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I learn from somewhere dylib should be signed in iOS, otherwise dyld will refuse to execute it.

But when I test the example of this post. The dylib is not signed by default in this project, but it just running well when try to open it. I have verified the dylib's binary doesn't contain lc_code_signature.

Anyone can explain this?


I test on a jailbreak device with 3 cases for a dylib without lc_code_signature.

  1. Dynamic linked with the binary, so the dylib is list in LOAD COMMAND.
  2. Copy to the main bundle, and dlopen it.
  3. Copy to the Document dir and open it.

They all succeed.

  • well, jailbreaks usually disable signature checking completely so no wonder it works. – Igor Skochinsky Oct 24 '16 at 7:55
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EDIT: The below applies only to non-jailbroken devices. Jailbreaks usually disable signature checking completely to allow unsigned homebrew software to run.


Apparently only libraries which are listed as dependencies of the executable in the header (LC_LOAD_DYLIB and similar commands) are checked, and those loaded with dlopen are not.

This is mentioned indirectly in the Code Signing Guide:

As with other Gatekeeper functions, this check is performed the first time the app is run. It does not apply to libraries that the app loads itself using the dlopen function, although those libraries are still potentially subject to library validation, as described in Using Library Validation.

Following the link, we see this passage:

Starting in iOS 8 and macOS 10.10, the system offers library validation as a policy for the dynamic libraries that a process links against.

The policy is simple: A program may link against any library with the same team identifier in its code signature as the main executable, or with any Apple system library. Requests to link against other libraries are denied.

In iOS, an Apple system library is a library that Apple mastered into the OS image. In macOS, an Apple system library is a library shipped in /System/Library. The team identifier is the 10-character alphanumeric string, such as YH9SZ5LKR4, associated with your developer account, and recorded in your Apple-issued signing certificate. In iOS, library validation is always enabled for all apps. There is nothing you need to do to opt in.

So it would seem that dlopen should not be able to load unsigned libraries in iOS 8 and higher, though I have not checked this.

  • Thanks for you detail explain. I use iOS9 to test, and still can load unsigned binary. – Karl Oct 22 '16 at 9:19
  • @Karl: and you're not using a jailbroken device? In that case it seems either the doc is wrong, or there is a bug. – Igor Skochinsky Oct 22 '16 at 13:00

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