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