Read and Write UEFI Firmware Variables from .NET Code

UEFI firmware variables are small, named key-value records stored in NVRAM. Each variable belongs to a namespace identified by a vendor GUID, carries a set of attribute flags that control its lifetime and access, and holds a binary payload. The UEFI specification defines a set of globally-scoped variables (boot order, secure boot state, OS indications, and more) under the well-known GlobalVariable GUID. OEM vendors extend the same storage mechanism with their own GUIDs for platform-specific settings. The FirmwareEnvironment class and its pre-configured GlobalFirmwareEnvironment subclass give you typed, marshal-free access to all of these.

Using GlobalFirmwareEnvironment

FirmwareEnvironment.Global is a lazily initialised singleton of type GlobalFirmwareEnvironment. It is scoped to the standard UEFI global namespace (8BE4DF61-93CA-11D2-AA0D-00E098032B8C) and exposes commonly needed variables as strongly typed properties.

Reading Secure Boot state

SecureBoot is read-only — it is set by the firmware during boot and indicates whether Secure Boot enforcement is currently active:

using Unified.Firmware;

bool isSecure = FirmwareEnvironment.Global.SecureBoot;
Console.WriteLine("Secure Boot: {0}", isSecure ? "Enabled" : "Disabled");

Reading and writing the boot timeout

Timeout is the number of seconds the firmware waits before auto-selecting the first boot entry. It can be read and written:

using Unified.Firmware;

// Read the current timeout
ushort currentTimeout = FirmwareEnvironment.Global.Timeout;
Console.WriteLine("Boot timeout: {0}s", currentTimeout);

// Change to 5 seconds
FirmwareEnvironment.Global.Timeout = 5;

Reading the BootOrder array

BootOrder exposes the ushort[] array of ordered boot indices. For most boot-management scenarios, prefer FirmwareBootService.LoadOrder (which wraps this in BootOptionIndex[]), but the raw array is useful for diagnostics:

using Unified.Firmware;

ushort[] order = FirmwareEnvironment.Global.BootOrder;
Console.WriteLine("Boot order: {0}", string.Join(", ", order.Select(x => $"Boot{x:X4}")));

OsIndications — reading and writing

OsIndications allows the OS to signal requests to the firmware across the next reboot. The flag is a ulong-backed [Flags] enum:

using Unified.Firmware;

// Read which OsIndications the firmware supports
OsIndications supported = FirmwareEnvironment.Global.OsIndicationsSupported;
Console.WriteLine("Supports boot-to-UI: {0}", supported.HasFlag(OsIndications.BOOT_TO_FW_UI));

// Read current value
OsIndications current = FirmwareEnvironment.Global.OsIndications;
Console.WriteLine("Current OsIndications: {0}", current);

// Set the boot-to-firmware-UI bit
FirmwareEnvironment.Global.OsIndications |= OsIndications.BOOT_TO_FW_UI;

Trigger boot to firmware UI

FirmwareInterface.BootToUserInterface() is a convenience wrapper that checks capability support and sets the BOOT_TO_FW_UI bit in one call. The system will enter the firmware setup screen on the next reboot — the call does not restart the computer.

using Unified.Firmware;

try
{
    FirmwareInterface.BootToUserInterface();
    Console.WriteLine("The firmware UI will open on next reboot.");
}
catch (PlatformNotSupportedException ex)
{
    Console.Error.WriteLine("Not supported: {0}", ex.Message);
}

Custom vendor namespace

Instantiate FirmwareEnvironment directly with any GUID to access vendor-specific variables. The constructor takes a backend (use FirmwareInterface.CurrentBackend) and the target namespace GUID:

using System;
using Unified.Firmware;
using Unified.Firmware.EnvironmentVendor;

// Access Microsoft-namespace variables
var msEnv = new FirmwareEnvironment(
    FirmwareInterface.CurrentBackend,
    FirmwareVendors.MicrosoftVendor
);

// Read a string variable from that namespace
string? policy = msEnv.ReadStringVariable("CurrentPolicy", out VariableAttributes attrs);
Console.WriteLine("CurrentPolicy attrs: {0}", attrs);

Reading from or writing to the wrong vendor GUID will either return null / a default value (if the variable does not exist under that GUID) or corrupt an unrelated variable (if a different variable happens to share the same name in that namespace). Always verify you are using the correct GUID before writing. If a read returns null or default, the variable does not exist in that namespace rather than having been deleted from a different one.

FirmwareVendors — known namespace GUIDs

FirmwareVendors is a static class that collects well-known vendor GUIDs as Guid fields:

Field	GUID	Description
`GlobalVariable`	`8BE4DF61-93CA-11D2-AA0D-00E098032B8C`	Standard UEFI global namespace. Stores `BootOrder`, `BootNext`, `Timeout`, `SecureBoot`, `SetupMode`, etc.
`ImageSecurityDatabase`	`D719B2CB-3D3A-4596-A3BC-DAD00E67656F`	Secure Boot key databases (`db`, `dbx`, `dbt`).
`HardwareErrorVariable`	`414E6BDD-E47B-47CC-B244-BB610208F4EF`	WHEA hardware error records (`HwErrRec####`).
`CapsuleReport`	`39B68C46-F7FB-441B-B6D1-E15C1B773062`	Firmware update capsule result variables.
`MicrosoftVendor`	`77FA9ABD-0359-4D32-BD60-28F4E78F784B`	Microsoft-specific variables such as `CurrentPolicy`, BitLocker state, early-boot telemetry.
`LenovoVendor`	`C020489E-6DB2-4EF2-9AA5-CA06FC11D36A`	Lenovo / ThinkPad BIOS settings accessible via WMI-to-UEFI bridge.
`DellVendor`	`4BCFDDBD-65F0-4FC7-BF96-981FBB0EBFF4`	Dell-specific variables for keyboard backlight, power profiles, CBOM.
`HpVendor`	`577FA4AD-1A3E-4BCE-A268-3F173B00DBEC`	HP SureStart, security settings, and hardware tokens.
`AsusVendor`	`20C731A8-79C0-4E80-AFDE-0C3DEB22CDAE`	ASUS ROG / Prime memory timings, Aura LED, and overclocking profiles.
`AppleVendor`	`7C436110-AB2A-4BBB-A880-FE41995C9F82`	Apple Mac EFI variables such as `StartupMute`, Bluetooth peripheral pairings, and `efi-boot-device`.

VariableAttributes

Every firmware variable is stored with a bitmask of VariableAttributes that controls its lifetime and who can read or write it. The most common combination for OS-facing variables is NON_VOLATILE | BOOTSERVICE_ACCESS | RUNTIME_ACCESS.

Flag	Value	Meaning
`NON_VOLATILE`	`0x01`	Persists across power cycles. Omit for variables that only exist during a single boot session.
`BOOTSERVICE_ACCESS`	`0x02`	Readable and writable during EFI boot services (before `ExitBootServices()`). All standard variables carry this flag.
`RUNTIME_ACCESS`	`0x04`	Readable and writable at OS runtime after `ExitBootServices()`. Required for variables that the OS or applications need to access.
`AUTHENTICATED_WRITE_ACCESS`	`0x10`	Writes must be accompanied by a valid authentication descriptor. Used for Secure Boot key management.

See the VariableAttributes API reference for the full set of flags.

Reading a struct variable

FirmwareEnvironment.ReadVariable<T> marshals the raw bytes of an NVRAM variable into any unmanaged struct:

using Unified.Firmware;

// Read the raw Timeout value and inspect its attributes
ushort timeout = FirmwareEnvironment.Global.ReadVariable<ushort>("Timeout", out VariableAttributes attrs);
Console.WriteLine("Timeout: {0}s, Attributes: {1}", timeout, attrs);

Writing a string variable

FirmwareEnvironment.WriteStringVariable serialises a .NET string as a null-terminated UTF-16LE string (the encoding expected by UEFI):

using Unified.Firmware;

var env = new FirmwareEnvironment(
    FirmwareInterface.CurrentBackend,
    FirmwareVendors.GlobalVariable
);

env.WriteStringVariable(
    "PlatformLang",
    "en-US",
    VariableAttributes.NON_VOLATILE |
    VariableAttributes.BOOTSERVICE_ACCESS |
    VariableAttributes.RUNTIME_ACCESS
);

Wrap all FirmwareEnvironment calls in a try/catch for FirmwareEnvironmentException. This exception surfaces OS-level errors such as missing privileges, unsupported variable names, or hardware write failures. On Windows, the calling process must hold the SE_SYSTEM_ENVIRONMENT_NAME privilege; on Linux, root access is required. Catching FirmwareEnvironmentException separately from PlatformNotSupportedException lets you distinguish a privilege error from a system that does not support UEFI at all.

Get Started

Core Concepts

Guides

Read and Write UEFI Firmware Variables from .NET Code

Using GlobalFirmwareEnvironment

Reading Secure Boot state

Reading and writing the boot timeout

Reading the BootOrder array

OsIndications — reading and writing

Trigger boot to firmware UI

Custom vendor namespace

FirmwareVendors — known namespace GUIDs

VariableAttributes

Reading a struct variable

Writing a string variable

Build docs developers (and LLMs) love

Get Started

Core Concepts

Guides

Documentation Index

​Using GlobalFirmwareEnvironment

​Reading Secure Boot state

​Reading and writing the boot timeout

​Reading the BootOrder array

​OsIndications — reading and writing

​Trigger boot to firmware UI

​Custom vendor namespace

​FirmwareVendors — known namespace GUIDs

​VariableAttributes

​Reading a struct variable

​Writing a string variable

Build docs developers (and LLMs) love

Using GlobalFirmwareEnvironment

Reading Secure Boot state

Reading and writing the boot timeout

Reading the BootOrder array

OsIndications — reading and writing

Trigger boot to firmware UI

Custom vendor namespace

FirmwareVendors — known namespace GUIDs

VariableAttributes

Reading a struct variable

Writing a string variable