Attack Protection for iOS In-App Purchases

Apple's iOS in-app purchase system has been attacked in the past by people who have tricked apps into giving them content for free. They have since improved the systems involved to try to limit this kind of thing.

I've read through the StoreKit reference documents available from Apple and I have a general idea of the workflow and the checks that need to be done, and so on. However, there may be security issues that I'm not aware of.

Can anyone provide a full list of theft-attacks that can be attempted against In-App purchase mechanisms, how developers might mistakenly allow these attacks, and what are best practices for preventing them?

These are the attacks that I am aware of, past and present:

Fake App Store

Made famous by the Russian programmer Alexey Borodin, this attack only affects apps that verify purchase receipts directly with the App Store. By modifying the DNS settings of the device, and installing forged security certificates, the verification requests are sent to a fake App Store server, which automatically returns that the purchase is valid. Unsuspecting apps will accept these verification calls and deliver the content to the user.

Commentary

After this exploit was made known in July of 2012, Apple issued updated documentation and advice for developers to ensure this kind of attack would not continue to occur. Borodin has been quoted in various web articles as stating that the "game is over" based on Apple's updated APIs and best practices guidelines.

Prevention

Apple has an entire document dedicated to this loophole here. (EDIT: Link is down, Wayback if you want...although the document covered iOS 5.1 and earlier.) The biggest point they bring up is having your app send the receipt to an external server that you own, and then having your server verify the receipt with Apple. However, if you do send the receipt directly from the app to the App Store, they recommend the following checks:

Check that the SSL certificate used to connect to the App Store server is an EV certificate.
Check that the information returned from validation matches the information in the SKPayment object.
Check that the receipt has a valid signature.
Check that new transactions have a unique transaction ID.

Fake Verification Server

If your app sends the transaction receipts to your server, which then forwards them to the App Store, one option is for the attacker to fake your verification server. By some method (altering the DNS table, changing the URL, etc.) the receipt is sent to an alternate location and a "successful verification" is returned. In this way the receipt never reaches your server and you never have a chance to check it with the App Store.

Commentary

Apparently there are a variety of apps in the Cydia store that serve to run in the background, monitor receipt traffic, and redirect it for this purpose. Source: Hussulinux Blog

Prevention

If you immediately deliver content as soon as a receipt is verified, there is no known way to prevent this kind of attack. However, take this scenario: you have a user account system managed on your own server. If the purpose of the In-App Purchase is to notify your server that a particular user account has purchased an item, and the app downloads that item from your server, you are immune to the attack. Redirecting the receipt to another server will accomplish nothing for the attacker, because your server will never mark the user account as owning an item, as it never sees the receipt.

Fake Receipts

An attacker can fake the purchase process and then send a forged receipt to your verification server. Unlike the previous attack, the receipt's outbound location is not changed, but it is replaced with an imposter. This spoofed receipt is, in fact, a valid receipt from a previous App Store transaction, and the App Store will verify it as such. By faking the purchase process and then sending a spoofed receipt to your server, the content is never actually paid for.

Commentary

Apparently there are a variety of Cydia apps that do this sort of thing. You can spot fake receipts because their product_id is totally different from anything you use in your app. Apparently the most famous spoofed id is com.zeptolab.ctrbonus.superpower1. Source: Hussulinux Blog.

Prevention

In the link where I found this attack, the blogger recommended that you unpack the receipt at your verification server (base64_decode) and check the product_id before sending the receipt to the App Store. However, in this article Apple recommends that you first send the receipt to the App Store, and then read the returned information to be certain that the receipt is valid.

(Also, correct me if I'm wrong, but Apple's recommended technique could also be used to prevent this kind of attack even if you don't have a verification server. If your app sends the receipt directly to the App Store, it could examine the contents of the JSON response to ensure it's valid. But this goes against Apple's recommended best practices of using an external verification server, so I wouldn't advocate it.)

In Closing

These are the attacks that I'm aware of, feel free to correct me if I'm wrong on any point or to put forth additional attacks and fixes.

As a note, there's this website: http://www.in-appstore.com/ which claims to allow in-app purchases for free, either on iOS 5 or with a jailbroken iOS 6 device, and is active as of July 5th, 2013. While I'm not 100% sure how they are doing it, it definitely seems to involve DNS rerouting and faked security certificates, which would imply Fake App Store or Fake Verification Server, which would additionally imply that there are still apps out there that are not secured against these attacks.

Resources

Apple iOS in-app purchase hacking
My Experiences With Verifying In-App Purchase Receipts
How to detect "IAP Crackers"?

EDIT:

Additional

It seems like one or two people have swung by here and found this post useful, and I'm glad.

There's more information that can be had on this subject, either in other posts, books, or, if you're up to it, scouring the underbelly of the internet. Here's just a couple of websites and posts and so forth that I want to look into, but haven't had a chance yet. I'll add more links later when I find interesting tidbits.

http://www.se7ensins.com/forums/threads/tut-how-to-hack-ios-games-and-apps.701845/ http://www.iapphacks.com/

A couple of immediate takeaways: don't store your player's data in a simple plist unless you want it to be edited by some teenager. People don't have to hack your IAP system if they can just give themselves gold or something similar by editing the files on the disk. Perhaps by encrypting these files, you could discourage a certain segment of attackers.

Based on the se7ensins link, it seems as though an attacker can also pry apart your binary and mess with it to achieve the same ends as editing a plist file, or even more, but this will require a slightly higher skill level. Perhaps setting up jailbreak detection would suffice to deter most people who would resort to this.

Again, this section is mostly speculation, but it may help someone. Really, the level of protection you have depends on how far a developer is willing to go (down the rabbit hole of security and encryption) to protect their bottom line.