Apple's macOS Tahoe 26.4 update introduced a security change that prevents users from manually copying the login Keychain file to another Mac. The login Keychain, stored as a SQLite database in the user's home folder, is now bound to device-specific Secure Enclave keys, making it inaccessible on other machines. This change, which took effect in March but only recently gained attention, means that copying the Keychain file to a new Mac will not allow it to open, as the Secure Enclave must be accessed to decrypt the metadata key protecting the Keychain attributes.

While the file can still be copied, it will not function on a different Mac due to the missing Secure Enclave binding. Non-login Keychain files remain unaffected and can be used on other Macs normally. The issue impacts administrators performing bulk migrations and users attempting manual Keychain transfers, though backups to the same Mac, Migration Assistant, iCloud Keychain synchronization, exporting items, or using custom keychains and third-party password managers offer workarounds.

Critics, including Rich Trouton and Jeff Johnson, have labeled the change a "ticking time bomb" and "incredibly irresponsible," noting users may only discover the failure during emergencies, such as a Mac death, when recovery is most needed.