CVE-2021-41258 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in getkirby/cms. A fix is available for getkirby/cms — see the affected versions and patch details below.
Cross-site scripting (XSS) from image block content in the site frontend
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-41258 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
getkirby/cmsReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
Impact
Kirby's blocks field stores structured data for each block. This data is then used in block snippets to convert the blocks to HTML for use in your templates. We recommend to escape HTML special characters against cross-site scripting (XSS) attacks.
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 other users. If the user is logged in to the Panel, a harmful script can for example trigger requests to Kirby's API with the permissions of the victim.
The default snippet for the image block unfortunately did not use our escaping helper. This made it possible to include malicious HTML code in the source, alt and link fields of the image block, which would then be displayed on the site frontend and executed in the browsers of site visitors and logged in users who are browsing the site.
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.
You are not affected if you don't use the blocks field or specifically the image block in any of your blueprints. You are also protected if you use a custom block snippet that either escapes the printed values or doesn't use them. The attack can only be performed by logged-in users and only surfaces in the site frontend (i.e. in your templates). The Panel itself is unaffected and will not execute JavaScript that was injected into the image block content.
Patches
We have patched the vulnerability in Kirby 3.5.8 by escaping special HTML characters in the output from the default image block snippet. Please update to this or a later version to fix the vulnerability.
Credits
Thanks to Azrul Ikhwan Zulkifli (@azrultech) from BAE Systems AI Vulnerability Research Team for responsibly reporting the identified issue.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | getkirby/cms | ≥ 3.5.0&&< 3.5.8 | 3.5.8composer require getkirby/cms:^3.5.8 |
Affected Products
kirbygetkirbyDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for getkirby/cms, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update getkirby/cms to 3.5.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41258 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Frequently Asked Questions
Is CVE-2021-41258 in your dependencies?
Find it across Packagist, including transitive dependencies.