What is AddressBookSourceSync, and why is it online?

Last updated: 2026-07-31

What it is

AddressBookSourceSync is an Apple-supplied helper for Contacts account synchronization. It is not the main Contacts application, even though its name still uses the older “Address Book” terminology. Its documented job is to synchronize local Contacts data with iCloud, CardDAV, and Exchange servers.

The system instance runs from /System/Library/Frameworks/AddressBook.framework/Versions/A/Helpers/AddressBookSourceSync.app/Contents/MacOS/AddressBookSourceSync. Apple has used the bundle identifiers com.apple.AddressBookSourceSync and com.apple.AddressBook.SourceSync for the relevant components and launch configuration.

AddressBookSourceSync is managed by launchd and starts when macOS needs it. It is not intended to be launched directly by the user, and closing the Contacts application does not necessarily stop it. The application and the synchronization helper have separate roles: one provides the interface for viewing and editing contacts, while AddressBookSourceSync performs account-related background work.

Why it talks to the network

AddressBookSourceSync needs network access because the contact records on the Mac must be compared and synchronized with records held by an enabled account provider. It may run after a Contacts account is added or enabled, when a contact changes on the Mac, when the server has a change to deliver, or when macOS schedules routine background synchronization.

The installed LaunchAgent includes a background activity interval of one hour and registers services associated with periodic synchronization and push notifications. That interval is only a chance for the system to schedule work. The actual wake-up time and transfer time remain subject to macOS scheduling, and an hourly opportunity does not show that data was transmitted.

The documented server categories are Apple iCloud, configured CardDAV services, and Exchange services. macOS also lets users configure contact accounts from providers such as Google and Yahoo. However, it has not been verified that AddressBookSourceSync directly performs every transfer for every such provider. If Google contact syncing is enabled, communication with Google can be expected as part of that account’s synchronization, but the exact division of work among macOS helper processes is not fully published.

How much traffic is normal

Apple does not publish a normal byte range, daily allowance, or megabyte baseline for AddressBookSourceSync. A fixed number would therefore be misleading.

With no pending contact changes, traffic may be close to zero. When records change, AddressBookSourceSync can produce short, intermittent transfers containing contact records and synchronization metadata. Enabling an account for the first time, resynchronizing a large contact collection, or transferring contact pictures can create a larger burst. Even then, its traffic would normally be expected to remain substantially smaller than transfers involving a photo library, video, or a system update.

That description is qualitative, not a measured threshold. There is no reliable basis for declaring AddressBookSourceSync abnormal merely because it exceeds a particular daily MB total. Sustained high traffic is a reason to investigate whether an account is being resynchronized, whether the contact collection is unusually large, or whether repeated failures are causing more attempts. It is not, by itself, proof of malware or a complete address-book upload.

Can you turn it off

AddressBookSourceSync is optional when you do not want network-account contacts synchronized on the Mac. The supported control is account-specific; macOS does not provide a supported master switch for the AddressBookSourceSync process itself.

For a regular Internet Account, open Apple menu  > System Settings > Internet Accounts > [account], then turn off Contacts. For iCloud, open Apple menu  > System Settings > [your name] > iCloud > See All next to Saved to iCloud, then turn off Contacts. If Contacts is enabled for several accounts, repeat the change for each one to stop all corresponding synchronization.

After Contacts is disabled for an account, that account’s contacts stop updating between the Mac and its server. They may also be removed from the Contacts application on that Mac. Turning the setting back on will usually make them available again. Contacts stored locally and not associated with one of these network accounts are not the same thing as the account data being synchronized.

Killing AddressBookSourceSync is not a durable substitute for changing the account setting. launchd can start it again when macOS requests synchronization work. Deleting its LaunchAgent or permanently blocking the process can instead leave contact uploads, downloads, or synchronization state coordination delayed or failed. Apple does not provide or promise a safe process-level throttling mechanism either; limiting the connection may cause timeouts or delayed synchronization.

What people get wrong

• “AddressBookSourceSync is a virus or advertising tracker.” The system instance in the protected /System/Library location, with the bundle identifier com.apple.AddressBookSourceSync, is an Apple component. A same-named executable found somewhere else should still be checked on its own merits.

• “AddressBookSourceSync only synchronizes iCloud.” Apple’s supplied manual page also names CardDAV and Exchange. Its documented scope is broader than iCloud alone.

• “AddressBookSourceSync handles Mail, Calendar, Photos, or a complete Mac cloud backup.” Apple’s public description is limited to Contacts data. Other categories are handled by their respective applications or background components.

• “Quitting Contacts stops AddressBookSourceSync.” AddressBookSourceSync is an independent helper managed by launchd. To stop an account’s contact synchronization, disable Contacts for that account under Internet Accounts or iCloud settings.

• “Killing AddressBookSourceSync, deleting its LaunchAgent, or blocking it permanently is a harmless optimization.” macOS can launch the process again on demand. Interfering with it can delay or prevent contact uploads, downloads, and synchronization-state coordination.

• “If AddressBookSourceSync connects to Google, Apple must be uploading my contacts to Google.” When a Google Contacts account has been enabled by the user, communication with that provider is an expected part of account synchronization. The available evidence does not establish that AddressBookSourceSync directly performs every Google transfer, because Apple has not fully documented how that work is divided among helpers.

• “AddressBookSourceSync uploads the entire address book every hour.” The one-hour LaunchAgent interval represents a scheduling opportunity. It does not prove that each scheduled run uses the network, much less that every run performs a complete upload.

• “A fixed number of megabytes per day tells me whether AddressBookSourceSync is behaving normally.” Apple provides no such baseline. Persistent large transfers should be investigated in context, including account resynchronization, collection size, contact pictures, and repeated failures, rather than judged against an invented universal limit.

Seeing what it actually used

If the concern is traffic rather than the process name, the next step is to measure AddressBookSourceSync over the period in question. Bytetally’s per-process statistics can show how much network traffic was attributed to AddressBookSourceSync, so you can compare isolated bursts with sustained activity. Use that record alongside recent account changes or resynchronization events; it does not by itself reveal the contents of a transfer.

Common questions