Solving OWA Attachment Security with Citrix NetScaler

What’s the problem exactly?

With the rise of the availability of web based applications, web based versions of their client-server counterparts, and workforce mobility, comes the increased risk of potentially leaving behind sensitive information on remote devices not under the control of corporate IT.  While there are no 100% solutions to this problem, organizations can implement numerous preventative measures to deal with it which are available from the developer as well as 3rd party technologies.

Microsoft Outlook Web Access is no exception to this problem and is often at the forefront of the concerns of security teams because of it’s popularity and the fact it is very often exposed to the public internet so that mobile work forces can remain productive from anywhere.  This is of course exacerbated by the fact that OWA and other web based email interfaces like it provide access to email attachments.  These attachments, when opened, are cached in a browser’s Temporary Internet Files folder or worse perhaps saved to a folder location that is easily accessible by anyone.  End-users could potentially walk-away from these devices leaving corporate information behind to possibly be exploited for malicious purposes. 

As mentioned, there is no solution that is 100% safe to the aforementioned problem.  There are numerous solutions to these issues, ranging from VPN Cleanup Agents, to access via Virtualization technologies such as Citrix XenApp or Citrix XenDesktop, to proxy devices such as Citrix NetScaler.  Each method has their own set of pros and cons and have varying levels of cost and preventative .  This article focuses on utilizing the power of Citrix NetScaler ADC

How can one solve this problem?

Starting with Exchange Server 2007, Microsoft introduced the concept of Public and Private computers allowing end-users to select whether their end device was to be trusted or not. 

OWA 2007 Login Form	OWA 2010 Logon Form

On the Exchange Server end, these options pertain to settings that can be used to control the behaviour of how attachments are dealt with.  These include the ability to block all attachment access or allow attachment access but only via the built-in document conversion utilities which convert the attachment to HTML for view right in the browser.

Disallow all attachment access or only allow file preview	Allow attachments with restrictions or certain file types only or force use of file preview

These settings in turn control the end-user experience with regards to attachment access. 

Public computer with no access to attachment

Private computer with attachment access and preview

Giving end-users the control is not the answer

All of the above are great options if you actually trust your end users to select the correct option at login to classify the type of device or location they are accessing your network from. But a more realistic solution is to profile the device, user, or location and enforce the behaviours based on this discovery information.  Some examples of the information you could base a decision on include:

• Client IP Source Address
• SSL Certificate Auth/Details
• Browser/Device Type
• User Identity or Group Membership
• Presence of Anti-Virus or Anti-Spam software

How can Citrix NetScaler ADC help? 

NetScaler has three core features which can be leveraged to secure Outlook Web Access attachment functionality based on the methods described above. The NetScaler AAATM feature (Authentication, Authorization, and Accounting for Traffic Management) enables the ability to use the NetScaler to perform authentication to user account directories based on LDAP, RADIUS, TACACS+, or Client SSL certificates.  Secondly, Access Gateway functionality provides bi-directional SSL VPN tunnelling and ICA Proxy capabilities and adds the ability to perform endpoint analysis to the aforementioned AAA features provided by AAATM.  And lastly, the NetScaler Rewriting feature allows us to alter or inject html in Requests and Responses based on conditions we define by the very extensible AppExpert policy engine. 

Instead of letting the End User control the attachment behavior, the NetScaler can be inserted in front of the solution to provide

• DMZ Authentication and Authorization
• Single Sign-On to Outlook Web Access
• Attachment control based on end user, location, or device attributes

In the configuration example below, Source Address and Group Membership are chosen for example to determine the level of attachment security applied to the session.

In addition to providing attachment security, Citrix NetScaler can be leveraged to provide High Availability, High Scalability, and Consolidation through additional on-box features such as SSL Offload, Content Switching, Load Balancing, Content Compression, and Integrated Cache. 

Sample NetScaler AAATM Configuration for OWA 2010

Sample Access Gateway Configuration (coming soon)

More Information

• Citrix NetScaler Deployment Guide for Microsoft Exchange 2010
• Citrix NetScaler AppExpert Template for Outlook Web Access
• How to Configure Single Sign-On for Outlook Web Access 2010
• How to Configure Single Sign-On for Outlook Web Access 2007

 

del.icio.us Tags: OWA Attachment Security,Citrix NetScaler,AAA Traffic Management,Access Gateway

Server Name Indication and SSL VPN

The Problem

Many People are unfamiliar with Server Name Indication (SNI) despite having been introduced as an extension to the TLS protocol back in 2005. In a nutshell, when client computer browsers or SSL based VPNs are negotiating encryption with a server, there is no information which can be gleaned by the server in order to determine which virtual host the client is actually requesting.  This is due to the fact that the hostname of the subsequent request is contained in the encrypted header which would not be visible until after the received data was decrypted as it made its way down the stack.  This is problematic with respect to virtual hosting since each server or appliance can serve many hosts through the same address.  If it is desired to secure the data of that host through SSL, then a 1:1 mapping of hostname to IP address is currently required.

Client: (TLS Handshake) Hello, I support XYZ Encryption.
Server: (TLS Handshake) Hi There, Here is my Public Certificate, and lets use this encryption algorithm.
Client: (TLS Handshake) Sounds good to me.
Client: (Encrypted) HTTP Request
Server: (Encrypted) HTTP Reply

What about ‘STARTTLS’ or TLS ‘Upgrade’ in HTTP/1.1?

STARTTLS is another standard which is commonly used by protocols such as SMTP, POP, IMAP, and LDAP.  Back in the day, it was common practice to have parallel secure ports for most protocols.  For example,  with SMTP, POP, IMAP, and LDAP, and HTTP you have 25/465 110/995 145/993 389/636, and 80/443 respectively. The idea of  STARTTLS was born when the IETF which governs internet assigned numbers and ports decided back in 1997 at some meeting that the issuing of paralell “secure” ports for all protcols should be depricated.   With STARTTLS, when the connection to the server host is established, the client sends a plantext command with the virtual host name.  This has enough information for the server to decide which certificate to offer for the SSL/TLS handshake.

Client: (TLS Handshake) Hello, I support XYZ Encryption.

Client: (Cleartext) I am using server ‘access.mycompany.com‘

Server: (Cleartext) By The Way, I also support TLS Encryptionn.

Client: (Cleartext) Lets use Encryption, aka ‘STARTTLS’.

Client: (TLS Handshake) Hello, I support XYZ Encryption.

Server: (TLS Handshake) Hi There, Here is my Public Certificate, and lets use this encryption algorithm.

Client: (TLS Handshake) Sounds good to me.

Client & Server: (Encrypted) Exchange Data

A similar method for web browsers, and SSL VPN clients was derived in the HTTP/1.1 specification and is called TLS Upgrade. HTTP/1.1 TLS Upgrade method can be applied to upgrade an open HTTP connection. In a nutshell, the client would include this in a request:

GET http://access.mycompany.com/securestuff HTTP/1.1
Host: access.mycompany.com
Upgrade: TLS/1.0
Connection: Upgrade

The server in turn might respond with:

HTTP/1.1 101 Switching Protocols
Upgrade: TLS/1.0, HTTP/1.1
Connection: Upgrade

The main benefit with these methods are that you can have both naked and secure traffic traversing through the same  port.  Main problems to this and likely why these methods have not been adopted are that all methods would require that any proxy servers in between the client and server also support this method.  A proxy server that did not acknowledge it or perhaps strips the command (could also happen on a legacy firewall), would potentially present a man-in-the-middle attack.  A lesser issue might be that you have a user perception issue as there is a certain familiarity with port 443 being the “secure” port.  On the server end of things, you would also need to have the unsecure port open for the application in question which may not be the case.

The Solution

An extension to SSL/TLS called Server Name Indication (SNI) addresses this issue by sending the name of the virtual host as part of the SSL/TLS negotiation. This enables the server to bind the correct virtual host early and present the browser with the certificate containing a CN matching that in the SNI header.  This method also has far fewer complications associated with it as compared to TLS Upgrade or STARTTLS.  The SNI extension is described in gross detail here. With SNI, you would have a sequence like:

Client: (TLS Handshake) Hello, I support XYZ Encryption, and I am trying to connect to
‘access.mycompany.com‘.
Server: (TLS Handshake) Hi There, Here is my Public Certificate, and lets use this encryption algorithm.
Client: (TLS Handshake) Sounds good to me.
Client: (Encrypted) HTTP Request
Server: (Encrypted) HTTP Reply

But Don’t Browser’s and Servers need to support this extension in order for this to work?

Yup, that is the idea.  As with any RFC, extension, or modification you have to have adoption by the software developers, and hardware vendors which in turn are driven by the adoption or request of the technology by the IT community.  The latter is only done through education and practical application examples which is one of my main drivers for writing this blog post.  At the time of this writing, there are no known Remote Access Appliances which support this RFC extension.  Below is the list of known browsers, servers, and SSL application libraries which do support the SNI extension:

Browsers

• Mozilla Firefox 2.0 or later
• Opera 8.0 or later (the TLS 1.1 protocol must be enabled)
• Internet Explorer 7 (Vista, not XP) or later
• Google Chrome
• Safari 3.2.1 Mac OS X 10.5.6

Servers

• Apache with mod_gnutls or mod_ssl
• Cherokee if compiled with TLS support
• New versions of lighttpd 1.4.x and 1.5.x
• Nginx with an accompanying OpenSSL built with SNI support
• OS X 10.5.6

Libraries

• Mozilla NSS
• OpenSSL
  • 0.9.8f – not compiled in by default, can be compiled in with config option ‘–enable-tlsext’.
  • Unreleased 0.9.9 is likely to include SNI compiled in by default.
• GNU TLS

Unsupported Operating Systems Browsers, and Libraries

The following combinations do not support SNI.

• Windows XP and Internet Explorer 6 or 7
• Konqueror/KDE in any version
• Microsoft Internet Information Server IIS
• Sun Java System Web Server
• Microsoft.Net
• Sun Java JSEE

What SNI could add to SSL-based VPN Solutions?

So what does this mean with respect to Remote Access Solutions such as Citrix Access Gateway, F5 Firepass, or Juniper Secure Access remote access solutions?  The benefits of adopting support for SNI are wide an varying, but here is my first pass at a few:

• Since the SNI would be presented to the access appliance before the SSL negotiation finalized, specific SSL policies such as supported cipher suites, could be bound to the session.   This would be useful where you needed to meet more stringent security requirements such as FIPS level 1/2 for specific hosts, or where you had a client application which used a specific type of encryption that you needed to utilize.
• As cloud computing is becoming more prevalent, service providers are going to need a means to offer customers secure access to their applications and content.  Since many cloud services are based on anycast addresses (floating IP), CNAME records, and also servicing 1000’s of users, a 1:1 option for customer:IP is not practical or possible. SNI presents a cheap, workable alternative to having no secure offering.
• Enterprises who wish to publicly expose their intranet or line of business applications securely may want to do so through a remote access appliance, but not want to allocate multiple public IP addresses.
• Businesses who have only been allocated a single IP address and are using Port Address Translation (PAT) to serve up multiple applications.  This is actually pretty common since many businesses are provisioned with ADSL which uses DHCP assign IP addresses. Most companies use a remote access device as an all-in-one solution for outbound RNAT, inbound VPN, and line of business applications, and firewall.

I hope to see one of these vendors include support for this little-known, but extremely-practical extension to the TLS protocol in the near future.  If you manage to find your way to this post, and you are an IT professional, I would love to hear your comments.