TL;DR
This vulnerability affects all Kirby sites that have not disabled the REST API with the 'api' => false option.
It was possible to trigger a PHP error in the API backend that would expose the full filesystem path of the Kirby installation on the server. This could be used to guess the default content.salt or prepare specialized attacks.
----
Introduction
An information exposure occurs when system data or debugging information leaves the program through an output stream or logging function that makes it accessible to unauthorized parties. Using other weaknesses, an attacker could cause errors to occur; the response to these errors can reveal detailed system information, along with other impacts. An attacker can use messages that reveal technologies, operating systems, and product versions to tune the attack against known vulnerabilities in these technologies. A product may use diagnostic methods that provide significant implementation details such as stack traces as part of its error handling mechanism.
Affected components
Kirby's REST API at /api returns JSON data for each request. If an error occurs during processing of the request, a special error handler converts this error to JSON data that API frontends can handle.
The returned data for errors depends on the debugging mode. If the debug option is enabled, the JSON data contains fields for the path and line of the affected file, the internal exception class and the API route that caused the error. This data is omitted from error responses in production (when the debug option is disabled).
Impact
Some internal errors may contain sensitive information in the error message itself. This is often the case with PHP errors.
In affected releases, the REST API error handler did not sanitize error messages for sensitive information.
This exposed system information like the full source path to external API users, including users without authentication. This could be used to guess the default content.salt or prepare specialized attacks.
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 has hardened the API error handler to only expose full error messages for exceptions in the Kirby\Exception namespace. All other errors are only exposed in debug mode (with disguised paths, keeping only the paths relative to the Kirby installation). Outside of debug mode, general exceptions and PHP errors are replaced with a generic error message.
Credits
Thanks to Peter Levashov (@petersevera) for responsibly reporting the identified issue.
TL;DR
This vulnerability affects all Kirby sites where users of a particular role have no permission to create pages, files or users (pages.create, files.create or users.create permission is disabled). This can be due to configuration in the user blueprint(s), via options in the model blueprint(s) or via a combination of both settings.
This vulnerability is of high severity for affected sites.
Developers' Kirby sites are not affected if they intend all users of their site to be able to create pages, files and users. The vulnerability can only be exploited by authenticated users.
----
Introduction
An authorization bypass allows authenticated users to perform actions they should not be allowed to perform based on their configured permissions, thereby causing a privilege escalation.
The effects of an authorization bypass can include unauthorized access to sensitive information as well as unauthorized changes to content or system information.
Impact
Kirby's user permissions control which user role is allowed to perform specific actions to content models in the CMS. These permissions are defined for each role in the user blueprint (site/blueprints/users/...). It is also possible to customize the permissions for each target model in the model blueprints (such as in site/blueprints/pages/...) using the options feature. The permissions and options together control the authorization of user actions.
Kirby provides the pages.create, files.create and users.create permissions (among others). These permissions can again be set in the user blueprint and/or in the blueprint of the target model via options. In affected releases, Kirby allowed to override the options during the creation of pages, files and users by injecting custom dynamic blueprint configuration into the model data. The injected options could include 'create' => true, which then caused an override of the permissions and options configured by the site developer in the user and model blueprints.
Patches
The problem has been patched in Kirby 4.9.0 and Kirby 5.4.0. Please update to one of these or a later version to fix the vulnerability.
In all of the mentioned releases, we have updated the normalization code that is used during the creation of pages, files and users to include a filter for the blueprint property. This prevents the injection of dynamic blueprint configuration into the creation request.
Credits
Kirby thanks @offset for responsibly reporting the identified issue.