Accepted CDHash

A bit over a month ago, Apple announced big changes to the way Mac OS X versions 10.9.5 and 10.10 will recognize the code signatures of 3rd party applications, hinting very strongly that consequences would be dire for any developer neglecting to re-sign their apps:

Important: For your apps to run on updated versions of OS X they must be signed on OS X version 10.9 or later and thus have a version 2 signature.

Few details were given as to why the old signatures would no longer be respected, leaving developers with little to go on except to take Apple’s word that we should drop everything and update our code signing processes, which for some of us was a non-trivial amount of work. Those of us willing to grant Apple the benefit of the doubt assumed that there was some greater purpose to our collective suffering. Surely this nuisance is in the name of preserving or increasing security for all of Apple’s and our mutual customers.

That, and the fact that Apple stated bluntly that if we don’t make these changes, our apps will not pass Gatekeeper’s assessment and thus will not be allowed to launch without considerable work by users. This is an unacceptable user experience to any developer worth her or his salt, so of course the vast majority of us complied.

Some of us complied even when the logic of doing so was comically bent. For example Apple implied that developers of apps for the Mac App Store also needed to update their code signatures, in spite of the fact that all Mac App Store apps are signed by Apple, not by the original developers. The process of submitting software for sale on the App Store does include a code signing phase, but the signature is replaced by Apple before it is distributed to customers. So, if there is a security issue with version 1 code signatures, Apple is in a position to remedy the problem without the involvement of 3rd party developers. Some of us sought clarification from Apple on this point. Questions in the Apple Developer Forums along the lines of “do we really have to re-sign apps and submit a new version just to accommodate this new requirement?” were met by terse restatements from Apple along the lines of “all apps must be signed with version two code signatures.”


To make matters yet more confusing, a developer who has signed off on the chore of complying with Apple’s requests would not necessarily be able to verify the job was done right, because for example on pre-release builds of 10.9.5 and 10.10, many apps with “old and busted” version 1 signatures unexpectedly passed the system’s Gatekeeper check, contrary to the firm indication from Apple that they shouldn’t. Apple’s documentation provided a specific command to run from the Terminal that would verify or reject any specific app’s code signing:

spctl -a -t exec -vv

For many of us with very old version 1 code signatures, the command line came back with a cryptic “accepted cdhash”, and the system happily opened and ran the apps without issue.

I reported the problem as a security bug because it seemed to suggest that apps with broken, insecure code signatures would still be allowed to run. I waited to see whether, with each subsequent beta release, Apple would finally ratchet things down to give me a taste of how customers would experience trying to run these apps with outdated signatures. But each beta release continued to allow them to run without incident.

Finally, a few days ago I got my bug sent back to me to be closed. Oh, they finally fixed it? Nope. It was returned with the classification “Behaves as intended.” Then yesterday, 10.9.5 shipped and lo and behold, contrary to every warning and veiled threat from Apple, many of these apps with old, version 1 code signatures, of the variety that yield a cryptic “accepted cdhash” assessment from the spctl tool, well, they just work. They’re fine. This happens to apply to my entire line of apps and this is probably also the case for many other developers. In short? I dropped everything, spent hours revising my code signing process, investigating unexpected results, contacting Apple for clarification, trying to make sense of Apple’s terse replies, and finally doing my best to comply regardless of doubt. And it turns out I didn’t actually have to lift a finger.

Which is not to say that nobody had to lift a finger. Any developer whose code signatures involved the use of “custom resource rules,” and (I think) any developer whose version 1 signature was signed with a new enough version of Xcode to escape the “accepted cdhash” loophole, but not new enough to be a “version 2” signature, did need to re-sign their apps.

This was a classic case of Apple communicating far too poorly about a situation that purported to affect potentially every Mac developer. Many of us spent way too much time trying to decode and make sense of the situation when Apple could have done so for us through careful clarification of the specific code signatures that needed updating, how they could be reliably verified, and what the actual consequences of inaction would be.