The following Citrix ADC builds contain enhancements to help customers mitigate HTTP smuggling attacks:

• NetScaler ADC and NetScaler Gateway 14.1-72.61 and later releases
• NetScaler ADC and NetScaler Gateway 13.1-63.18 and later releases of 13.1
• NetScaler ADC 14.1-FIPS 14.1-72.61 FIPS and later releases of 14.1-FIPS
• NetScaler ADC 13.1-FIPS and 13.1-NDcPP 13.1.37.272 and later releases of 13.1-FIPS and 13.1-NDcPP

In order to allow a wide variety of client/server transactions, Citrix ADC has a default lenient setting for parsing HTTP requests. However, an enhanced level of protection against HTTP Request Smuggling attacks can be provided by binding an in-built HTTP profile, nshttp_default_strict_validation, to one or more virtual servers by using the following commands on the CLI: 

set lb vserver <vserver_name> -httpProfileName nshttp_default_strict_validation 
set cs vserver <vserver_name> -httpProfileName nshttp_default_strict_validation 

Please note that customers are strongly advised to test these settings prior to deployment in a production environment. 

Customers who encounter any problems in deploying the above HTTP profile can alternatively disable HTTP multiplexing.  This can be done globally, across all services, using:  

set ns httpParam -conMultiplex DISABLED

OR, alternatively, per profile using: 

set httpProfile <profile_name> -conMultiplex DISABLED 

Please note that, depending on the capability of the back-end web server, there may be a minor performance impact as a result of disabling HTTP multiplexing because a separate TCP connection will be created for each client connection. Customers are advised to test this configuration prior to deployment in a production environment. 

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HTTP request smuggling is a type of attack that takes advantage of differences between the way sequences of HTTP requests are processed by a web server and one or more intermediate devices, like a Citrix ADC load balancer. HTTP Request smuggling attacks are possible when requests from different clients are received by the intermediate device and then multiplexed, pipelined and forwarded on to a back-end server over the same network connection. If the back-end server does not process these sequences of requests in the same way as the intermediate device, a maliciously crafted HTTP request could interfere with other legitimate HTTP traffic, potentially allowing the attacker unauthorized access to other users’ data.  

What Citrix is Doing

Citrix is notifying customers and channel partners about this potential security issue through the publication of this security bulletin on the Citrix Knowledge Center at https://support.citrix.com/support-home/topic-article-list?trendingCategory=20&trendingTopicName=Security%20Bulletin  

Obtaining Support on This Issue

If you require technical assistance with this issue, please contact Citrix Technical Support. Contact details for Citrix Technical Support are available at https://support.citrix.com/support-home/home 

Subscribe to Receive Alerts

Citrix strongly recommends that all customers subscribe to receive alerts when a Citrix security bulletin is created or modified at https://support.citrix.com/wolken-support/view/aboutsupport/my-support-alerts  

Reporting Security Vulnerabilities to Citrix

Citrix welcomes input regarding the security of its products and considers any and all potential vulnerabilities seriously. For details on our vulnerability response process and guidance on how to report security-related issues to Citrix, please see the following webpage: https://www.cloud.com/trust-center/support 

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