Kirby: Access to image files outside of the site root via path traversal in the media handling

TL;DR

This vulnerability affects all Kirby sites that are deployed in a way that their index root on the server is next to a second directory that is read-accessible to PHP and shares the same name prefix (such as the site with the index root /var/www/site being next to /var/www/site2).

It was possible to create and access thumbnails from media files within such sibling directories that have a valid thumbnail configuration (JSON job file). This can affect staging sites, site backups or other internal sites.

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 to generate a thumbnail of. 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 a thumbnail.

The media handler uses filesystem containment checks in several places that are supposed to prevent breakout of the media and index roots of the site.

Impact

In affected releases, the containment checks were incomplete and did not cover the case of a sibling directory next to the containment directory that starts with the same prefix. E.g. a directory site2 passed the containment check of directory site. This allowed attackers to access media files with prepared job files that are stored within such sibling directories of the site's index root, opening the potential for information leaks from sensitive files stored within them, and 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, we have hardened the containment helpers Kirby\Filesystem\Dir::realpath() and Kirby\Filesystem\F::realpath() to require either an exact match or a DIRECTORY_SEPARATOR boundary, so that a sibling path sharing the same name prefix can no longer pass the check. This prevents access to files outside the index root of the active site. We have also hardened the Asset class to block paths that contain the ../ sequence.

Credits

Thanks to @0x1saac for responsibly reporting the identified issue.