CVE-2026-21896 — getkirby/cms
Fix: getkirby/kirby@f5ce134CVE-2026-21896 is a CWE-863 vulnerability in getkirby/cms. A fix is available for getkirby/cms — see the affected versions and patch details below.
Kirby is missing permission checks in the content changes API
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-21896.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
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
TL;DR
This vulnerability affects all Kirby sites where user permissions are configured to prevent specific role(s) from performing write actions, specifically by disabling the update permission with the intent to prevent modifications to site content.
If developers haven't configured any user permissions that deviate from the default of allowing all actions, their site is not affected.
Introduction
Kirby allows to restrict the permissions of specific user roles. Users of that role can only perform permitted actions.
Permissions for updating content have already existed and could be configured for each model type, but were not enforced by Kirby's API backend code during operations to the changes version.
The changes version is the content version that contains unsaved changes of existing models (pages, users, files or the site).
Impact
The missing permission checks allowed attackers with Panel access to create or discard a changes version or update the content fields in an existing changes version. All of these actions could affect arbitrary models.
This could cause the following impact:
- Attackers could maliciously create changes versions for all models of the site, creating editing locks that would prevent other authenticated users from making content changes until those locks were cleared.
- Attackers could update the content in a malicious way, for example by adding defamatory or spam content or by including malicious links or scripts. While this updated content would not immediately be published to the site, an inattentive editor with update permissions could inadvertently publish these changes in the belief that an authorized user has made them.
- Attackers could discard extensive changes, making editors lose their content work.
Patches
The problem has been patched in Kirby 5.2.2. Please update to this or a later version to fix the vulnerability.
In the mentioned release, we have added checks for the model update permissions that ensure that users without this permission cannot create, edit or discard the changes version of the respective model.
A future Kirby release will add separate edit and save permissions that will make it possible to control write actions to model content more granularly.
Credits
Thanks to Lukas Kleinschmidt (@lukaskleinschmidt) for responsibly reporting the identified issue.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | getkirby/cms | ≥ 5.0.0&&< 5.2.2 | 5.2.2composer require getkirby/cms:^5.2.2 |
Detection & 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 5.2.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-21896 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-21896 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-21896. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-21896 in your dependencies?
O3 Security finds CVE-2026-21896 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.