Steve on Security

Registration for the Security Development Conference just went live not too long ago and I just registered. Early bird pricing is $300.

Check it out.

3 TRACKS & 24 SESSIONS

The session tracks for this industry event will target three important roles for any security organization: security engineers, business decision makers, and lifecycle process managers. The sessions in these tracks will include experts representing over 30 organizations from a variety of industries. Visit the event website to see our current list of speakers, spread the word and register early to join us at the Security Development Conference 2012 on May 15 & 16!

WHY YOU SHOULD ATTEND

• Accelerate Adoption – Hear from leaders across a variety of organizations and learn from their experiences on how to accelerate SDL adoption in your own organization
• Gain Efficiencies – Learn effective ways to align SDL practices across engineering, business, and management
• Networking – Interact with peers, vendors and sponsors who provide SDL services, training, and tools
• Affordable Training – This is an affordable training opportunity that can benefit your entire security team
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REGISTRATION FEES

During the Toronto stop of the TechDays tour in Canada Richard Campbell was in town talking to a bunch of really smart people about the latest and greatest technologies they've been working on.

And then me for some reason.

We got to talk about ADFS and associates:

Richard talks to Steve Syfuhs at TechDays Toronto about IT Pros providing security services for developers using Active Directory Federated Services. IT and development talking to each other willingly? Perish the thought! But in truth, Steve makes it clear that ADFS provides a great wrapper for developers to access active directory or any other service that has security claims that an application might require. Azure depends on it, even Office 365 can take advantage of ADFS. Steve discusses how IT can work with developers to make the jobs of both groups easier.

You can listen to it here: http://www.runasradio.com/default.aspx?showNum=240

I need to work on using fewer vague analogies.

Every once in a while you need to make a life-altering decision.

Last night I sent an email to the ObjectSharp team telling them I had resigned (I had spoken to the bosses in person prior).

Boy, talk about blunt, eh?

Every once in a while you are offered a once in a lifetime opportunity to do something pretty amazing. I’ve had three of these opportunities. The first was Woodbine Entertainment where I got my start in the Toronto development world. The second was ObjectSharp where I have been able to learn so much from some of the brightest minds in the industry. The third was two weeks ago in Vancouver.

Two weeks ago I was offered a position to lead development of a product for an ISV in BC.

Me? A leader? Wait. Huh?

Well okay, not quite. Cue the screeching cut-away-from-sappy noise.

So what's the deal? I'm not saying, yet. At this point I'm sure a few people could guess though.  

Suffice to say I'll be moving to BC at the end of the year. I'll be at ObjectSharp until December 16th and then will be moving right around the new year.

I'm not sure I can really describe how excited I am about this new position. I'll be working on an awesome product with an awesome group of people.

Of course, it's a little sad leaving ObjectSharp. They have a such a great team of people, and some of the smartest people in the industry.

So it should be an interesting experience.

Good morning class!

Pop quiz: How many of you do proper input validation in your ASP.NET site, WebForms, MVC, or otherwise?

Some Background

There is an axiom in computer science: never trust user input because it's guaranteed to contain invalid data at some point.

In security we have a similar axiom: never trust user input because it's guaranteed to contain invalid data at some point, and your code is bound to contain a security vulnerability somewhere, somehow. Granted, it doesn't flow as well as the former, but the point still stands.

The solution to this problem is conceptually simple: validate, validate, validate. Every single piece of input that is received from a user should be validated.

Of course when anyone says something is a simple concept it's bound to be stupidly complex to get the implementation right. Unfortunately proper validation is not immune to this problem. Why?

The Problem

Our applications are driven by user data. Without data our applications would be pretty useless. This data is usually pretty domain-specific too so everything we receive should have particular structures, and there's a pretty good chance that a few of these structures are so specific to the organization that there is no well-defined standard. By that I mean it becomes pretty difficult to validate certain data structures if they are custom designed and potentially highly-complex.

So we have this problem. First, if we don't validate that the stuff we are given is clean, our application starts behaving oddly and that limits the usefulness of the application. Second, if we don't validate that the stuff we are given is clean, and there is a bug in the code, we have a potential vulnerability that could wreak havoc for the users.

The Solution

The solution as stated above is to validate all the input, both from a business perspective and from a security perspective. We want it to go something like this:

In this post we are going to look at the best way to validate the security of incoming data within ASP.NET. This requires looking into how ASP.NET processes input from the user.

When ASP.NET receives something from the user it can come from four different vectors:

• Within the Query String (?foo=bar)
• Within the Form (via a POST)
• Within a cookie
• Within the server variables (a collection generated from HTTP headers and internal server configuration)

These vectors drive ASP.NET, and you can potentially compromise an application by maliciously modifying any of them.

Pop quiz: How many of you check whether custom cookies exist before trying to use them? Almost everyone, good. Now, how many of you validate that the data within the cookies is, well, valid before using them?

What about checking your HTTP headers?

The Bypass

Luckily ASP.NET has some out-of-the-box behaviors that protect the application from malicious input. Unfortunately ASP.NET isn't very forgiving when it comes to validation. It doesn't distinguish between quasi-good input and bad input, so anything containing an angle bracket causes a YSoD.

The defacto fix to this is to do one of two things:

• Disable validation in the page declaration within WebForms, or stick a [ValidateInput(false)] attribute on an MVC controller
• Set <pages validateRequest="false"> in web.config

What this will do is tell ASP.NET to basically skip validating the four vectors and let anything in. It was assumed that you would do validation on your own.

Raise your hand if you think this is a bad idea. Okay, keep your hands up if you've never done this for a production application. At this point almost everyone should have put their hands down. I did.

The reason we do this is because as I said before, ASP.NET isn't very forgiving when it comes to validation. It's all or nothing.

What's worse, as ASP.NET got older it started becoming pickier about what it let in so you had more reasons for disabling validation. In .NET 4 validation occurs at a much earlier point. It's a major breaking change:

The request validation feature in ASP.NET provides a certain level of default protection against cross-site scripting (XSS) attacks. In previous versions of ASP.NET, request validation was enabled by default. However, it applied only to ASP.NET pages (.aspx files and their class files) and only when those pages were executing.

In ASP.NET 4, by default, request validation is enabled for all requests, because it is enabled before the BeginRequest phase of an HTTP request. As a result, request validation applies to requests for all ASP.NET resources, not just .aspx page requests. This includes requests such as Web service calls and custom HTTP handlers. Request validation is also active when custom HTTP modules are reading the contents of an HTTP request.

Since backwards compatibility is so important, a configuration attribute was also added to tell ASP.NET to revert to the 2.0 validation mode meaning that it occurs later in the request lifecycle like in ASP.NET 2.0:

<httpRuntime requestValidationMode="2.0" />

If you do a search online for request validation almost everyone comes back with this solution. In fact, it became a well known solution with the Windows Identity Foundation in ASP.NET 4.0 because when you do a federated sign on, WIF receives the token as a chunk of XML. The validator doesn't approve because of the angle brackets. If you set the validation mode to 2.0, the validator checks after the request passes through all HttpModules, which is how WIF consumes that token via the WSFederationAuthenticationModule.

The Proper Solution

So we have the problem. We also have built in functionality that solves our problem, but the way it does it kind of sucks (it's not a bad solution, but it's also not extensible). We want a way that doesn't suck.

In earlier versions of ASP.NET the best solution was to disable validation and within a HttpModule check every vector for potentially malicious input. The benefit here is that you have control over what is malicious and what is not. You would write something along these lines:

public class ValidatorHttpModule : IHttpModule
{
    public void Dispose() { }

    public void Init(HttpApplication context)
    {
        context.BeginRequest += new EventHandler(context_BeginRequest);
    }

    void context_BeginRequest(object sender, EventArgs e)
    {
        HttpApplication context = (HttpApplication)sender;

        foreach (var q in context.Request.QueryString)
        {
            if (CheckQueryString(q))
            {
                throw new SecurityException("Bad validation");
            }
        }

        foreach (var f in context.Request.Form)
        {
            if (CheckForm(f))
            {
                throw new SecurityException("Bad validation");
            }
        }

        foreach (var c in context.Request.Cookies)
        {
            if (CheckCookie(c))
            {
                throw new SecurityException("Bad validation");
            }
        }

        foreach (var s in context.Request.ServerVariables)
        {
            if (CheckServerVariable(s))
            {
                throw new SecurityException("Bad validation");
            }
        }
    }

    // <snip />
}

The downside to this approach though is that you are stuck with pretty clunky validation logic. It executes on every single request, which may not always be necessary. You are also forced to execute the code in order of whenever your HttpModule is initialized. It won't necessarily execute first, so it won't necessarily protect all parts of your application. Protection from an attack that doesn't protect everything from that particular attack isn't very useful.  <Cynicism>Half-assed protection is only good when you have half an ass.</Cynicism>

What we want is something that executes before everything else. In our HttpModule we are validating on BeginRequest, but we want to validate before BeginRequest.

The way we do this is with a custom RequestValidator. On a side note, this post may qualify as having the longest introduction ever. In any case, this custom RequestValidator is set within the httpRuntime tag within the web.config:

<httpRuntime requestValidationType="Syfuhs.Web.Security.CustomRequestValidator" />

We create a custom request validator by creating a class with a base class of System.Web.Util.RequestValidator. Then we override the IsValidRequestString method.

This method allows us to find out where the input is coming from, e.g. from a Form or from a cookie etc. This validator is called on each value within the four collections above, but only when a value exists. It saves us the trouble of going over everything in each request. Within an HttpModule we could certainly build out the same functionality by checking contents of each collection, but this saves us the hassle of writing the boilerplate code. It also provides us a way of describing the problem in detail because we can pass an index location of where the problem exists. So if we find a problem at character 173 we can pass that value back to the caller and ASP.NET will throw an exception describing that index. This is how we get such a detailed exception from WIF:

A Potentially Dangerous Request.Form Value Was Detected from the Client (wresult="<t:RequestSecurityTo...")

Our validator class ends up looking like:

public class MyCustomRequestValidator : RequestValidator
{
    protected override bool IsValidRequestString(HttpContext context, string value, RequestValidationSource requestValidationSource, string collectionKey, out int validationFailureIndex)
    {
        validationFailureIndex = 0;

        switch (requestValidationSource)
        {
            case RequestValidationSource.Cookies:
                return ValidateCookie(collectionKey, value, out validationFailureIndex);
                break;

            case RequestValidationSource.Form:
                return ValidateFormValue(collectionKey, value, out validationFailureIndex);
                break;

            // <snip />
        }

        return base.IsValidRequestString(context, value, requestValidationSource, collectionKey, out validationFailureIndex);
    }

    // <snip />
}

Each application has different validation requirements so I've just mocked up how you would create a custom validator.

If you use this design you can easily validate all inputs across the application, and you don't have to turn off validation.

So once again, pop quiz: How many of you do proper input validation?

The biggest detractor to Single Sign On is the same thing that makes it so appealing – you only need to prove your identity once. This scares the hell out of some people because if you can compromise a users session in one application it's possible to affect other applications. Congratulations: checking your Facebook profile just caused your online store to delete all it's orders. Let's break that attack down a little.

• You just signed into Facebook and checked your [insert something to check here] from some friend. That contained a link to something malicious.
• You click the link, and it opens a page that contains an iframe. The iframe points to a URL for your administration portal of the online store with a couple parameters in the query string telling the store to delete all the incoming orders.
• At this point you don't have a session with the administration portal and in a pre-SSO world it would redirect you to a login page. This would stop most attacks because either a) the iframe is too small to show the page, or b) (hopefully) the user is smart enough to realize that a link from a friend on Facebook shouldn't redirect you to your online store's administration portal. In a post-SSO world, the portal would redirect you to the STS of choice and that STS already has you signed in (imagine what else could happen in this situation if you were using Facebook as your identity provider).
• So you've signed into the STS already, and it doesn't prompt for credentials. It redirects you to the administration page you were originally redirected away from, but this time with a session. The page is pulled up, the query string parameters are parsed, and the orders are deleted.

There are certainly ways to stop this as part of this is a bit trivial. For instance you could pop up an Ok/Cancel dialog asking "are you sure you want to delete these?", but for the sake of discussion lets think of this at a high level.

The biggest problem with this scenario is that deleting orders doesn't require anything more than being signed in. By default you had the highest privileges available.

This problem is similar to the problem many users of Windows XP had. They were, by default, running with administrative privileges. This lead to a bunch of problems because any application running could do whatever it pleased on the system. Malware was rampant, and worse, users were just doing all around stupid things because they didn't know what they were doing but they had the permissions necessary to do it.

The solution to that problem is to give users non-administrative privileges by default, and when something required higher privileges you have to re-authenticate and temporarily run with the higher privileges. The key here is that you are running temporarily with higher privileges. However, security lost the argument and Microsoft caved while developing Windows Vista creating User Account Control (UAC). By default a user is an administrator, but they don't have administrative privileges. Their user token is a stripped down administrator token. You only have non-administrative privileges. In order to take full advantage of the administrator token, a user has to elevate and request the full token temporarily. This is a stop-gap solution though because it's theoretically possible to circumvent UAC because the administrative token exists. It also doesn't require you to re-authenticate – you just have to approve the elevation.

As more and more things are moving to the web it's important that we don't lose control over privileges. It's still very important that you don't have administrative privileges by default because, frankly, you probably don't need them all the time.

Some web applications are requiring elevation. For instance consider online banking sites. When I sign in I have a default set of privileges. I can view my accounts and transfer money between my accounts. Anything else requires that I re-authenticate myself by entering a private pin. So for instance I cannot transfer money to an account that doesn't belong to me without proving that it really is me making the transfer.

There are a couple ways you can design a web application that requires privilege elevation. Lets take a look at how to do it with Claims Based Authentication and WIF.

First off, lets look at the protocol. Out of the box WIF supports the WS-Federation protocol. The passive version of the protocol supports a query parameter of wauth. This parameter defines how authentication should happen. The values for it are mostly specific to each STS however there are a few well-defined values that the SAML protocol specifies. These values are passed to the STS to tell it to authenticate using a particular method. Here are some most often used:

When you pass one of these values to the STS during the signin request, the STS should then request that particular type of credential. the wauth parameter supports arbitrary values so you can use whatever you like. So therefore we can create a value that tells the STS that we want to re-authenticate because of an elevation request.

All you have to do is redirect to the STS with the wauth parameter:

https://yoursts/authenticate?wa=wsignin1.0&wtrealm=uri:myrp&wauth=urn:super:secure:elevation:method

Once the user has re-authenticated you need to tell the relying party some how. This is where the Authentication Method claim comes in handy:

http://schemas.microsoft.com/ws/2008/06/identity/claims/authenticationmethod

Just add the claim to the output identity:

protected override IClaimsIdentity GetOutputClaimsIdentity(IClaimsPrincipal principal, RequestSecurityToken request, Scope scope)
{
    IClaimsIdentity ident = principal.Identity as IClaimsIdentity;
    ident.Claims.Add(new Claim(ClaimTypes.AuthenticationMethod, "urn:super:secure:elevation:method"));
    // finish filling claims...
    return ident;
}

At that point the relying party can then check to see whether the method satisfies the request. You could write an extension method like:

public static bool IsElevated(this IClaimsPrincipal principal)
{
    return principal.Identity.AuthenticationType == "urn:super:secure:elevation:method";
}

And then have a bit of code to check:

var p = Thread.CurrentPrincipal as IClaimsPrincipal;
if (p != null && p.IsElevated())
{
    DoSomethingRequiringElevation();
}

This satisfies half the requirements for elevating privilege. We need to make it so the user is only elevated for a short period of time. We can do this in an event handler after the token is received by the RP.  In Global.asax we could do something like:

void Application_Start(object sender, EventArgs e)
{
    FederatedAuthentication.SessionAuthenticationModule.SessionSecurityTokenReceived 
        += new EventHandler<SessionSecurityTokenReceivedEventArgs> (SessionAuthenticationModule_SessionSecurityTokenReceived);
}
void SessionAuthenticationModule_SessionSecurityTokenReceived(object sender, SessionSecurityTokenReceivedEventArgs e)
{
    if (e.SessionToken.ClaimsPrincipal.IsElevated())
    {
        SessionSecurityToken token = new SessionSecurityToken(e.SessionToken.ClaimsPrincipal, e.SessionToken.Context, e.SessionToken.ValidFrom, e.SessionToken.ValidFrom.AddMinutes(15));
        e.SessionToken = token;
    }
}

This will check to see if the incoming token has been elevated, and if it has, set the lifetime of the token to 15 minutes.

There are other places where this could occur like within the STS itself, however this value may need to be independent of the STS.

As I said earlier, as more and more things are moving to the web it's important that we don't lose control of privileges. By requiring certain types of authentication in our relying parties, we can easily support elevation by requiring the STS to re-authenticate.

Got this email just now.  I was amused (mainly because it got past the spam filters)…

Dear Email user,

This message is from Administration center Maintenance Policy verified that your mailbox exceeds (10.9GB) its limit, you may be unable to receive new email or send mails, To re- set your SPACE on our database prior to maintain your IN BOX, You must Reply to this email by Confirming your account details below.

Surname:

First Name:

User-Name:

Password:

Date Of Birth:

Failure to do this will immediately render your Web-email address deactivated from our database.

Thanks

Admin Help Desk© 2011

And my Reply…

Dear Admin Help Desk© 2011,

Thank you for the warning that my SPACE on your database prior to maintain my IN BOX exceeds its limit. Please find the following information handy for future reference.

Surname: Jobs

First Name: Steve

User-Name: Jobbers1

Password: CenterOfTheUniverse

Date Of Birth: December 25^th, 0001

If for whatever reason you cannot reset my SPACE please email me back at tips@fbi.gov and I will be sure to help you.

Over on the Canadian Solution Developer's blog I have a series on the basics of writing secure applications. It's a bit of an introduction to all the things we should know in order to write software that doesn't contain too many vulnerabilities. This is part five of the series, unedited for all to enjoy.

There are only a few certainties in life: death, taxes, me getting this post in late, and one of your applications getting attacked.  Throughout the lifetime of an application it will undergo a barrage of attack – especially if it's public facing.  If you followed the SDL, tested properly, coded securely, and managed well, you will have gotten most of the bugs out.

Most.

There will always be bugs in production code, and there will very likely always be a security bug in production code.  Further, if there is a security bug in production code, an attacker will probably find it.  Perhaps the best metric for security is along the lines of mean-time-to-failure.  Or rather, mean-time-to-breach.  All safes for storing valuables are rated in how long they can withstand certain types of attacks – not whether they can, but how long they can.  There is no one-single thing we can do to prevent an attack, and we cannot prevent all attacks.  It's just not in the cards.  So, it stands to reason then that we should prepare for something bad happening.  The final stage of the SDL requires that an Incident Response Plan is created.  This is the procedure to follow in the event of a vulnerability being found.

In security parlance, there are protocols and procedures.  The majority of the SDL is all protocol.  A protocol is the usual way to do things.  It's the list of steps you follow to accomplish a task that is associated with a normal working condition, e.g. fuzzing a file parser during development.  You follow a set of steps to fuzz something, and you really don't deviate from those steps.  A procedure is when something is different.  A procedure is reactive.  How you respond to a security breach is a procedure.  It's a set of steps, but it's not a normal condition.

An Incident Response Plan (IRP - the procedure) serves a few functions:

• It has the list of people to contact in the event of the emergency
• It is the actual list of steps to follow when bad things happen
• It includes references to other procedures for code written by other teams

This may be one of the more painful parts of the SDL, because it's mostly process over anything else.  Luckily there are two wonderful products by Microsoft that help: Team Foundation Server.  For those of you who just cringed, bear with me.

Microsoft released the MSF-Agile plus Security Development Lifecycle Process Template for VS 2010 (it also takes second place in the longest product name contest) to make the entire SDL process easier for developers.  There is the SDL Process Template for 2008 as well.

It's useful for each stage of the SDL, but we want to take a look at how it can help with managing the IRP.  First though, lets define the IRP.

Emergency Contacts (Incident Response Team)

The contacts usually need to be available 24 hours a day, seven days a week.  These people have a range of functions depending on the severity of the breach:

• Developer – Someone to comprehend and/or triage the problem
• Tester – Someone to test and verify any changes
• Manager – Someone to approve changes that need to be made
• Marketing/PR – Someone to make a public announcement (if necessary)

Each plan is different for each application and for each organization, so there may be ancillary people involved as well (perhaps an end user to verify data).  Each person isn't necessarily required at each stage of the response, but they still need to be available in the event that something changes.

The Incident Response Plan

Over the years I've written a few Incident Response Plans (Never mind that I was asked to do it after an attack most times – you WILL go out and create one after reading this right?).  Each plan was unique in it's own way, but there were commonalities as well.

Each plan should provide the steps to answer a few questions about the vulnerability:

• How was the vulnerability disclosed?  Did someone attack, or did someone let you know about it?
• Was the vulnerability found in something you host, or an application that your customers host?
• Is it an ongoing attack?
• What was breached?
• How do you notify the your customers about the vulnerability?
• When do you notify them about the vulnerability?

And each plan should provide the steps to answer a few questions about the fix:

• If it's an ongoing attack, how do you stop it?
• How do you test the fix?
• How do you deploy the fix?
• How do you notify the public about the fix?

Some of these questions may not be answerable immediately – you may need to wait until a postmortem to answer them.

This is the high level IRP for example:

1. The Attack – It's already happened
2. Evaluate the state of the systems or products to determine the extent of the vulnerability
   • What was breached?
   • What is the vulnerability
3. Define the first step to mitigate the threat
   • How do you stop the threat?
   • Design the bug fix
4. Isolate the vulnerabilities if possible
   • Disconnect targeted machine from network
   • Complete forensic backup of system
   • Turn off the targeted machine if hosted
5. Initiate the mitigation plan
   • Develop the bug fix
   • Test the bug fix
6. Alert the necessary people
   • Get Marketing/PR to inform clients of breach (don't forget to tell them about the fix too!)
   • If necessary, inform the proper legal/governmental bodies
7. Deploy any fixes
   • Rebuild any affected systems
   • Deploy patch(es)
   • Reconnect to network
8. Follow up with legal/governmental bodies if prosecution of attacker is necessary
• Analyze forensic backups of systems
9. Do a postmortem of the attack/vulnerability
   • What went wrong?
   • Why did it go wrong?
   • What went right?
   • Why did it go right?
   • How can this class of attack be mitigated in the future?
   • Are there any other products/systems that would be affected by the same class?

Some of procedures can be done in parallel, hence the need for people to be on call.

Team Foundation Server

So now that we have a basic plan created, we should make it easy to implement.  The SDL Process Template (mentioned above) creates a set of task lists and bug types within TFS projects that are used to define things like security bugs, SDL-specific tasks, exit criteria, etc..

While these can (and should) be used throughout the lifetime of the project, they can also be used to map out the procedures in the IRP.  In fact, a new project creates an entry in Open SDL Tasks to create an Incident Response Team:

A bug works well to manage incident responses.

Once a bug is created we can link a new task with the bug.

And then we can assign a user to the task:

Each bug and task are now visible in the Security Exit Criteria query:

Once all the items in the Exit Criteria have been met, you can release the patch.

Conclusion

Security is a funny thing. A lot of times you don't think about it until it's too late. Other times you follow the SDL completely, and you still get attacked.

In the last four posts we looked at writing secure software from a pretty high level.  We touched on common vulnerabilities and their mitigations, tools you can use to test for vulnerabilities, some thoughts to apply to architecting the application securely, and finally we looked at how to respond to problems after release.  By no means will these posts automatically make you write secure code, but hopefully they have given you guidance to start understanding what goes into writing secure code.  It's a lot of work, and sometimes its hard work.

Finally, there is an idea I like to put into the first section of every Incident Response Plan I've written, and I think it applies to writing software securely in general:

Something bad just happened.  This is not the time to panic, nor the time to place blame.  Your goal is to make sure the affected system or application is secured and in working order, and your customers are protected.

Something bad may not have happened yet, and it may not in the future, but it's important to plan accordingly because your goal should be to protect the application, the system, and most importantly, the customer.

Over on the Canadian Solution Developer's blog I have a series on the basics of writing secure applications. It's a bit of an introduction to all the things we should know in order to write software that doesn't contain too many vulnerabilities. This is part four of the series, unedited for all to enjoy.

Before you start to build an application you need to start with a design of it.  In the last article I stated that bugs that are introduced at this stage of the process are the most expensive to fix throughout the lifetime of the project.  It is this reason that we need to have a good foundation for security, otherwise it'll be expensive (not to mention a pain) to fix. Keep in mind this isn't an agile verse the world discussion because no matter what at some point you still need to have a design of the application.

Before we can design an application, we need to know that there are two basic types of code/modules:

• That which is security related; e.g. authentication, crypto, etc.
• That which is not security related (but should still be secure nonetheless); e.g. CRUD operations.

They can be described as privileged or unprivileged.

Privileged

Whenever a piece of code is written that deals with things like authentication or cryptography, we have be very careful with it because it should be part of the secure foundation of the application. Privileged code is the authoritative core of the application.  Careless design here will render your application highly vulnerable.  Needless to say, we don't want that.  We want a foundation we can trust.  So, we have three options:

• Don't write secure code, and be plagued with potential security vulnerabilities.  Less cost, but you cover the risk.
• Write secure code, test it, have it verified by an outside source.  More cost, you still cover the risk.
• Make someone else write the secure code.  Range of cost, but you don't cover the risk.

In general, from a cost/risk perspective, our costs and risks decrease as we move from the top of the list to the bottom.  This should therefore be a no-brainer: DO NOT BUILD YOUR OWN PRIVILEGED SECURITY MODULES.  Do not invent a new way of doing things if you don't need to, and do not rewrite modules that have already been vetted by security experts.  This may sound harsh, but seriously, don't.  If you think you might need to, stop thinking.  If you still think you need to, contact a security expert. PLEASE!

This applies to both coding and architecture.  In part 2 we did not come up with a novel way of protecting our inputs, we used well known libraries or methods.  Well now we want to apply this to the application architecture. 

Authentication

Let's start with authentication.

A lot of times an application has a need for user authentication, but it's core function has nothing to do with user authentication.  Yes, you might need to authenticate users for your mortgage calculator, but the core function of the application is calculating mortgages, and has very little to do with users.  So why would you put that application in charge of authenticating users?  It seems like a fairly simple argument, but whenever you let your application use something like a SqlMembershipProvider you are letting the application manage authentication.  Not only that, you are letting the application manage the entire identity for the user.  How much of that identity information is duplicated in multiple databases?  Is this really the right way to do things?  Probably not.

From an architectural perspective, we want to create an abstract relationship between the identity of the user and the application.  Everything this application needs to know about this user is part of this identity, and (for the most part) the application is not an authority on any of this information because it's not the job of the application to be the authority.

Let's think about this another way.

Imagine for a moment that you want to get a beer at the bar. In theory the bartender should ask you for proof of age. How do you prove it? Well, one option is to have the bartender cut you in half and count the number of rings, but there could be some problems with that. The other option is for you to write down your birthday on a piece of paper to which the bartender approves or disapproves. The third option is to go to the government, get an ID card, and then present the ID to the bartender.

Some may have laughed at the idea of just writing your birthday on a piece of paper, but this is essentially what is happening when you are authenticating users within your application because it is up to the bartender (or your application) to trust the piece of paper. However, we trust the government's assertion that the birthday on the ID is valid, and the ID is for the person requesting the drink.  The bartender knows nothing about you except your date of birth because that's all the bartender needs to know.  Now, the bartender could store information that they think is important to them, like your favorite drink, but the government doesn't care (as it isn't the authoritative source), so the bartender stores that information in his own way.

Now this begs the question of how do you prove your identity/age to the government, or how do you authenticate against this external service?  Frankly, it doesn't matter as it's the core function of this external service and not our application.  Our application just needs to trust that it is valid, and trust that it is a secure authentication mechanism.

In developer speak, this is called Claims Based Authentication.  A claim is an arbitrary piece of information about an identity, such as age, and is bundled into a collection of claims, to be part of a token.  A Security Token Service (STS) generates the token, and our application consumes it.  It is up to the STS to handle authentication.  Both Claims Based Authentication and the Kerberos Protocol are built around the same model, although they use different terms.  If you are looking for examples, Windows Live/Hotmail use Claims via the WS-Federation protocol.  Google, Facebook, and Twitter use Claims via the OAuth protocol.  Claims are everywhere.

Alright, less talking, more diagramming:

The process goes something like this:

• Go to STS and authenticate (this is usually a web page redirect + the user entering their credentials)
• The STS tells the user's browser to POST the token to the application
• The application verifies the token and verifies whether it trusts the the STS
• The Application consumes the token and uses the claims as it sees fit

Now we get back to asking how the heck does the STS handle authentication?  The answer is that it depends (Ah, the consultants answer).  The best case scenario is that you use an STS and identity store that already exist.  If you are in an intranet scenario use Active Directory and Active Directory Federation Services (a free STS for Active Directory).  If your application is on the internet use something like Live ID or Google ID, or even Facebook, simplified with Windows Azure Access Control Services.  If you are really in a bind and need to create your own STS, you can do so with the Windows Identity Foundation (WIF).  In fact, use WIF as the identity library in the diagram above.  Making a web application claims-aware involves a process called Federation.  With WIF it's really easy to do.

Accessing claims within the token is straightforward because you are only accessing a single object, the identity within the CurrentPrincipal:

private static TimeSpan GetAge()
{
    IClaimsIdentity ident = Thread.CurrentPrincipal.Identity as IClaimsIdentity;

    if (ident == null)
        throw new ApplicationException("Isn't a claims based identity");

    var dobClaims = ident.Claims.Where(c => c.ClaimType == ClaimTypes.DateOfBirth);

    if(!dobClaims.Any())
        throw new ApplicationException("There are no date of birth claims");

    string dob = dobClaims.First().Value;

    TimeSpan age = DateTime.Now - DateTime.Parse(dob);

    return age;
}

There is secondary benefit to Claims Based Authentication.  You can also use it for authorization.  WIF supports the concept of a ClaimsAuthorizationManager, which you can use to authorize access to site resources.  Instead of writing your own authorization module, you are simply defining the rules for access, which is very much a business problem, not technical.

Once authentication and authorization are dealt with, the two final architectural nightmares problems revolve around privacy and cryptography.

Privacy

Privacy is the control of Personally Identifiable Information (PII), which is defined as anything you can use to personally identify someone (good definition, huh?).  This can include information like SIN numbers, addresses, phone numbers, etc.  The easiest solution is to simply not use the information.  Don't ask for it and don't store it anywhere.  Since this isn't always possible, the goal should be to use (and request) as little as possible.  Once you have no more uses for the information, delete it.

This is a highly domain-specific problem and it can't be solved in a general discussion on architecture and design.  Microsoft Research has an interesting solution to this problem by using a new language designed specifically for defining the privacy policies for an application:

Preferences and policies are specified in terms of granted rights and required obligations, expressed as assertions and queries in an instance of SecPAL (a language originally developed for decentralized authorization). This paper further presents a formal definition of satisfaction between a policy and a preference, and a satisfaction checking algorithm. Based on the latter, a protocol is described for disclosing PIIs between users and services, as well as between third-party services.

Privacy is also a measure of access to information in a system.  Authentication and authorization are a core component of proper privacy controls.  There needs to be access control on user information.  Further, access to this information needs to be audited.  Anytime someone reads, updates, or deletes personal information, it should be recorded somewhere for review later.  There are quite a number of logging frameworks available, such as log4net or ELMAH.

Cryptography

Finally there is cryptography.

For the love of all things holy, do not do any custom crypto work.  Rely on publically vetted libraries like Bouncy Castle and formats like OpenPGP.  There are special circles of hell devoted to those who try and write their own crypto algorithms for production systems.  Actually, this is true for anything security related.

Be aware of how you are storing your private keys.

Don't store them in the application as magic-strings, or store them with the application at all. 

If possible store them in a Hardware Security Module (HSM). 

Make sure you have proper access control policies for the private keys.

Centralize all crypto functions so different modules aren't using their own implementations.

Finally, if you have to write custom encryption wrappers, make sure your code is capable of switching encryption algorithms without requiring recompilation.  The .NET platform has made it easy to change.  You can specify a string:

public static byte[] SymmetricEncrypt(byte[] plainText, byte[] initVector, byte[] keyBytes)
{
    if (plainText == null || plainText.Length == 0)
        throw new ArgumentNullException("plainText");

    if (initVector == null || initVector.Length == 0)
        throw new ArgumentNullException("initVector");

    if (keyBytes == null || keyBytes.Length == 0)
        throw new ArgumentNullException("keyBytes");

    using (SymmetricAlgorithm symmetricKey 
             = SymmetricAlgorithm.Create("algorithm")) // e.g.: 'AES'
    {               
        return CryptoTransform(plainText, symmetricKey.CreateEncryptor(keyBytes, initVector));
    }
}

private static byte[] CryptoTransform(byte[] payload, ICryptoTransform transform)
{
    using (MemoryStream memoryStream = new MemoryStream())
    using (CryptoStream cryptoStream = new CryptoStream(memoryStream, transform, CryptoStreamMode.Write))
    {
        cryptoStream.Write(payload, 0, payload.Length);

        cryptoStream.FlushFinalBlock();

        return memoryStream.ToArray();
    }
}

Microsoft provides a list of all supported algorithms, as well as how to specify new algorithms for future use.

By following these design guidelines you should have a fairly secure foundation for your application.  Now lets look at unprivileged modules.

Unprivileged

In Part 2 there was a single, all encompassing, hopefully self evident, solution to most of the vulnerabilities: sanitize your inputs.  Most vulnerabilities, one way or another, are the result of bad input.  This is therefore going to be a very short section.

Don't let input touch queries directly.  Build business objects around data and encode any strings.  Parse all incoming data.  Fail gracefully on bad input.

Properly lock down access to resources through authorization mechanisms in privileged modules, and audit all authorization requests.

If encryption is required, call into a privileged module.

Finally, validate the need for SSL.  If necessary, force it.

Final Thoughts

Bugs during this phase of development are the most costly and hardest to fix, as the design and architecture of the application is the most critical step in making sure it isn't vulnerable to attack.

Throughout the first four articles in this series we've looked at how to develop a secure application.  In the last article, we will look at how to respond to threats and mitigate the damage done.