSP rules defined for the page prevent the browser from loading a specific resource, such as a script, stylesheet, image, font, or other types of content. To resolve this issue, you can take the following steps:

  • Identify the Blocked Resource: The error message should specify the type of resource that is being blocked and potentially the URL from which it's trying to load. This information is crucial for understanding what needs to be adjusted.
  • Review the CSP Policy: Look at the CSP policy defined for the page. You can usually find this in the HTTP response headers or in a meta tag in the HTML. Identify the directive(s) that are blocking the resource.
  • Adjust the CSP Policy:
  • Allow the Resource: If the resource is essential for the functionality of your page, you may need to modify the CSP policy to allow loading it. Determine the appropriate CSP directive to adjust, such as script-src, style-src, img-src, etc., and add the necessary source(s) or keyword(s) to allow the resource to load.
  • Review Trusted Sources: Ensure that the sources from which your page is loading resources are trustworthy. Avoid adding overly permissive CSP directives that might compromise security.
  • Test and Iterate: After making adjustments to the CSP policy, thoroughly test your page to ensure that the necessary resources load as expected without triggering CSP errors. Iterate on the adjustments if needed until the page functions correctly.
  • Consider Security Implications: While resolving CSP errors, it's crucial to maintain a balance between security and functionality. Avoid overly permissive CSP policies that may expose your page to security risks. Regularly review and update your CSP policy as needed to adapt to changes in your page's requirements and security best practices. By following these steps, you can effectively resolve CSP errors related to blocked resources and ensure that your web page functions correctly while maintaining a strong security posture.

Refused to Load the Script Because it Violates the Following Content Security Policy Directive

The steps to fix Content Security Policy script blocking is the same as for any object as above. However, as a script, the focus should be on the script-src tag.

Because an Ancestor Violates the Following Content Security Policy Directive: "Frame-Ancestors 'Self'"

Frames allow you to embed websites within websites. There are a whole host of security considerations to consider, which is why the Content Security Policy (CSP) Frame-Ancestors header requires that you allow list websites which are allowed to embed your website. The specific directive mentioned in your error message, "frame-ancestors," controls which web pages are allowed to embed the current page via frames, iframes, or similar technologies. When the browser evaluates this directive, it checks if the ancestor (the page embedding or framing your content) complies with the policy. If not, it blocks the content from being framed or embedded. If you're seeing this error, it means that the page you're trying to load is attempting to be embedded or framed by another page that doesn't comply with the CSP directive specified. This can happen if the framing page has a stricter CSP policy that doesn't allow framing of external content, or if it violates other CSP directives. To resolve this issue, you'll need to adjust the CSP policy of the framing page to allow framing of the content from the page you're trying to load. This typically involves adding the domain of the framed content to the frame-ancestors directive in the CSP header of the framing page. However, if you don't control the framing page, you may need to reach out to the owner of that page to ask them to adjust their CSP policy accordingly. For security reasons they are very unlikely to do this unless you have a close, trusted relationship with them.

Refused to Load the Image Because it Violates the Following Content Security Policy Directive: "img-src 'self'"

The Content Security Policy (CSP) Directive; "img-src 'self' means there's an issue with the Content Security Policy (CSP) directive related to loading images (img-src). The CSP directive 'img-src 'self' restricts the URLs from which images can be loaded to only those that originate from the same origin as the current page. The 'self' token refers to the origin of the current page. If you encounter this error, it suggests that the page you're accessing is attempting to load images from a source that is not allowed by its CSP policy. This could happen if:

  • The image source is not hosted on the same domain as the page itself.
  • The image source is not included in the allowed sources specified by the CSP policy. To resolve this issue, you can:
  • Host the images on the same domain as the page.
  • Add the domain from which the images are loaded to the CSP policy. For example, if images are loaded from "example.com", you would modify the CSP policy to include "img-src 'self' example.com".
  • Make sure to update the CSP policy of the page accordingly to allow loading images from the necessary sources while maintaining security.

Refused to Execute Inline Script Because it Violates the Following Content Security Policy Directive

Depending on the browser, the error message may also read: Refused to Apply Inline Style Because it Violates the Following Content Security Policy Directive. This message indicates that a website's Content Security Policy (CSP) is restricting the execution of scripts embedded directly within HTML markup. CSP is a security standard aimed at mitigating various web vulnerabilities, such as cross-site scripting (XSS) attacks. To comply with CSP guidelines and enhance security, it's advisable to avoid embedding scripts directly within HTML markup. Instead, consider externalizing scripts into separate files and then referencing them in the HTML. This approach allows CSP to better monitor and control script execution while maintaining website functionality.

Configuring a Content Security Policy can be challenging, but it is an important step to reducing unnecessary privilege on your site, especially if you handle payments and are looking to conform to standards such as PCI-DSS 4.0. In fact, Content Security Policy is mentioned directly in PCI-DSS 4.0 6.4.3, and 11.6.1. Get in touch today to become compliant, improve your security, and earn a higher Stellastra Cyber Security Score.


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