How does Apple know you are using private API?

Question

I submitted a binary file to Apple without any source code.

Apart from manually checking the source code how does Apple know what was used and what APIs you have called?

Solution 1

There are 3 ways I know. These are just some speculation, since I do not work in the Apple review team.

1. otool -L

This will list all libraries the app has linked to. Something clearly you should not use, like IOKit and WebKit can be detected by this.

2. nm -u

This will list all linked symbols. This can detect

• Undocumented C functions such as _UIImageWithName;
• Objective-C classes such as UIProgressHUD
• Ivars such as UITouch._phase (which could be the cause of rejection of Three20-based apps last few months.)

3. Listing Objective-C selectors, or strings

Objective-C selectors are stored in a special region of the binary, and therefore Apple could extract the content from there, and check if you've used some undocumented Objective-C methods, such as -[UIDevice setOrientation:].

Since selectors are independent from the class you're messaging, even if your custom class defines -setOrientation: irrelevant to UIDevice, there will be a possibility of being rejected.

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You could use Erica Sadun's APIKit to detect potential rejection due to (false alarms of) private APIs.

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(If you really really really really want to workaround these checks, you could use runtime features such as

• dlopen, dlsym
• objc_getClass, sel_registerName, objc_msgSend
• -valueForKey:; object_getInstanceVariable, object_getIvar, etc.

to get those private libraries, classes, methods and ivars. )

Solution 2

You can list the selectors in a Mach-O program using the following one-liner in Terminal:

otool -s __TEXT __objc_methname "$1" |expand -8 | cut -c17- | sed -n '3,$p' | perl -n -e 'print join("\n",split(/\x00/,scalar reverse (reverse unpack("(a4)*",pack("(H8)*",split(/\s/,$_))))))'

Solution 3

Let's say you want to use some private API; objective C allows you to construct any SEL from a string:

   SEL my_sel = NSSelectorFromString([NSString stringWithFormat:\
@"%@%@%@", "se","tOr","ientation:"]);
    [UIDevice performSelector:my_sel ...];

How could a robot or library scan catch this? They would have to catch this using some tool that monitors private accesses at runtime. Even if they constructed such a runtime tool, it is hard to catch because this call may be hidden in some rarely exercised path.

Solution 4

I imagine they look at all symbols your binary's trying to import (info no doubt easily available to them in the symbol table thereof) and ding you if any of those symbols are found in their "private API list". Pretty easy to automate, in fact.

Solution 5

otool -L somebinary

View more solutions

Comments

[Chance]

changed title - I suppose you meant "How does Apple know.."

[Sniggerfardimungus]

I've frequently wondered: if you were to write your binary to self-modify, only generating the code to import a private API after some criteria had been met (say, after the publication date of your app) whether Apple would catch it. They certainly report back to us some interesting statistics, like the number of our games that are being run on jailbroken phones.

[L̲̳o̲̳̳n̲̳̳g̲̳̳p̲̳o̲̳̳k̲̳̳e̲̳̳]

@user30997, The privileged code can probably only be accessed through a system call; the executing thread switches into a higher privilege and checks if the previous privilege has permissions to execute the code or not. That's just an example though, there are other ways of doing it, but I highly doubt the developers were naive enough to leave out a basic runtime privilege checking mechanism such as this, it would definately have been publicized by now.

[JUST MY correct OPINION]

Knowing Apple their review process upon encountering any uncertainty involves breaking out a set of dice for maximum whimsy.

[Matthew Frederick]

Great answer. I'll just add that if your application is doing something that is extremely difficult to do without using a private API, I'm sure your app gets extra scrutiny.

[an0]

I'm curious about the workaround of calling private methods. I think the compiler will generate call to objc_msgSend(foo, @selector(privateMethod)) for [foo privateMethod], so if Apple can detect the direct call of privateMethod they can also detect the indirect call via objc_msgSend(or performSelector:).

[hjaltij]

I'm wondering why you say that you shouldn't link against IOKit and WebKit?

[Rup]

user1203764 comments that these kind of calls can actually be detected

[jim.huang]

otool -ov just show the classes/selectors of the app it's self. Is there any way we can show the symbols it's linked to ? nm -u just show the objective c classes, i want show the objective-c selectors too.

[serverpunk]

What do you execute otool on? The .app file?

[Eric]

Couldn't apple just compile a version of iOS with guards and run your app on it ?

[Dan Rosenstark]

@Rup want to put an opinion on my question here about using valueForKey, please? stackoverflow.com/questions/11923597/…

[Rup]

@Yar interesting question! But I'm not enough of an expert to comment sorry. What Farcaller said seems reasonable to me

[Dan Rosenstark]

Thanks @Rup, no one apparently is enough of an expert in this field :)

[Nate]

Actually, I can say with certainty that this is not the case (at least it's not all they do), based on private API usage that I know has gotten through. If this is all that was needed, private API usage wouldn't slip through. My experience suggests that KennyTM's answer is almost certainly correct. This is an area where Objective-C is fundamentally different than other languages, like C.

[Nate]

@Eric, they could, although such an instrumented version would probably impact performance. Regardless, seeing private APIs get through into the App Store repeatedly, it's clear that they don't do this, or at least don't do it all the time.

[Eric]

If someone is interested, Erica Sadun's tool is now expired. You can fake it by using libfaketime (brew install libfaketime), and calling faketime '2008-12-12' ./apiscanner your.app

[Mangesh]

+1, @Robert Diamond, Can you describe more for the same. I need to check Google analytic uses UDID call or not. Thanks

[Daniel]

You mentioned that -valueForKey: should be safe to use. Can you comment on setting the value of a private key. Is -setValue:forKey: safe in your opinion?

[kennytm]

@Daniel: Should be the same.

[nomenas]

Based on these post I created tool which check if library uses some specific/private API. You can find this script in the following location github.com/nomenas/APIChecker. Developers who will discover more MacOS private APIs are welcomed to update this list with APIs in this repository.

[bugloaf]

Someone I know says he just got a call like this into the App Store.

[Motti Shneor]

Of course, the very use of NSSelectorFromString can raise suspicion on the Apple reviewer side, and if so - he can use other tools to reconstruct the original code (disassemble) and understand the original intent of the use.

[Motti Shneor]

That won't be enough, because program can only call this private call at an arbitrary time in the future, depending on other logic. To do this scrutiny Apple must actually block private APIs altogether, or have the frameworks report private API calls to Apple automatically - which significantly harms performance.