Securing The .Net Cookies

Posted by on October 13, 2015

I remember years ago when we talked about cookie poisoning, the act of modifying cookies to get the application to act differently.  An example was the classic cookie used to indicate a user’s role in the system.  Often times it would contain 1 for Admin or 2 for Manager, etc.  Change the cookie value and all of a sudden you were the new admin on the block.   You really don’t hear the phrase cookie poisoning anymore, I guess it was too dark. 

 

There are still security risks around the cookies that we use in our application.  I want to highlight 2 key attributes that help protect the cookies for your .Net application: Secure and httpOnly.

 

Secure Flag

The secure flag tells the browser that the cookie should only be sent to the server if the connection is using the HTTPS protocol.  Ultimately this is indicating that the cookie must be sent over an encrypted channel, rather than over HTTP which is plain text.

 

HttpOnly Flag

The httpOnly flag tells the browser that the cookie should only be accessed to be sent to the server with a request, not by client-side scripts like JavaScript.  This attribute helps protect the cookie from being stolen through cross-site scripting flaws.

 

Setting The Attributes

There are multiple ways to set these attributes of a cookie. Things get a little confusing when talking about session cookies or the forms authentication cookie, but I will cover that as I go.  The easiest way to set these flags for all developer created cookies is through the web.config file.  The following snippet shows the httpCookies element in the web.config.

  
<system.web>
    <authentication mode="None" />
    <compilation targetframework="4.6" debug="true" />
    <httpruntime targetframework="4.6" />
    <httpcookies httponlycookies="true" requiressl="true" />
 </system.web>

 

As you can see, you can set httponlycookies to true to se the httpOnly flag on all of the cookies.  In addition, the requiressl setting sets the secure flag on all of the cookies with a few exceptions.

 

Some Exceptions

I stated earlier there are a few exceptions to the cookie configuration.  The first I will discuss is the session cookie. The session cookie in ASP.Net is defaulted/hard-coded to set the httpOnly attribute.  This should override any value set in the httpCookies element in the web.config.  The session cookie does not default to requireSSL and setting that value in the httpCookies element as shown above should work just find for it.

 

The forms authentication cookie is another exception to the rules.  Like the session cookie, it is hard-coded to httpOnly.  The Forms element of the web.config has a requireSSL attribute that will override what is found in the httpCookies element.  Simply put, if you don’t set requiressl=”true” in the Forms element then the cookie will not have the secure flag even if requiressl=”true” in the httpCookies element.

 

This is actually a good thing, even though it might not seem so yet.  Here is the next thing about that Forms requireSSL setting.. When you set it, it will require that the web server is using a secure connection.  Seems like common sense, but…  imagine a web farm where the load balancers offload SSL.  In this case, while your web app uses HTTPS from client to server, in reality, the HTTPS stops at the load balancer and is then HTTP to the web server.   This will throw an exception in your application. 

 

I am not sure why Microsoft decided to make the decision to actually check this value, since the secure flag is a direction for the browser not the server.  If you are in this situation you can still set the secure flag, you just need to do it a little differently.  One option is to use your load balancer to set the flag when it sends any responses.  Not all devices may support this so check with your vendor.  The other option is to programmatically set the flag  right before the response is sent to the user.  The basic process is to find the cookie and just sent the .Secure property to “True”. 

 

Final Thoughts

While there are other security concerns around cookies, I see the secure and httpOnly flag commonly misconfigured.  While it does not seem like much, these flags go a long way to helping protect your application.  ASP.Net has done some tricky configuration of how this works depending on the cookie, so hopefully this helps sort some of it out.   If you have questions, please don’t hesitate to contact me.  I will be putting together something a little more formal to hopefully clear this up a bit more in the near future.

Comments

Comments are closed.