Apple has a unique identifier called the Identifier For Vendor (IDFV) discussed in this developers guide.
http://possiblemobile.com/2013/04/unique-identifiers/
I was thinking to would be possible to use with a Restful API so only requests that have a valid IDFV are accepted. But here is the problem I don't know any way to verify the IDFV. I could make it so when the app is installed it registers with my service. But ideally there would be a way I can verify this identifier.
I just had a brain storm on this: It's not all that simple, but what if you have an In-App Purchase for the app where that IAP is on a free tier. Have to investigate this more, but I think you can have a Non-Consumable purchase type that is free. The client can then send the purchase receipt (which is encrypted, as of iOS7) to the server side and the server side can then validate this receipt-- Apple does provide a RESTful means of doing this. Thoughts? Of course, you can also make the IAP have some > 0 price. This would give the app developer the means to add a payment mechanism.
I am not sure what your problem is. The app grabs the identifierForVendor and sends it to your server. What else can happen? You could certainly code the transmission so it also sent a hash of the identifierForVendor and some secret string (like "my secret 25671566") to test that the transmission was coming from your app.
You should also know that 1) the identifierForVendor will change if the app is deleted and reinstalled. 2) the keychain might solve the problem of #1 3) The app has a receipt, all apps do whether or not they have purchased an IAP. The receipt is encoded using the identifierForVendor. You could send the receipt from the device along with the purported identifierForVendor and see if it can be decoded. But see paragraph 1 above.
Related
In the SKReceiptRefreshRequest docs it says:
Use this API to request a new receipt if the receipt is invalid or missing.
When would the receipt be missing or invalid?
I found an answer that says apps downloaded from the App Store come with a receipt. Is this correct? And if so, does this receipt contain all the IAPs the user made on other devices?
Debug builds and apps installed via TestFlight do not necessarily contain a receipt.
The receipt can be somewhat invalid when the user made a purchase on another device or gained access via family sharing.
We have a successful app on the iOS app store with in-app purchases. Every time a purchase is completed we send the receipt to our server, our server than checks the receipt with Apple's servers and logs apple's response(including whether the purchase was valid and that they come from our app in that same time and date).
We have quite a few users who use iap cracks that send us receipts that apple says are invalid. However we started now to see cheaters that have receipts that apple replies that are VALID. What is strange in these cheats, that when such a cheater user purchases in our app, he usually purchases all of the purchases with the exact same receipt.
Have you heard of such a way to 'fool' apple receipt validation?(to generate receipts that apple will say they are from our app in the time of the 'purchase')
Is there something we can do to find those cheaters in their 1st purchase (for the next purchases we can simply check times of the next receipts or make sure that our receipts are unique)
Thanks!
Is there something we can do to find those cheaters in their 1st purchase
Actually, if this is the same hack I've seen discussed as a proof of concept recently, the first purchase is legitimate. The "innovation" is in decoding that legitimate receipt and rejigging its IAP ID with a different one while the receipt overall still appears valid. So simply avoiding the duplicates is enough. Didn't think that one was anywhere near production-ready though, so this might be something different.
We also faced similar issue while developing a game of iOS app store where business model was based on In App Purchase only.
Initially we used to check with Apple Servers for the receipts directly from the device. But some hacker has created a hack for the users where they can install the DNS server certificate on their device which spoofs the response from Apple.
The way to do this is let web server check for the receipts from Apple directly with some kind of hashing or md5 check to make sure the response if from Apple.
here is a link which have a detailed information on this https://www.objc.io/issues/17-security/receipt-validation/
Hope this helps.
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.
A client application successfully performs an in-app-purchase and receives a receipt from iTunes. The client then sends this receipt to the server, which verifies it with Apple and, if successful, unlocks some content for the client.
Is it possible to insert extra data in the IAP purchase request such that the receipt or the receipt validation contains this value?
I'm trying to prevent a man-in-the-middle/packet-sniffing type hack i.e. If someone was to discover another person's receipt and validate it against my server, how is the server to know that this receipt wasn't generated by this, the validating person?
From Apple's IAP documentation there are a number of values returns in the validation response. Is it possible that I could use version_external_identifier value? What is it's use or value?
Thanks
version_external_identifier
An arbitrary number that uniquely identifies a revision of your application. This key is missing in receipts created by the sandbox.
That really answers your question; its a way to determine what revision of your application made the purchase. You can have the same version number, but multiple revisions, and this allows for that and IAP.
Is it possible to insert extra data in the IAP purchase request such that the receipt or the receipt validation contains this value?
Ultimately, no. Apple have control of the receipt, and its contents. If you follow the guidelines in the documentation (see this for more info along with the link you posted), you will be ok against a man in the middle attack (Apple would have considered this). Make sure connections to your server and to Apple are HTTPS.
A man in the middle to make a purchase on someones behalf is slightly moot anyway; the purchase request is tied to an Apple ID, meaning the man-in-the-middle would need to know their credentials.
First of all, I am not talking about calling https://buy.itunes.apple.com/verifyReceipt/, instead, what I am asking is how to verify an iTuneStore receipt from one of our users.
We have in-app purchase in our app, and we verify in-game receipts on our server before we grant the product. However, this particular user claimed that he got charged by iTunes store but did not get the purchased products, and sent us his receipt as a proof.
So I am wondering if there is anyway we can verify such a receipt. There is "receipt no" and "order number" in the iTunes Store receipt, however, with in-game receipt verification we get transaction_id, and these numbers are quite different. Any idea?
Thanks in advance.
You should contact Apple's developer support. I doubt there is a way to verify it by yourself if you can't find a respective transaction in your own systems.
There is no manual way to do this, unless you can somehow get the receipt data from his device, in which case you can use Apple's API that you mentioned to verify his claims. If what he says is true then suggest him to claim a refund and then re-purchase. He should be able to do this here.
Now, the Lookup Order ID of App Store Server API could get a customer’s in-app purchases from a receipt using the order ID.
GET https://api.storekit.itunes.apple.com/inApps/v1/lookup/{orderId}
You could find the corresponding transactionId from this API response JWSTransactionDecodedPayload
For how to do the Lookup Order ID request, please refer to https://github.com/richzw/appstore#look-up-order-id
There is source code at https://github.com/roddi/ValidateStoreReceipt/blob/master/validatereceipt.m which will validate a receipt, and let you parse it and dump out all the transactions.
If the IAP transaction is there you would see it.
You'd have to write a special-purpose iOS app which uses that code and feed it the receipt. It seems like a lot of work.
I am not talking about calling https://buy.itunes.apple.com/verifyReceipt/
Actually u have online receipt verification tool, accessible by link above.
There is no better way to verify Your receipt. Offline verification is possible, but if even it succeeded, nobody can give 100% warranty that Your receipt is valid(maybe just same signature, but still fake one).
Apple processing every purchase on its servers and if it successful, receipt saved to database. If u want to check receipt u must access to that database and request for Your receipt. So You have a tool, but don't want to use it? No reliable way then.