Kirby CMS vulnerable to cross-site scripting (XSS) from links in KirbyTags and image blocks in the site frontend

TL;DR

This vulnerability affects all Kirby sites that allow the use of the (link: …) KirbyTag, the link: parameter of the (image: …) KirbyTag, the built-in image block with a link or the HTML importer for blocks, when content is authored by users who may not be fully trusted. The attack requires an authenticated Panel user with update permission to any textarea or blocks field, or write access to content files through another vector (e.g. a frontend form or content sync pipeline). Another attack vector is the use of Html::a() or Html::link() with untrusted user input.

This vulnerability is of high severity for affected sites.

Kirby sites are not affected if none of the mentioned KirbyTags or block types are used, or if every user who can edit content is fully trusted. The attack only surfaces in the site frontend (i.e. in its templates). The Panel itself is unaffected and will not execute JavaScript that was injected into the textarea or blocks field content.

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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 site frontend or 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.

In a stored XSS attack, the malicious payload is saved into the content data and has the potential to affect other users or site visitors.

Such vulnerabilities are critical if a consuming application might have potential attackers in its group of authenticated Panel users. They can escalate their privileges if they get access to the Panel session of an admin user. Depending on the site, other JavaScript-powered attacks are possible.

A specific class of stored XSS exploits the javascript: URI scheme in HTML <a href> attributes. When a browser processes a click action on a link with href="javascript:…", it executes the value as JavaScript in the origin of the current page. Because the site usually runs on the same origin as the Panel API, a successful exploit in the site frontend can give the attacker full control of the victim's Panel session.

Affected components

Kirby provides four first-party renderers that produce <a href="…"> output from editor-supplied field values:

1. The (link: …) KirbyTag
2. The link: parameter of the (image: …) KirbyTag, when the parameter does not resolve to a known file or