You can now downgrade BridgeOS on your T2 Intel Mac (2017-2020) to a previous version with Apple Configurator 2!
Ever since Apple released the very first T2 equipped Mac, some users wished that they had a way to downgrade to the previous version of BridgeOS.
The problem?
Updating BridgeOS on a T2 Mac was a one way street. You could only upgrade not downgrade.
Why would anyone want to downgrade BridgeOS?
Downgrading BridgeOS for troubleshooting & macOS Update workflow testing.
Let’s go over a quick example of why you might want to downgrade BridgeOS. You updated to macOS Big Sur 11.4 from 11.3. After the update, you’ve found that something is not working right. The T2 chip handles many things including, fan speed, battery, power, charging & sound (speakers & microphone). So in this example, maybe you are getting audio cracking noise (has happened in the past). Now you thinking that the new version of bridgeOS might be causing problems on your T2 Mac.
The 2nd example is macOS Update testing. If you are a system administrator in charge of updating a large fleet of Mac testing is very important. Part of that macOS Update process is updating BridgeOS. An example of this is if you upgrade a T2 Mac from 11.3 to 11.4. BridgeOS is updated in the process. Now that this T2 Mac is on the latest version of BridgeOS it normally can not be downgraded. Even if you boot back to recovery and install 11.3, BridgeOS will remain the same updated version. Set this Mac on the shelf because you will never be able to take it through a full update process again until 11.5.
Now you want to downgrade to a previous version of BridgeOS so you can single out an issue or test the full update process again.
You can now Downgrade BridgeOS!
When did this happen?
You could always upgrade BridgeOS via automatic download with Apple Configurator 2. –> https://mrmacintosh.com/how-to-restore-bridgeos-on-a-t2-mac-how-to-put-a-mac-into-dfu-mode/
I’ve also talked about downgrading BridgeOS before, but this was ONLY for beta versions of BridgeOS to get back to production/final versions.
Apple Configurator took care of all the work in both above examples, downloading the prod/final version of BridgeOS for your hardware.
This is different.
We can now download full BridgeOS IPSW Files directly from Apple the same way we do now with Apple Silicon M1 Macs. We can then use the BridgeOS IPSW File to restore/revive BridgeOS to your T2 Mac. The difference here is that Apple WILL stop signing for previous versions of BridgeOS. The signing process follows iOS and is canceled usually about one week after the release of a new update. Apple leaves ONE previous version signed (for 7 days) so you can now downgrade to that version!
BridgeOS and macOS Version Caveats
The first thing to know about BridgeOS is that it has macOS version requirements. BridgeOS supports the macOS version that’s released alongside it. The BridgeOS version also supports previous versions macOS. Let’s go over an example.
After updating to Big Sur version 11.4, you are now are having issues with your speakers (sound processed through T2 chip). You want to downgrade to the 11.3 compatible version of BridgeOS to see if that fixes the issue. You can now downgrade! The only problem is, the 11.3 version of BridgeOS does NOT support newer versions of macOS. You will need to downgrade to the minimum version of macOS that is supported by the version of BridgeOS that you are trying to downgrade to. You now need to downgrade macOS to version 11.3 to support the previous version of BridgeOS. If you don’t downgrade macOS to a compatible version and attempted to boot your Mac, you would be immediately prompted to update BridgeOS. The Mac would boot into “macOS Update Assistant” and would then download the latest compatible BridgeOS update and install it for you.
After downgrading BridgeOS you will need to reinstall/downgrade 11.3 over the top of 11.4 or erase and install 11.3 again. You will now be running 11.3 and the matching bridgeOS version 18P4556.
BridgeOS Version Compatibility Continued
I built a table to help visualize the current situation.
| T2 Mac BridgeOS IPSW Download | MacOS | Signed | Version | Date |
| 11.6 | NO | |||
| 11.5 | NO | |||
| 11.4 | NO | 18.16.14663 | 5/24 | |
| No BridgeOS update | 11.3.1 | YES | ||
| 5.3_18P4556_Restore.ipsw | 11.3 | YES | 18.16.14556 | 4/26 |
| 11.2 | YES | 18.16.14347 | 2/25 | |
| 11.1 | YES | 18.16.14346 | 2/01 | |
| 11.0.1 | YES | 18.16.13030 | 12/14 |
This table shows how BridgeOS version 18P4556 is compatible with macOS version 11.0.1 – 11.3.1. A forced BridgeOS update will be triggered if macOS 11.4 or newer is installed.
NOTE: Point/dot releases (example 11.3.1) may or may not come with an associated BridgeOS update. Big Sur 11.3.1 is compatible with the 11.3 version of BridgeOS
| T2 Mac BridgeOS IPSW | MacOS | Signed | Version | Date |
| 5.4_18P4663_Restore.ipsw | 11.4 | YES | 18.16.14663 | 5/24 |
| 5.3_18P4556_Restore.ipsw | 11.3 | NO | 18.16.14556 | 4/26 |
| 5.2_18P4347_Restore.ipsw | 11.2.2 | NO | 18.16.14347 | 2/25 |
| 5.2_18P4346_Restore.ipsw | 11.2 | NO | 18.16.14346 | 2/01 |
| 5.1_18P3030_Restore.ipsw | 11.1 | NO | 18.16.13030 | 12/14 |
| 5.0.1_18P2561_Restore.ipsw | 11.0.1 | NO | 18.16.12561 | 11/12 |
The table above lists the IPSW file version and build number. The next section is the maximum minor release that is compatible.
You can can download BridgeOS IPSW files here –> https://mrmacintosh.com/mac-intel-t2-bridgeos-download-database-upgrade-or-downgrade/
The BridgeOS Version in the 4th section matches what is listed in System Profiler (System Firmware Version), followed by the release date and size.
You can only downgrade back one previous version. Apple usually supports T-1, current minus one version. Normally when a new version of BridgeOS is released the the previous version of BridgeOS is unsigned after 7 days and cannot be installed.
How to Downgrade BridgeOS
Downgrading BridgeOS is just like restoring IPSW files on M1 Apple Silicon Macs & restoring/updating BridgeOS –> https://mrmacintosh.com/restore-macos-firmware-on-an-apple-silicon-mac-boot-to-dfu-mode/ & https://mrmacintosh.com/how-to-restore-bridgeos-on-a-t2-mac-how-to-put-a-mac-into-dfu-mode/
My T2 Intel Mac BridgeOS Download Database –> LINK
- 1. Download the version of BridgeOS IPSW file that you want to install
- 2. Boot the target T2 Mac into DFU Mode
- 3. Connect both Macs with USB-C white Apple charge cable.
- 4. Open Apple Configurator 2
- 5. Drag BridgeOS IPSW File over to DFU icon.
- 6. You will be prompted to Restore or Revive
- 7. Restore = Click Restore (erases all data)- downgrade BridgeOS (you can then reinstall macOS from Recovery
- 8. Revive = Click Revive (etains all data) – downgrades BridgeOS (you will need to install previous compatible version of macOS over the top of the current version)
- 9. Done! You are now on a previous version of BridgeOS.
This is what it looks like to drag a BridgeOS IPSW file over the DFU icon in Apple Configurator 2.
The Restore/Revive menu after dragging the BridgeOS IPSW file over to Apple Configurator 2.
This is what happens when you attempt to restore/revive with an unsigned version of BridgeOS.
The System cannot be restored on this device. The system install could not be authorized for this device. The build may not be approved for production installs.
How to downgrade BridgeOS
