Crashlytics error: Undefined symbols for architecture arm64

I solved this issue by just adding $(inherited) to other linker flags in Build Settings.

Crashlytics and Fabric require you to link your target against the following:

• Security.framework
• SystemConfiguration.framework
• libc++
• libz

Just select your target -> Build Phases -> Link Binary with Libraries -> add the ones missing.

I hope it helped.

I run into a similar issue, when I was updating an app that was created before Xcode 5.

Since Xcode 5 new projects build with modules enabled by default. In my old app "Enable Modules" was set to "No". So this was the solution.