Advisory ID:               SYSS-2020-004
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:        Information Exposure Through Caching (CWE-512)
Risk Level:                Information Disclosure
Solution Status:           Open
Manufacturer Notification: 2020-01-31
Solution Date:             no solution
Public Disclosure:         2020-03-05
CVE Reference:             CVE-2020-10110
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 shows how long an entry
was in the cache.

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Vulnerability Details:

Some responses of the web based solution are handled with an included
caching system.  This system can be seen in the HTTP response header.
The header also contains information, how long the response was in the
cache.  The number of seconds will never exceed the value of 112.
After that period of time, the cache will be refreshed with an
additional client request.

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Proof of Concept (PoC):

$ curl --include --url https://$NSGATEWAYHOST/logon/themes/Default/resources/config.xml
HTTP/1.1 200 OK
Age: 21        
Date: Fri, 31 Jan 2020 11:53:17 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: "12a-59c921b8df640"
Server: Apache
X-Frame-Options: SAMEORIGIN
Last-Modified: Mon, 20 Jan 2020 13:17:05 GMT
Accept-Ranges: bytes
Content-Length: 298
X-Citrix-Application: Receiver for Web
Pragma: no-cache
Keep-Alive: timeout=15, max=95
Content-Type: application/xml

<?xml version="1.0" encoding="utf-8" ?>
<ResourceManager>
  <SupportedLanguageList>
    <Language>en</Language>
    <Language>de</Language>
    <Language>es</Language>
    <Language>fr</Language>
    <Language>ja</Language>
  <Language>zh-CN</Language>
  </SupportedLanguageList>
</ResourceManager>

The "Via" header indicates that there is a caching system in place.  The
"Age" header shows how many seconds the initial request was processed.
A value of 1 indicates that the response is sent from the web server
instead of from the cache.

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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-2020-004
    https://www.syss.de/fileadmin/dokumente/Publikationen/Advisories/SYSS-2020-004.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