SecureBoot flaw for all WP devices

First of all, Welcome to 2019, and happy new year everyone!

Edit: This post has been updated by Rafael Rivera because it was very rough, thank you!

Rene Lergner's WPinternals tool supports Microsoft Lumia devices out of the box. But thanks to a newly surfaced production-signed EFI application, all the others - like the Alcatel Idol 4S, HP Elite x3, and HTC One M8 - can join the party too.

Recently, an efi file has been leaked to the internet

This file is one of many production signed developermenu.efi files, but what does this mean?

developermenu.efi is an EFI application that can alter EFI variables and kickstart the EFI mass storage application on all newer generation Qualcomm-based devices. Of course, mass storage access is only step one of many to successfully disable SecureBoot.

Generally, to disable SecureBoot on a device, one needs to:

• Install developermenu.efi. This can be completed by exploiting access to file system in some manner (e.g. via SSH, MTP or File Explorer). I use the File Explorer approach on Windows 10 Mobile 10586 to backup/replace resetphone.efi. I do not recommend using MTP due to its inherent flakiness. Detailing how to do this is out of scope for this blog post, but remember: Interop Unlock requires Windows 10 Mobile 14393 or earlier so a reflash will most certainly be required.
• Enter mass storage mode via the newly installed app triggering a phone reset. (Remember, we replaced the resetphone.efi application with our developer menu application.)
• Exploit EFI variable storage (thanks Rene!) and patch mobilestartup.efi to prevent policy reinstalls.

Before we can exploit EFI variable storage, however, some additional set up is required:

• Disable integrity checking (nointegritychecks) in the Boot Configuration Data (BCD) store for the mobile startup entry. (This store is normally located in EFIESP/efi/microsoft/boot/BCD.)
• Optional) Enable test signing support as well (testsigning) to both the mobile startup and Windows Loader entries.
• Obtain EFIESP/Windows/System32/boot/mobilestartup.efi from an unlocked Microsoft Lumia device.
• Replace mobilestartup.efi on your device with the aforementioned copy.

Now, we just need to make a quick change to Rene's WPInternals tool to fiddle with variable storage:

• Clone the WPinternals repository. (We'll be working straight out of the master branch.)
• Open the project with Visual Studio and open TestCode.cs.
• Add the following code to the first function (TestCode):

PhoneNotifierViewModel Notifier = new PhoneNotifierViewModel();
Notifier.Start();

await SwitchModeViewModel.SwitchTo(Notifier, PhoneInterfaces.Lumia_MassStorage);
MassStorage MassStorage = (MassStorage)Notifier.CurrentModel;

bool GPTChanged = false;
var GPTChunk = MassStorage.ReadSectors(0, 256);
var GPT = new GPT(GPTChunk);

// Now add NV partition
Partition BACKUP_BS_NV = GPT.GetPartition("BACKUP_BS_NV");
Partition UEFI_BS_NV;
if (BACKUP_BS_NV == null)
{
                BACKUP_BS_NV = GPT.GetPartition("UEFI_BS_NV");
                Guid OriginalPartitionTypeGuid = BACKUP_BS_NV.PartitionTypeGuid;
                Guid OriginalPartitionGuid = BACKUP_BS_NV.PartitionGuid;
                BACKUP_BS_NV.Name = "BACKUP_BS_NV";
                BACKUP_BS_NV.PartitionGuid = Guid.NewGuid();
                BACKUP_BS_NV.PartitionTypeGuid = Guid.NewGuid();
                UEFI_BS_NV = new Partition();
                UEFI_BS_NV.Name = "UEFI_BS_NV";
                UEFI_BS_NV.Attributes = BACKUP_BS_NV.Attributes;
                UEFI_BS_NV.PartitionGuid = OriginalPartitionGuid;
                UEFI_BS_NV.PartitionTypeGuid = OriginalPartitionTypeGuid;
                UEFI_BS_NV.FirstSector = BACKUP_BS_NV.LastSector + 1;
                UEFI_BS_NV.LastSector = UEFI_BS_NV.FirstSector + BACKUP_BS_NV.LastSector - BACKUP_BS_NV.FirstSector;
                GPT.Partitions.Add(UEFI_BS_NV);
                GPTChanged = true;
}

if (GPTChanged)
{
            GPT.Rebuild();
            MassStorage.WriteSectors(0, GPTChunk);
}

Partition TargetPartition = GPT.GetPartition("UEFI_BS_NV");
string SBRes = "WPinternals.SB";
var streamdata = new SeekableStream(() =>
{
            var assembly = System.Reflection.Assembly.GetExecutingAssembly();

            // Magic!
            // The SB(A) resource is a compressed version of a raw NV-variable-partition.
            // In this partition the SecureBoot variable is disabled.
             // It overwrites the variable in a different NV-partition than where this variable is stored usually.
            // This normally leads to endless-loops when the NV-variables are enumerated.
            // But the partition contains an extra hack to break out the endless loops.
            var stream = assembly.GetManifestResourceStream(SBRes);

             return new DecompressedStream(stream);
});

MassStorage.WriteSectors((UInt32)TargetPartition.FirstSector, new BinaryReader(streamdata).ReadBytes((int)streamdata.Length));
LogFile.Log("Phone is unlocked!", LogType.FileAndConsole);

• Connect your phone, booted into mass storage mode, to the PC and verify functionality before continuing.
• Build and launch WPinternals (Debug-Test configuration).
• Verify "Phone is unlocked!" is displayed in the console.

At this point, SecureBoot is off, regardless of what the OS wants/reports. Beware, however: Your only eMMC access is via developermenu.efi. Be very careful on devices with no flashing/recovery mechanism other than the "lightning cog". You can easily brick your phone. For safer reflashes, the tool already backed up the original variable storage.

Happy 2019 and happy hacking!