Kirby vulnerable to Cross-site scripting (XSS) in the link field "Custom" type
TL;DR
This vulnerability affects Kirby sites that use the new [link field](https://getkirby.com/docs/reference/panel/fields/link) and output the entered link without additional validation or sanitization.
The attack commonly requires user interaction by another user or visitor.
The link dialog of the writer field is *not* affected as the writer field content is automatically sanitized by the Kirby backend code. You are also already protected if you limit the acceptable link types with the options field property.
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Introduction
Cross-site scripting (XSS) is a type of vulnerability that allows to execute any kind of JavaScript code inside the Panel session of the same or other users. In the Panel, a harmful script can for example trigger requests to Kirby's API with the permissions of the victim.
Such vulnerabilities are critical if you might have potential attackers in your group of authenticated Panel users. They can escalate their privileges if they get access to the Panel session of an admin user. Depending on your site, other JavaScript-powered attacks are possible.
Impact
The new link field introduced in Kirby 4 allows several different link types that each validate the entered link to the relevant URL format. It also includes a "Custom" link type for advanced use cases that don't fit any of the pre-defined link formats.
As the "Custom" link type is meant to be flexible, it also allows the javascript: URL scheme. In some use cases this can be intended, but it can also be misused by attackers to execute arbitrary JavaScript code when a user or visitor clicks on a link that is generated from the contents of the link field.
Patches
The problem has been patched in [Kirby 4.1.1](https://github.com/getkirby/kirby/releases/tag/4.1.1). Please update to this or a [later version](https://github.com/getkirby/kirby/releases) to fix the vulnerability.
In the patch release, we have updated the link field to hide the "Custom" link type by default and added a warning to our [documentation](https://getkirby.com/docs/reference/panel/fields/link#custom-link-type) that this link type should only be enabled if additional validation is performed or no protection against XSS attacks is needed.
Credits
Thanks to Natwara Archeepsamooth (@PlyNatwara) for responsibly informing us about the javascript: attack vector.
