Directly writing error messages to a webpage without sanitization allows for a cross-site scripting vulnerability if parts of the error message can be influenced by a user.

To guard against cross-site scripting, consider using contextual output encoding/escaping before writing user input to the page, or one of the other solutions that are mentioned in the references.

The following example shows an exception being written directly to the document, and this exception can potentially be influenced by the page URL, leaving the website vulnerable to cross-site scripting.

This second example shows an input being validated using the JSON schema validator ajv, and in case of an error, the error message is sent directly back in the response.

This is unsafe, because the error message can contain parts of the input. For example, the input {'<img src=x onerror=alert(1)>': 'foo'} will generate the error data/<img src=x onerror=alert(1)> should be number, causing reflected XSS.

  • OWASP: DOM based XSS Prevention Cheat Sheet.
  • OWASP: XSS (Cross Site Scripting) Prevention Cheat Sheet.
  • OWASP DOM Based XSS.
  • OWASP Types of Cross-Site Scripting.
  • Wikipedia: Cross-site scripting.