CVE-2021-29460

Cross-site scripting (XSS) from unsanitized uploaded SVG files in Kirby

<?php

@include_once __DIR__ . '/kirby/bootstrap.php';

if (version_compare(kirby()->version(), '3.5.4', '<') === true) {
  die('This script requires Kirby 3.5.4+.');
}

$objects = [
  // the site itself
  site(),

  // all pages and drafts
  ...site()->index(true)->values(),

  // all users
  ...kirby()->users()->values()
];

$errors = false;
foreach ($objects as $object) {
  foreach ($object->files() as $file) {
    try {
      // validate the contents lazily
      // (if a validator exists)
      $file->validateContents(true);
    } catch (\Kirby\Exception\InvalidArgumentException $e) {
      echo $file->id() . ': ' . $e->getMessage() . "<br>\n";
      $errors = true;
    }
  }
}

if ($errors === false) {
  echo 'No validation errors found.';
}

An editor with write access to the Kirby Panel can upload an SVG or XML file that contains harmful content like <script> tags. The direct link to that file can be sent to other users or visitors of the site. If the victim opens that link in a browser where they are logged in to Kirby, the script will run and can for example trigger requests to Kirby's API with the permissions of the victim.

This vulnerability is 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.

Visitors without Panel access can only use this attack vector if your site allows SVG or XML file uploads in frontend forms and you don't already validate or sanitize uploaded SVG or XML files.

The attack requires user interaction by another user or visitor and cannot be automated.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2021-29460 has a CVSS score of 7.6 (High). The vector is network-reachable, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.5.4); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

1. What is CVE-2021-29460? CVE-2021-29460 is a high-severity cross-site scripting (XSS) vulnerability in getkirby/cms (composer), affecting versions < 3.5.4. It is fixed in 3.5.4. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2021-29460? CVE-2021-29460 has a CVSS score of 7.6 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of getkirby/cms are affected by CVE-2021-29460? getkirby/cms (composer) versions < 3.5.4 is affected.
4. Is there a fix for CVE-2021-29460? Yes. CVE-2021-29460 is fixed in 3.5.4. Upgrade to this version or later.
5. Is CVE-2021-29460 exploitable, and should I be worried? Whether CVE-2021-29460 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-29460 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-29460? Upgrade getkirby/cms to 3.5.4 or later.

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