macOS 15 and 26 VMs capped to 1.75 GB RAM regardless of actual RAM size

Hi,

we're running into an issue on our virtual machines (running in UTM, but based on internet posts in other places it happens elsewhere as well) where they are limited to 1.75 GB RAM.

On macOS 15, this issue seemed to appear in 15.8 and was fixed in 15.8.2.

However, for the latest macOS 26 release (26.7.1), the issue is still there. The issue specifically seems to be related to the firmware version: 20457.1.29. My LLM figured out that you can swap out the firmware like this:

This relies on an undocumented file layout, so back up first.

With the VM stopped, copy bytes 0x4000–0x6000 (firmware slot descriptors) and 0x20000–0x420000 (two firmware slots) from the auxiliary storage file of an unaffected VM. The donor must have the same machine identifier (ECID) and either 18000.x firmware (such as a clone made before the update) or 20457.41.2. Keep the rest of the target file, which holds NVRAM and boot policy state. Replacing the whole file fails to boot when the two disks have different LocalPolicy nonces.

An update that ships affected firmware overwrites the splice, so it has to be reapplied after such updates.

I mostly wanted to share that workaround, but any input from others suffering from the same issue, or that have any additional information about this issue, would be appreciated. I am not so comfortable with running our macOS 26 regression tests on a VM with manually injected firmware that may or may not be fully compatible with the OS version.

The other internet posts about what seems like the same issue:

• https://github.com/utmapp/UTM/issues/7897
• https://github.com/cirruslabs/macos-image-templates/issues/385

I reported it to Apple as FB25121799