Vishful thinking…

One overlooked feature that is coming in ArcGIS Server 10

Posted in ArcGIS, ESRI, GIS by viswaug on March 29, 2010

At the ESRI devsummit last week, there were a lot of new features in ArcGIS Server (AGS) version 10 that were getting a lot of attention. There was a lot of fan-dare around geodatabase editing over the web with the new Feature Service in AGS 10 and the flashy demos with all the new RIA clients for editing with the REST API. There are also a lot of articles & blog posts that have been written to cover those features in details. So, I thought I would stay off the beaten path and direct your attention towards a new feature in AGS 10 that is getting little to no attention. That new feature in AGS v10 is support for exporting PNG32 (32 bit PNG) map images from the REST API and other APIs offered by AGS. This should provide the complete support 8-bit transparency in PNGs and should help us avoid issues like the ones here. Printing maps functionality should also benefit from this new feature both in terms of quality and to avoid transparency issues when layering images.

Just check out the ‘Image Format’ drop-down here for proof of 32 bit PNG support . And as you can see here, AGS 9.3.1 only supported PNG24. Hope that helps someone… 🙂


Working with ESRI token secure services

Posted in ArcGIS, ESRI, GIS, Uncategorized, Web by viswaug on March 29, 2010

At the ESRI developer summit this past week, I ran into some people that were either having a hard time with using the ESRI token authentication or were leaving their systems vulnerable to hacks given their use/abuse of long lived tokens. I thought it might be useful to share one way that we have been using ESRI token secured services in our web mapping applications.

Token secure services require the client to request a token with their username & password which should then be used/included in all other future requests to access the services. The token provided to the user by AGS is also valid only for the time period requested by the user. The AGS server also applies a upper limit to how long the token can be valid.

One of the main reasons for troubles with using such token secure services in a web mapping application is that the user logs into the web application that he is using and not actually the AGS server(s) that the web application is using map services from. So, in order to use the map services in the web application, the user has to log-in (again) to the AGS server also. Having the user log in again after they have already logged into the web application is highly undesirable. To prevent the user from having to enter in the credentials to access AGS services again, some may decide to use a long lived token and hard-code the token into the web application or hard-code the username & password to access AGS services in the mapping client application. I don’t think I need to explain why hard-coding the username & password in the client web mapping application is dangerous. But this still leaves the application highly vulnerable to hacks since anybody who can read the URL being used to access the services have access to the long-lived token. Using the long-lived token, anybody can obtain access to the AGS services since the only defense is the ClientID (or the HTTP Referrer header) and that can be spoofed easily since it is never verified. Also, the long-lived token doesn’t expire often and leaves the hackers a lot of time to get the token and access the secure AGS services

To get around this, there is an easy way to setup the web application to use and better secure the AGS services. We might have two main ways of sharing username & password between the web application and the web application. The first way is to have AGS and the web application share the membership/permission/roles datastore. In this case, the web application can use the same username & password combination to obtain a token from the AGS server. The second way is to have all users of the web application use the same name username & password to access the AGS services. The second way could work because the user has already been authenticated by the web application and so he can be trusted to access the AGS services also. In this case, the username & password that will be used to log-in all web application authenticated users can be stored in the web application configuration file (web.config). This credential can be used to obtain a token from AGS. This is generally how Bing map services are also handled. The Bing credentials are stored in the web.config and used to obtain a Bing token when the page with the map is loaded.

So, once the user logs into the web application, the username & password from the shared datastore/web.config can be used to make a request to the AGS ‘GetToken’ URL endpoint and obtain a short-lived token for AGS access. This token can then be sent down to the client as a part of the HTML / ASPX page. Another technique is to write a HTTPHandler that accepts a GET request without a username & password and uses credentials from the shared datastore/web.config to obtain a token to access AGS services and sends the token down to the web application client. Is method is secure because the HTTPHandler itself can be secured by either windows/forms authentication of the host web application.

Another thing to note about AGS tokens is that AGS does NOT require a ‘Referrer’ (IP address/ Site URL) to generate a short-lived token (long-lived tokens do require them). If you are generating a token from the AGS web page to generate a token, you will have an option to not specify the ‘Referrer’ (ClientID), but if you are just making a HTTP request to the GetTokens endpoint, you can obtain a short-lived token without the ClientID. When using short-lived tokens obtained without the ClientID, AGS does NOT enforce checks on where the calls are originating from. Actually, this is the reason why Silverlight clients are currently able to consume token secure map / AGS services. Silverlight 3 & under clients do not include the ‘Referrer’ HTTP Header for all outgoing HTTP requests, so ClientID origin checks are not enforced on Silverlight API clients. This issue has been fixed in Silverlight 4.

Unfortunately, the authentication tokens generated by ASP.NET to secure web applications and the ones generated by AGS are generated using different techniques. The key used to generate the token is different, ASP.NET uses the machineKey from web.config and AGS token uses a key from the AGS configuration file. If this wasn’t the case, we could technically have the ASP.NET web application and AGS share the same token…