secunet Security Networks AG Security Advisory

Advisory: Typo3 Core sanitizeLocalUrl() Non-Persistent Cross-Site Scripting 

1. DETAILS
----------
Product:         Typo3 CMS
Vendor URL:      typo3.org
Type:            Cross-site Scripting[CWE-79]
Date found:      2015-07-30
Date published:  2015-09-14
CVSSv2 Score:    3,5 (AV:N/AC:M/Au:S/C:N/I:P/A:N)
CVE:             CVE-2015-5956 


2. AFFECTED VERSIONS
--------------------
Typo3 6.2.14 and below
Typo3 4.5.40 and below
and other older versions may be affected too.


3. INTRODUCTION
---------------
"With more than 500,000 installations TYPO3 CMS is the most widely used
Enterprise Content Management System, providing the basis for websites,
intranets and web & mobile applications worldwide."

(from the vendor's homepage)


4. VULNERABILITY DETAILS
------------------------
The Typo3 version branches 6.x and 4.x are vulnerable to an authenticated,
non-persistent Cross-Site Scripting vulnerability when user-supplied input
is processed by the sanitizeLocalUrl() function. While there is already a
XSS filter in place, it is possible to mitigate it by using a data URI with
a base64 encoded payload.

The payload is slightly different through the vulnerable branches, 6.x needs
a space in the data URI payload, while 4.x doesn't. In the following proof
of concepts, the javascript <script>alert('XSS')</script> is used as a
base64 encoded data URI in the "returnUrl" and "redirect_url" parameters,
which can be found throughout Typo3.

4.x Branch Proof-of-Concept:
The following request forges the "back" link in the Typo3 "Show record
history" backend module: 
https://example.com/typo3/show_rechis.php?returnUrl=data:text/html;base64,PH
NjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4=

6.x Branch Proof-of-Concept:
Typo3 uses the payload from the "redirect_url" parameter in the HTTP
Location header and therefore "redirects" the victim to the payload after
logging in: 
https://example.com/typo3/index.php?redirect_url=data:text/html;base64,%20PH
NjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4=
                                                         
7.x Branch:
The 7.x branch is basically vulnerable too, but the attacker additionally
needs to know a secret token (moduleToken), which is included in every
request in order to successfully exploit the vulnerability, which makes
exploitation unfeasible.

The following request forges the "back" link in the Typo3 "Show record
history" backend module:      
https://example.com/typo3/index.php?M=record_history&moduleToken=260ab28ad49
73d29e0a77d2f799e79ca3028de28&element=tt_content%3A1&returnUrl=&returnUrl=da
ta:%20text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4=

                    
5. SECURITY RISK
----------------
The vulnerability can be used to temporarily embed arbitrary script code
into the context of the Typo3 backend interface, which offers a wide range
of possible attacks such as stealing cookies or attacking the browser and
its components.

The exploit probability is different depending on the attacked parameters.
In order to exploit the "redirect_url" parameter, the victim only has to
login to the backend interface, while there is an even higher amount of user
interaction needed to exploit the "returnUrl" parameter, because the victim
needs to be logged in to the backend with appropriate access rights, follow
the prepared link and has to click on the "Back" button in the view.     


6. SOLUTION
-----------
Update to TYPO3 versions 6.2.15 or 7.4.0


7. REPORT TIMELINE
------------------
2015-07-30: Vulnerability discovered
2015-08-03: CVE requested from MITRE
2015-08-04: Vendor notified
2015-08-07: CVE-2015-5956 assigned
2015-08-07: Vendor acknowledges the vulnerability
2015-09-08: Vendor releases update and security advisory
2015-09-14: Advisory released


8. REFERENCES / CREDITS
----------------------- 
This vulnerability was discovered and researched by Julien Ahrens from
secunet Security Networks AG.

[0]
https://typo3.org/teams/security/security-bulletins/typo3-core/typo3-core-sa
-2015-009/


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----------------------------
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