TL;DR

This vulnerability affects all Kirby sites where users of a particular role have access to the REST API (access.panel permission is enabled) but no permission to upload any kind of file (files.create, files.replace and user/users.update permissions are all disabled).

It was possible to fill up the temporary directory for chunked uploads with unfinished chunks even as a user without any upload permission.

This vulnerability is of high severity for affected sites.

Your Kirby sites are not affected if you intend all users of your site to be able to upload files. The vulnerability can only be exploited by authenticated users. It was not possible to bypass the actual permission checks for any files that end up in the content or site/accounts folders.

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Introduction

Missing authorization allows authenticated users to perform actions they are not intended to have access to.

The effects of missing authorization can include unauthorized access to sensitive information as well as unauthorized changes to content or system information.

Affected components

Kirby's REST API provides routes to upload files, specifically to create content files, replace existing content files and to create and replace user avatars.

Each upload route takes either full file upload requests or chunked upload requests that can be continued in subsequent requests. During a chunked upload, the incomplete state of the uploaded file is stored in a temporary directory until the last chunk completes the file. At this time, the final permission and business logic checks are performed before the complete file is moved to its final destination.

Impact

In affected releases, the chunk upload handler did not check for the user's file upload permissions before storing incomplete chunk data in the temporary directory.

This allowed attackers without upload permissions to upload multiple large files in chunks. If the final chunk was never provided, Kirby would keep the incomplete files for 24 hours. This could cause attacker-controlled storage consumption until the temporary files were automatically cleaned up or manually removed, potentially preventing other users from uploading files, or site logic from storing data.

Patches

The problem has been patched in Kirby 5.5.2. Please update this or a later version to fix the vulnerability.

In all of the mentioned releases, we have added preflight checks to the upload chunk processor that verify the relevant system permission before storing the chunk data in the temporary directory.

Credits

Thanks to @alcls01111 for responsibly reporting the identified issue.

TL;DR

This vulnerability affects all Kirby sites that are deployed to a server that allows requests for URLs with encoded slashes (%2f), such as nginx, PHP's built-in server or Apache setups that have the option AllowEncodedSlashes enabled. It was possible to create and access thumbnails from media files in arbitrary accessible directories on the server that have a valid thumbnail configuration (JSON job file). It was also possible to detect the presence of files with the .json file extension anywhere on the server. This vulnerability is of high severity for affected sites. Server setups using Apache's default configuration or other servers that have been hardened against encoded slashes in URLs are not affected.

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Introduction

A path traversal (also known as directory traversal) vulnerability occurs when untrusted input is used to build a filesystem path without properly confining the result to an intended base directory. By injecting sequences such as ../, an attacker can escape that directory and reach files elsewhere on the server.

Affected components

Kirby's media handler processes requests for files in the media directory that have not been generated yet. It parses the provided path and finds the correct file or asset from which to generate a thumbnail. Each thumbnail needs to have a prepared job file (a metadata file with file extension .json) in order to allow the media handler to generate it. Each parent (such as a page) has its own media directory, which in turn contains the individual files. Kirby's media handler searches for the file within the parent's media directory.

Impact

In affected releases, Kirby did not prevent path traversal in the filenames that were searched within the parent directory. In affected server setups where the attacker can provide encoded slashes (%2f) in the request, Kirby allowed the request to traverse away from the parent's media directory. Because the response differs between existing and non-existing thumbnail configurations, attackers were able to tell whether an arbitrary JSON file exists on the server (addressed by a relative path from the media directory of an arbitrary existing parent, including a relative path that points outside of the site's index root). For existing files with the file extension .json and a valid filename key (resulting in a valid job file), it was possible to generate thumbnails of the referenced media files, deleting the job file in the process. Patches

The problem has been patched in Kirby 4.9.5 and Kirby 5.5.2. Please update to one of these or a later version to fix the vulnerability.

In all of the mentioned releases, Kirby\Cms\Media::thumb() now rejects any filename that contains path information (anything other than a plain filename) before it is appended to the validated root. We have also hardened the file::version component to block paths that contain the ../ sequence.

Credits

Thanks to Jorge González Milla (@Pig-Tail) for responsibly reporting the identified issue.

Kirby is an open-source content management system. Prior to 4.9.5 and 5.5.2, depending on the release line, Kirby's media handler used incomplete filesystem containment checks in src/Filesystem/Dir.php and src/Filesystem/F.php through Kirby\Filesystem\Dir::realpath() and Kirby\Filesystem\F::realpath(). The checks accepted a sibling directory whose path shared the intended root's string prefix, such as /var/www/site2 next to /var/www/site, because they did not require an exact match or a DIRECTORYSEPARATOR boundary. A remote attacker could use Kirby\Cms\Media::thumb() to create and access thumbnails from image files in a PHP-readable sibling directory when that directory contained a valid .json thumbnail job file, potentially exposing staging sites, backups, or other internal sites and deleting the job file during processing. This issue is fixed in versions 4.9.5 and 5.5.2.

A stored cross-site scripting (XSS) vulnerability in the Edit Content Layout module of Kirby CMS v4.1.0 allows attackers to execute arbitrary web scripts or HTML via a crafted payload injected into the Link field. NOTE: the vendor's position is that this issue did not affect any version of Kirby CMS. The only effect was on the trykirby.com demo site, which is not customer-controlled.

Summary

Kirby CMS through version 5.1.4 allows an authenticated user with Editor permissions to cause a persistent Denial of Service (DoS) via a malformed image upload.

Details

The vulnerability is caused by improper validation of the return value of PHP's getimagesize() function. When a malformed file is uploaded with a valid image extension (e.g., .jpg), the function returns false instead of an expected array.

The application fails to handle this condition properly and proceeds with image processing, resulting in a fatal TypeError. This leads to persistent application crashes when the affected file is accessed.

Impact

- Persistent Denial of Service (DoS) - Affected pages return HTTP 500 errors - Requires manual removal of the malformed file to restore functionality - Exploitable by authenticated users with Editor permissions