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Fixing DOM-Based XSS

Published: 25 October 2020    Last Updated: 03 November 2022

Whilst Reflected and Stored XSS can generally be addressed through server-side user input encoding (such as through the PHP htmlentities() function) or with browser protections such as Content-Security-Policy – this is not sufficient for DOM-XSS.

Where a dangerous function is used, user input into that function should be limited through user input filtering. An allow-list approach of restricting user input to only known-good input should be used. For example, limiting input to the smallest number of characters possible (such as alphanumerics only) and checking the expected data type (such as limiting input to integers only). This is in contrast to a block-list of known-bad inputs being blocked, which is often less effective due to the large degree of flexibility that JavaScript allows. For a good example of this flexibility, consider something like JScrewIt.


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Fixing Cross-site Scripting (XSS)

Published: 25 October 2020    Last Updated: 03 November 2022

This issue comes about where user supplied input is included within server responses without filtration or encoding.

One very effective method of preventing this attack is to use an allow-list (sometimes called a whitelist) which will allow only known good content. For example, if your expected input is an integer and the user supplies anything other than an integer you can simply reject that input – and perhaps supply a message to inform the user what the issue is, without including the original payload.


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Finding Cross-site Scripting (XSS)

Published: 19 October 2020    Last Updated: 03 November 2022

Cross-site Scripting (XSS) issues occur where user supplied input is insecurely included within a server response, or insecurely processed by a client-side script. If the payload is included with the response that immediately follows the request containing the payload then this is known as Reflected XSS. It is also sometimes referred to as Non-persisted XSS. If the payload is stored by the server and returned in a later response, it is known as Stored XSS, or Persistent XSS. Where the issue is due to insecure client-side processing it is known as DOM-Based XSS. Finding and exploiting DOM-Based XSS is quite different to stored or reflected, so we’ve separated it into its own article: Finding DOM-XSS.


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