Creating Secure Applications

Session 204 WWDC 2010

Secure coding helps protect a user's data from theft or corruption and prevents the compromise of secrets, loss of service, or damage to your users' systems. Gain insight into emerging security threats and learn powerful techniques to securely code, review, and install your software. Learn about hot topics in security such as the use of SSL, privacy, URL handlers, WebKit views, and privilege separation with Service Management.

Welcome to Creating Secure Applications.

I'm Matt Murphy.

I'm an engineer on the Product Security Team.

Thank you for coming.

So why are you here?

You probably know if you're in the audience but, you know, it helps to go over it.

So you're here because you want to avoid the consequences of security issues.

Things like negative press, lost revenue, and so on.

You realize that security is a complicated business.

We're dealing with more and more connected devices, you know, things like MacBooks, iPhones, iPads and so on.

And those environments demand a lot of special care and attention.

You're here because you wanted to determine the optimal ways to prevent security issues.

And if you're developing for our platform, who better to ask?

Finally, you want to maximize the benefits in terms of security with your available resources because we all know that security is not really what you want to do unless you're writing security software.

You want to develop cool applications and cool features for your customers.

And finally, if you make a security mistake, it's really expensive.

Believe me, we know.

[ Laughter ]

So in this part of presentation, I'm going to cover some design tips for secure software.

I'm going to give you some tools that you can use to help find bugs in your code.

And some tips to avoid frequently seen security issues.

And then later on, I'm going to hand it over to David who will discuss a little bit about Objective-C and Cocoa and we'll also give you some practical examples of how to apply some of the theoretical stuff that I'm going to discuss in the first part of the presentation.

So the discussion will flow in term of the Security Lifecycle.

The four distinct components of a secure piece of softwaredesign, code, test, and maintain.

Now, we're not going to cover the maintenance part here. That is, as we say, an exercise to the reader.

But that involves things like fixing bugs and delivering fixes to you customers.

Now, this is what I'm going to cover in the design portion of the discussion.

Now, as you'll note in the last bullet, there's quite a bit of information about designing security software that we don't have time to cover in an hour.

So we encourage you to look at the Secure Coding Guide which is available on Apple Developer web site.

And contrary to its name, it actually has some very useful design tips as well as cutting instructions that you can use.

First, we're going to talk a little bit about privilege separation.

Now, privilege separation is the art, shall we say, of separating your code into privileged and non-privileged components.

So that an ordinary user like me can walk up to your Mac and use it.

The first thing you want to know about supporting privilege separation is what not to do.

And the first thing you want to not do is you want to not use the AuthorizationExecuteWithPrivileges API, because I've seen many, many developers say, "Oh, but this is so easy."

And they use it and they introduce security bugs or they introduce complexity into their software.

What you want to do instead is want to factor privilege code into a background service.

And we'll discuss that in a little more detail here.

So you want to use launchd and you want to use the service management APIs to set up your background service, things like SMJobBless, SMJobSubmit, those are good reference points for you to look out if you're looking at writing privilege code.

You can also see, the SampleD example which gives you a starting point for how to write a launchd service.

And it's available from the Apple Developer web site.

Next, we want to cover a little bit about reduced privilege, which is sort of practical consequence of privilege separation but it's important enough that it gets its own section and here's why.

The most important thing about running with reduced privilege on the Mac OS is you want to test as a standard user.

So many developers don't do this and they unwittingly introduce dependencies that require the users to run as administrators to run their software.

You don't want to do that.

It is important for you to be a part of good security posture as it is for you to avoid introducing bugs in your own code.

When you're testing as a standard user, your application should just work.

I should be able to walk up to it and that should work the same as a standard user as it would as an administrator with perhaps a few exceptions if you have really truly privileged functionality.

If that doesn't happen, you found a bug and you should fix that bug because your user should be able to run your software as a standard user.

Now, a corollary to that is you don't want to rely on the special capabilities of administrators.

These are usually things like places on the file system that you can modify when you're running as an administrator.

And obviously, you can't if you're a standard user as we just noted.

But also these kinds of things might break in the future for administrators.

So we're constantly tightening the permissions of the file system and what administrator users can do without explicitly authorizing and gaining elevated privilege.

So what I mean when I say places on the file system you can write to /Applications including your own application bundle.

If you have one user who installs you application for example and another user who runs it, it won't be writable to that other user.

Also, you know, the administrator of the system may change the permissions on your bundle to be tighter.

For example, the ownership may be root.

And if your application attempts to write into that bundle, that is going to break.

/Applications/Utilities unless you are writing and you're are installing a system utility.

If you're writing a system utility you don't want to be modifying things in that directory at runtime, only at install time.

/Library and its sub-directories.

Now, that's a huge list but there're a couple of common culprits that we see people write to all the time.

One is /Library/Application Support and the other is /Library/Preferences.

But as I mentioned there's quite a few others.

So there are several reasons why you might write to one of those directories and they're not always obvious.

So three of the most common problems that we see that prevent applications from running properly in a reduced privilege environment are registration, things like serial numbers and license keys are installed in a global directory at the first run.

And so if the person running your application is a standard user oops, that just broke.

What you should do instead is if your you're going to ask for a serial number or license key during your application's launch.

You should do it at the install time because your installer has privilege and then you can save that file and then every run of the application can simply consult it and make sure that it's there.

Second, things like global preferences.

A lot of applications rely on /Library/Preferences, for example, being writable as I mentioned previously.

And this will break you know, if you're running as a non-administrator.

So if you have a preference that you truly want to have in effect for all users of the system and you want standard users to be able to use your application, you should have a background job and you should protect it with authorization as necessary.

Again, you can use launchd in the service management APIs to facilitate that.

Finally, and this is the most complicated one, is the case of a custom installer.

We'll see an application that you just double click and run and it throws a bunch of files around somewhere.

And most of the time that works because the user is running as an administrator but it doesn't always.

So what you want to do if you're going to write an installer.

The first thing we recommend that you do, obviously, is use PackageMaker and the Apple Installer or one of the third party installers that's already available.

But if you need to write a custom installer and some people do, you can also have your custom installer UI use the installer command under the hood, basically, making your installer UI around the package system that's already there.

And finally, if you can't do that, you can run your you can run your installer as a launchd job and then remove it when the install completes.

Now, that's a little bit hackie but it will work for you and it will get around the privilege issue and allow your application to run properly in a reduced privilege environment which is really important.

Next, I'm going to talk a little bit about avoiding setuid.

And setuid is a security curse because it's an attacker's dream.

So the attacker controls tons of things that are input to a setuid program file descriptors, the environment, and so on.

And because of the way setuid works, there can be bugs in your code that can be exploited by an attacker or there can be bugs in third party code or even our code that you expose by being setuid.

A great many of the system frameworks for example, don't expect to be loaded into a setuid binary.

You don't want to write self-repairing tools.

We've seen self-repairing tools written many, many times.

Hey, if I don't have the setuid bit AuthorizationExecuteWithPrivileges and then I'll restore the setuid bit.

That's great.

Right? No.

Please don't do it.

A local user can alter your self-repairing binary because usually if the setuid bit is gone, the permissions are wrong.

And once the local user can do that, your self-repair will happily set the setuid bit on that modified binary and elevate the malicious code to root the next time it runs.

It is great for an attacker.

Not so great for the user.

So what you want to do instead of self-repairing tool is you want to have an installer package and you want to have that installer package set the setuid bit on your file at install time requesting RootAuthorization to do so.

So that it does not subvert the privilege model.

Now, to give you an idea of why we want you to avoid setuid, we have a little diagram here.

Now, to user a setuid is very simple interaction.

Hey, I run a tool.

It does something.

I don't even really know what it does but it works.

So you might not even realize that you're running such a tool.

Now to an attacker, it's a lot more interesting.

An attacker has control of I/O and your setiud tool, command-line arguments, environment variables, the working directory, file descriptors, the file mode mask, the emask, interval timers, signal mask, mach ports, and there's more that I couldn't fit the diagram.

So, you know, it is a very complicated interaction.

You don't want to do it.

So finally, and perhaps the most important point of the security design discussion for a network task and particularly those of you developing on iPhone is to protect data in transit.

You have to assume users of applications are mobile, as I mentioned before.

You know, MacBooks, iPhones, iPod touches, iPads.

The days where users are sitting on a desktop, they're not quite long gone yet but they're on the way out the door.

So you have to be suspicious of things like the Domain Name System and the local network in general.

Now, you know, if I am sitting at Starbucks and I'm running your application, you know, I think, I think I am pretty safe.

You know, I am on a network that I know and, you know, I am sipping my latte and having great, great time.

But what's bad about those networks is that they can be traffic on those networks can be monitored and can be tampered with by third party from the network relatively easily and there are a lot of third parties on those networks.

So what you want to do is any kind of sensitive data that your application is transmitting or receiving protects it with SSL.

Now, there are two really easy ways to do that.

One is NSURLConnection with an https: URL and the other is CFReadStream with the SSL extensions.

Now, here is something you don't want to do if you're protecting data in transit.

Don't disable chain validation.

Now, that's really important because if you disable chain validation, you've basically told the certificate system, "Hey, any certificate is acceptable here.

I don't care where it came from, anything."

So I can get a certificate, present it to your application and the system will say, "Sure, OK."

So here's what that looks like.

I wanted to give you an example because I've seen it recommended a lot and people don't call it disabling chain validation but that's what it is.

And so if you see code the sets this constant a kCFStreamSSLValidatesCertificateChain, yes I know it is a mouthful, to false, no.

You need to rethink the design angle there.

Now, the one case that you may want to do this is if you are expecting to use a self sign-certificate for example.

But you have to validate specifically that the certificate you've been presented is the one you expected and that is a very complicated process.

But you cannot simply disable chain validation and plug and go.

You will have introduced a vulnerability and you'll have effectively neutered SSL.

It's also important you know, this isn't necessarily sensitive data per se, but it's integrity critical.

So if you have code or other content that your application will download on demand such as updates, it's really important that you sign that stuff so that the application can verify, "Hey, this hasn't been manipulated in transit."

It's very important when you're doing code signing that you verify the signing certificate, because this poses a similar problem to chain validation in SSL.

Anyone can get a code signing certificate and say, "Hey, this code is signed by me."

I can get a certificate that says, "Hey, Matthew Murphy signed this code."

And your application will accept it unless you explicitly validate that you've received a signature from a particular certificate.

You don't want me signing out on packages for your applications.

Trust me. So now we have talked a little bit about, you know, designing your application securely.

So there are some coding tips that I wanted to cover next.

And we're going to talk about a few different things in the coding section.

But again, highly recommend reading the Secure Coding Guide.

I really like it.

It's great stuff.

First, let's talk a little bit about safe file handling.

Now the biggest part of safe file handling is using a safe temporary or cache file directory.

Now what do I mean by safe?

Well, you can get them from confstr or NSTemporaryDirectory.

Those are going to be good safe directories for your application to use.

Now what's an unsafe directory?

It's a world-writable directory, something like say, /tmp or /Library/Caches where other users can dump files and can potentially interfere with your applications expectations.

If you have to use these directories, these world-writable directoriesand be very, very careful it's important to note that the higher level APIs like the writeToFile method on NSString and NSDictionary and NSFileManager, those are not safe to use in a world-writable directory.

You'll introduce race conditions.

So what you have to do is you have to use a lower level open API.

It's also important to note that you can't open existing files in a world-writable directory in a security critical way.

You have to create those files yourself to ensure that you're manipulating the file you expect.

And you have to use the exclusive flag I've highlighted on the slide here to ensure that you don't follow links for example.

Next let's cover a little bit about permissions.

Now on the Mac like most Unix-like platforms, files are actually world-readable by default.

Now this stuns some people but the most important thing to note here is that the directory structure is largely what controls what controls access to a file.

If you can't execute through the parent directory of a particular file, you can't see the file much less read it.

However, world-read permissions are not appropriate for everything.

So if you have a file where world-read permissions aren't appropriate or simply aren't necessary, we encourage you to set tighter permissions when you're creating a file.

Please, please, please, please, please, avoid creating world-writable files.

Don't do it.

The reason why you don't want to do it is because it's subject to race conditions.

Your file can be deleted and replaced for example.

It can be substituted with a link and your program will follow the link and so on and so forth.

Also, an unprivileged user can simply destroy the file if they want to, which is not good for your application stability.

What you want to do instead, again, you want to have a background service that your application can make calls to when it runs as any user that will edit a file under the hood and you don't want to have that file be world-writable.

Next, I'm going to talk a little bit about bounds checking.

So bounds checking is a classic defensive programming technique and it's designed to stop buffer overflow vulnerabilities.

Buffer overflow is a bug where you have a piece of data that's too large for the memory buffer you've allocated into it and your application attempts to copy it anyway.

So you have to perform a lot of sanity checks on any kind of untrusted or unvalidated input, things that come off the network from files and so on.

And you have to use the safe string functions.

The safe string functions are listed here for you, things like strlcat, strlcpy, snprintf and vsnprintf, and fgets.

Also the functions that we recommend you avoid using are on the left under the X here, strcat, strcpy.

Now you may be surprised to see strncat and strncpy on that list.

Gee, I thought those were bounded functions and they are but they're bounded in such a way that makes it really easy to introduce security bugs and we'll show you that on the next slide.

But also on the list are obvious suspects like sprintf and vsprintf, and of course the gets function, which is the source of one of the oldest compiler warnings in the book for security.

Now to give you an example of what I mean and why we don't recommend you use this strncpy and strncat functions, consider this example.

Here we have a 5 byte destination buffer and a 6 byte string.

Obviously, if we copy the entire string into the destination buffer, we're going to cause an overflow.

This is not good.

So here we have three prospective copies that we can make.

Of this source string into this destination buffer one, using each function.

First let's look at strcpy.

Notice how it copies right on pass the end of the buffer.

So you've just corrupted memory and potentially allowed an attacker to execute arbitrary code.

Not good. Now let's look at strncpy and see what it does.

Notice how it copies 5 bytes but the last byte is the E.

There's no null terminator.

That's not good either because the next time you manipulate that string, you're going to get a read right off the end of it.

And you may even introduce an exploitable security hole.

Now let's look at what strlcpy does.

You can see it also copies only 5 bytes, but the last byte is the null terminator so that you end up with a properly terminated string and this is why we recommend that you use strlcpy instead of strncpy and strcat and strlcat.

Next let's cover a little bit about integer overflows.

Now, an integer overflow occurs when an arithmetic operation produces a value that's larger than an integer type can hold.

Now that's a big complicated dictionary definition.

So let's explain it with a little bit of code.

Here, we have a structure that has an N entries value in it, that is potentially hostile.

We'll say it came from the network.

And then we have an NSData initializer which says dataWithLength, N entries times, you know, this size up here which is a constant.

Now if that N entries is really, really large that integer will overflow and the allocation will be unexpectedly small then the memcpy down here which is done in a loop will copy right on past the end of the allocated buffer.

To show you exactly why that happens, here's an example of what we get if we specify a particular large value.

And you can see we have an editor here that has 33 significant bits.

So the 1 that would be at the high end of that integer simply gets truncated away and we end up with a four by allocation and into which we're copying tens of millions of entries.

That's not good.

So what we recommend that you do to avoid integer overflows is use our checkint API which has been available since Leopard.

So you can see the code now includes the checkint.h but the meat of it is down here where we have this check_uint32_mul function which will set an error condition bit in the error value if the multiplication results in an overflow.

Now you can see it says CHECKINT_NO_ERROR here so we can succeed.

Now we know this multiplication was safe.

So that's integer overflows.

Now I can give you a little bit of background on how to test your application through security.

Now, there're obviously two components of testing.

One of which is manual auditing and David will cover a little bit more about manual auditing in his section of the talk.

But I want to give you some useful automated testing tools.

So we're going to cover two different type of tools.

We're going to cover static analysis and we're going to cover fuzzing.

So the Developer Tools include a static analyzer for you.

You can run it with Build and Analyze menu item in Xcode.

Checks your code for some really common bugs, memory management issues, things like reference counting, that sort of thing.

A small subset of buffer overflows, it won't catch them all, which is important and some non-security bugs, things like dead stores and so on.

There's really detailed warnings when it finds a bug that help you document the data flow through your application.

The rules aren't very detailed yet, but we're improving them with each updates the Developer Tools.

So you can see an example here of where the analyzer has found a bug.

I have an NSString value that I don't initialize and I pass to NSLog and then I built my code with Build and Analyze.

So Build and Analyze has flagged this and said, "Hey, you declared this but didn't initialize it and then you passed it to a by value function.

Now this example is obviously pretty contrive but the analyzer can catch some more details and more sophisticated example of this same type of bug.

So the important thing about the analyzer is that we recommend you use it often.

If you run it all the time, you're going to catch any new bugs that you introduced, at least bugs that the analyzer, again, has rules to catch.

So that's important.

Also, we keep adding rules.

So even if your code isn't changing.

It's important that you run this periodically to get the benefit of the new rules.

There's a project configuration option that you can use that will let you run the analyzer with every build of your project.

And you can see that here in this screen shot.

In the build options, there's a pretty self explanatory option called the Runs Static Analyzer and you can see I have checked it in my project which has made it turn fold because it's non-default.

But when I build my project now, the analyzer will run automatically.

Other important class of automated testing for you to be doing with your software is fuzzing.

So fuzzing is where you subtly alter valid program inputs.

If I have data that comes from a file or from the network, great candidates for fuzzing.

It doesn't have to complicated, change a bit here, right there, a couple of bytes there, you've got a fuzz file that could potentially cause havoc in your application.

And as you might guess, the most likely thing that your application is going to do if you find a bug is crash.

So if it crashes, there you go, you found a bug.

And our CrashWrangler tool can help you prioritize to determine which of those bugs are important for you or most important for you to fix.

So you can run CrashWrangler with either crash logs or with a live target and that is an application, it's running and it's being hit with data.

And it has a heuristic for identifying exploitable bugs things like out of bounds rights, double freeze, so on so forth, from nonexploitable simple crashes.

Things like null pointer to your references and the like.

And you can download it from the Apple Developer website at connect.apple.com you just search for CrashWrangler.

And it's available for you to use today.

So we've covered three of the four pieces of the security life cycle.

We've covered design, we've covered code and we've covered testing.

And now I want to hand it over to David for a little more detail on practical applications of these things and what you can do with Cocoa and Objective-C specifically.

[ Applause ]

Hi I'm David Remahl and I'm a product security engineer.

I think it will be instructive to try to apply some of the techniques that Matt has talked about to a real world application.

And the application we'll be talking about today is called Naivete.

It's a magical and revolutionary feed reader for Atom feeds.

It supports some of the Atom feeds on the web, it's not perfect yet, it's a version 1.0.

It has a ground breaking feature that's an industry first, it's document-based using the Cocoa document architecture.

It opens this new URL format that we've invented naive:.

It has a gorgeous icon and if it crashes that's a feature not a bug.

Now I don't anticipate that the App Store reviewers will take that excuse.

And that's why we've put this program app on the sample code sites for WWDC as an example of code not to write.

So now I want to demo some of the features of this application.

Let's start it here from the dark.

It opens up a blank document, and it has these bookmarks pop-up where we can for example load the hot news feed from apple.com.

Here we go, it's a simple RSS reader or Atom in this case.

You select the stories in the source list here.

It even supports sorting on the dates or the title and if you hit a link it will open it up in Safari.

I also prepared a feature feed that demonstrates some other unique features of this application.

It supports HTML5 and the proof of that is this.

[ Laughter ]

If you load an Atom feed that contains a podcast file it supports downloading that so if I hit download here you'll see the podcast file arrived in my Downloads folder and that works for pictures as well.

Oh, I forgot to show you the document support.

If I save this [ Pause ]

Here's the file, if we close this you'll be right back at this feed.

Now let's go back to the slides and think about the security of this application.

So as we're going through this part of the presentation we'll be filling out this security scorecard for the application.

I'll be returning to this periodically and will see how it fairs.

It's important when you're developing an application as early as possible, at the design phase to understand the attacks and the environment that your application will be exposed to.

And we call this the attack surface.

And one way of understanding the attack surface is to enumerate the entry points to the application.

In this case we have the naive: URLs.

If I click on a naive link in Safari, it will open up that feed in the application.

Documents, users love to share documents among each other so that's a way that an attacker might attack the user.

Feeds of course are downloaded from the web and could potentially be used in attacks against the application.

And finally in this case, we support enclosures.

In order to understand the attacks that might be used against us, one way is to enumerate the APIs that we use.

In this case, the stories are loaded in the WebView.

It's you need to know that a WebView that the entire security modeling of the WebView is based on the document origin.

Since this deals with HTML and other web formats, we need to be concerned with cross-site scripting.

This is normally an issue that web application developers or browser vendors are tasked to solve.

But when we make connected applications that embed WebViews for either for internal use or for viewing web contents, then cross-site scripting becomes important to desktop or iPhone iOS application developers.

External links, we, of course, need to be careful when we're dealing with those.

And we could fill an entire session on WebKit security.

So this will be some of the things we focus on today.

URL handlers naturally have to be written to expect some malicious input and do careful input validation.

And we also need for the documents, we need to think about the serialization format and make sure that that's aptly chosen to be secure.

And as Matt has often referred you to the Secure Coding Guide, this is where you will find the source of security concentrations for the APIs that you are using.

So let's go back to the application and see how it fares against some of these attacks.

I've prepared an attack feed here that tests some of the attacks we were talking about.

So as you've seen, the application handles links from feed contents.

But what if someone would try to link to an application on the local disk?

I've prepared this item here, and when I click this link, we will see what happens.

Uh-oh. That launched the application, and that's probably not something we intended in this application because we wanted it to open up in Safari.

So we've found a bug.

Let's go to the code and fix that.

So when the user clicks on a link in the WebView, this Web Policy Delegate gets evoked.

And we see here that if the source of the navigation was a user click, then we do this.

We use NSWorkspace openURL to open the URL of the request.

And if you read the NSWorkspace documentation, you'll see that openURL has a dual purpose.

One is to open safe URLs like as HTTP or AFP.

And another is to open local files.

So it's one of the recommended APIs for opening up a local document.

In this case, we only want the safe case, and we want to handle the file URLs separately.

So I've written some code here that addresses this vulnerability.

We check whether the scheme of the URL is equal to the file scheme.

And if so, we select the file and the finder instead of opening it up through NSWorkspace.

So we build and run this.

[ Pause ]

Attacks, and now, the file gets selected instead, which is what we want.

There is a bug still in this code.

I don't know if you spotted it, but here's a hint.

Someone saw it?

So if the scheme starts with all capital letters, then we still have a bug.

And we could solve this by doing a case insensitive comparison.

But a better way is to use the NSURL API as filed URL, which handles all these cases, regardless of capitalization or future changes or whatever.

It will do the right thing.

[ Pause ]

Now we handle that case correctly too.

I mentioned JavaScript injection and the importance of the origin.

In this case, I've wrote a great script that tries to first print the origin of the document, and we see that it's served from an applewebdata URL.

And then it tries to get the EDC password database or any other local file.

And since applewebdata live file is handled especially by WebKit, it was allowed to get this database.

And it could, potentially upload it to the web.

In order to fix this we need to do something about the origin.

[ Pause ]

So we're using this API loadHTMLString passing in the feed contents, and then we pass and analyze the base URL.

What we want to do is to pass in the URL of the original feed, and that's available as this property on the document.

So now, if we try this exploit again.

[ Pause ]

The origin is now an http: URL, and it was no longer able to access the user database, which is what we want.

The fourth attack I want to talk about now is against enclosures.

So I'm sure you've seen this feature in Safari where it tags files with the download source.

So when I downloaded this file in Safari, this is a shell script, and when I start it, the user gets this warning.

So if he wasn't expecting a shell script, maybe an image or something, then he is alerted to that something, something is strange.

Let's see if we get the same support from Naivete.

Downloading closure [ Pause ]

Running arbitrary code.

Yeah, so [ Laughter ]

This shell script just ran without warning the user at all.

Fortunately, the face is very simple.

You just go into the info.plist and check the file quarantine flag.

[ Pause ]

So, there is the warning.

If every bug was this easy to fix, we'd all be out of work.

[ Laughter ]

So let's go back to the slides and look at the Score Card.

Well, we had a fail for the Cross-Site Scripting initially.

Local URLs, both of the applewebdata variety and the file variety, were mishandled, and it didn't provide adequate protections against Trojans.

File URLs are special, both to the WebView and to NSWorkspace.

And the lesson of this is partially to be aware of that fact, but to read the document API documentation of every API you are using and understand the security consequences of the documentation.

And finally, if your application downloads files, you should be using file quarantine.

And the simple info.plist fix provides most of the security benefits, and it's very easy to turn on.

But if you want to provide the user with even more information, such as the source of the file, then you can use the Launch Services API on the slide.

Next talk let's talk about file formats.

You can consider file formats and documents from two levels.

First, there's the semantic content, which is the information that you want to encode in your document.

And then there's the serialization format, which describes how this semantic contents match the bytes on disk.

So what signifies a secure serialization format?

First of all, we want it to be simple and predictable because security issues tend to thrive in complexity.

We want the attack surface to be small, and that means that we don't want we want as few lines of code to be running basically, because there will be fewer lines to have bugs in them.

And we want to be sure that the format parser was written to expect malicious input, or potentially malicious input, and do the appropriate input audition.

Now, I want to look at the format path that Naivete uses.

So we already saved this features file, and I happen to know that this is a property list.

So we can view it in the Property List Editor.

This first line is a cue is a clue to what format the application is using.

It's says NSKeyedArchiver, and that means that it's a Cocoa Archive using NSKeyedArchiving.

Keyed archiving is a relatively complicated format that can encode an arbitrary graph of objects.

It's very easy to use.

You just pass it the root object that you want to encode.

But it has some curious properties that makes it unsuitable for use as a general purpose document format.

For example, here, we see the NSURL string, and you might wonder, "What happens if I change this to some other class?"

Let's use something that the application will not expect.

So this means that any object that implements initWithCoder, the NS coding protocol, will be exposed to potentially malicious data coming from your document.

And we can see what happens if we open up this document that I prepared that replaces the URL with a button.

In this case, we just got some warnings about an unrecognized selective being sent to the object, because the code was expecting an NSURL.

It found an NSButton.

This can be get worse, and I'm just experienced log message a document that doesn't load.

And I won't show you the details on how this works, but this document managed to start an application that could have run any code.

So the fix we want to apply in this case, is to replace the serialization format.

And it started out looking pretty good.

This was a Property List.

And Property Lists are fine to use for untrusted input.

The problem was that it was a keyed archive.

So we will replace this document serialization code and loading code with [ Pause ]

With some code that uses just a plain property list instead.

[ Pause ]

So if we open this up in Property List Editor, we'll see a much simpler hierarchy here, encoding just the information that we wanted to contain.

And there are no class names to replace, for example.

So for the document serialization, it was another X because it allowed arbitrary code execution from a document.

So, some of the formats that are safe to use, if you use them correctly, are XML Property List, Binary Property Lists, and spotless serialization basically.

There is NSXML which is made to deal with untrusted XML, and Core Data is also appropriate as a general purpose document format.

So you should use these for your document formats, your natural protocols, and other cases when you share data across privileged boundaries.

There are some things you shouldn't use, and we've already talked about the NSArchiver and NSKeyedArchiver.

And if you're using this class, it's way overdue to be replaced because it was deprecated in 10.2.

These APIs, with the exception on NSSerialization, might still be appropriate to use in your internal storage, like preference files, or if you're doing something similar to Interface Builder with frozen code, or if you have an IPC connection where you're passing serializing data to pass across a connection where there's no real trust boundary.

Now, let's move on to the code part of the development cycle.

And connect cleanups in here.

So first, we'll use the Build and Analyze menu item to see if the static analyzer will catch any bugs for free for us.

So we got one warning and one static analyzer warning.

Let's start with the warning.

In this case, we're calling NSLog in the handler for the naivete: URL format.

And we're taking the URL and passing it as the first argument to NSLog.

NSLog takes a format string.

And format strings are some of the oldest and most obnoxious security bugs out there.

In C, they are usually exploited using the %n specifier.

Cocoa does not support %n.

But what it does support is the %at operator which sends a message.

So first, I'll show an example of how this bug can trigger.

[ Pause ]

Actually, I will show you that later on a different bug.

The problem with sending messages to an object off of the stack when we are not passing any object beyond the first parameter here, so it will just take some random value off the stacks, send as message, and when Objective-C message send picks up that pointer, it does a series of pointer references, and finally arrives at the IMP which is a function pointer.

And if the attacker is able to control any of these pointers along that path that Objective-C message send traverses, then he might be able to point execution to his code, which would obviously be bad.

The fix is very simple.

We just use a static string as format specifier and pass the document the object that we want to print.

That is the second one.

Or in this case, it is just debug codes so we can remove it.

Remove your debug code before shipping.

The other warning we need to run the analyzer again.

Here's the analyzer warning.

We get an error on the return line here.

It says that an object had order release sent too many times, and of course, this is a stupid bug.

I called order release on an object that was returned already, or released.

Fix is very simple.

As Matt mentioned, the static analyzer will catch a lot, but it definitely would not catch everything.

Actually, there is another format string vulnerability on this line, and that is what I wanted to show you a minute ago.

When I load this format string URL, it contains some %x specifiers that's translated into random data off the sack in this very dialog when we are using NSAlert.

And as an example, when that causes a potential security vulnerability, if we are using this %at specifier, yep, we got a crash, and the debugger has broken.

That's bad.

The fix though is as simple.

Just like that.

The order release case was a reference counting mistake.

And reference counting mistakes can become pretty hairy.

And one example of that is when I'm using NSURLConnection in this document, I set the document as the delegates.

And as you will know, if you have read NSURLConnections documentation, the connection retains its delegate.

So even if the user closes the document, the object if the URLConnection finishes or causes an error, the object will still be around to get that message.

So we should be OK.

So let's see what happens if we try to connect to a fake web server here that will just hang.

We have the spinner here showing that something is happening.

The user gets impatient, closes the window, and then this connection closes.

Ah,we crashed.

Why did we crash?

The document is still around.

And indeed, the document did get the message about the connection failing.

The problem is that when the window was closed, the any IBOutlets to that window got invalid, because the IBOutlets are typically unless you have retained them specifically they're weak references.

So when we're sending a message to this release progress indicator, we get a bug.

And again, if the attacker is able to fill the address space with malicious data, he might be able to highjack execution here.

The solution in this case is too complicated to show in a demo, but it involves making sure that the connection gets canceled before any referenced objects get released.

And as I mentioned, this code is all up on the [inaudible] site of sample code.

So, there are some vulnerabilities that I won't be able to show you today but if you're curious, you can look at them in the sample code.

So, format strings and reference counting, another X there.

Static analyzing your code is a great aid.

It will find some bugs but it definitely won't catch everything.

So you need to stay vigilant and make sure that you code correctly.

Be careful with format strings.

It's very easy to whenever you don't pass a static string as a format string, it's likely that there might be a vulnerability there.

And the complier will give you warnings in most cases but not all format string methods and functions are annotated.

And reference counting and weak references are hard.

It's easy to get mixed up, especially around the edge cases like in this case when the download hang and the user close the document.

Using Garbage Collection on the Mac might help.

On the other hand, it might introduce other edge cases and it's not available on the phone.

Next apply let's play some fuzzing techniques that Matt talked about.

It's a very simple and effective technique and I wrote a fuzzer for a general in generic fuzzer for binary and XML property list in less than an hour.

It's about a hundred lines of code really simple Python.

It enumerates a plist and at each element, it does the number of permutations to it to try to confuse the application that will later be reading on the file.

And I won't be doing that now but you can run it with CrashWrangler in order to find duplicate crashes or to determine whether they are likely to be exploitable or not.

So, I'll just quickly run this fuzzer against one of our documents that we've created.

This is also available at sample code for this session.

And it's easy to run.

So now you'll see that it created 20 files here that are all subtle variations of the original document.

And when I open them up, you'll see some perhaps unexpected behavior from we're still crashed.

[ Pause ]

[ Laughter ]

Yeah, so some of these were not expected and I think there are some log messages emitted about in other selectors being invoked.

So there are other vulnerabilities in this code and they're all listed in the sample code.

Fuzzing, spending an hour fuzzing this application would have eliminated a number of security vulnerabilities.

So, fuzzing is important and should be part of your testing strategy and use multiple fuzzers because they excel at different things.

This one just generated a lot of valid XML, valid properties XML but you will want to try some binary fuzzers and you want to try a fuzzer that focuses on boundary values and maybe one that just does random permutations.

And if you are developing a protocol, then you might have a fuzzer that does that has a deep understanding of the protocol and another one that just does dumb manipulation of the traffic.

You shouldn't only rely on fuzzing, of course. You should also be writing unit tests and focus on the edge cases.

Not the ones that the user will most often hit but the ones that an attacker will focus on.

Penetration testing, which is pretty much what we started this part of the talk doing.

Trying some different attacks and seeing how the application responds to them.

Even if you think you've mitigated the vulnerability it doesn't hurt to verify it.

So, in summary you should think about security throughout the entire development process from design to code, to testing, and then post release.

Be aware of the security properties of the API CUs.

Read the Secure Coding Guide with the API DOCs, talk to others and have a deep understanding of the APIs.

Understand the attacks that affect your application space.

So, in this case we were dealing with a feed reader and the bugs we were looking at had a lot to do with cross-site scripting, URLs, input validation.

Your application will be different.

It might be running on different devices.

It might be in a completely different problem space and it's up to you to have a deep understanding of that space.

And take advantage of the hardening techniques and security APIs like key chained and content protection on the phone because why not?

It might save your users from being exploited.

What are some simple steps that you can go out and do today?

First of all, you can visit the Dev Forum Security sections.

There's a lot of helpful people there that will answers your questions.

Read the Secure Coding Guide if you haven't already and run the static analyzer against your application.

You might find one bug, you might find 20.

It's definitely worth the time investment.

And finally fuzz your application.

Use one of the commercially available fuzzers or write your own, it doesn't take a lot of work.

And that will put you on the path to giving your application a clean security score card.

Here are some other sessions that might be interesting to you for launchd or if you're writing network applications specifically for iOS.

And there's the iPhone securing your application data session which might be interesting to you.

Thank you.

[ Applause ]

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