Web MIDI API

The Web Midi API is a powerful feature that is identified by the name "midi". It integrates with Permissions by defining the following permission-related flags:

permission descriptor type

WebIDLdictionary MidiPermissionDescriptor : PermissionDescriptor {
  boolean sysex = false;
};

{name: "midi", sysex: true} is stronger than {name: "midi", sysex: false}.

The Web Midi API defines a policy-controlled feature named "midi" which has a default allowlist of 'self'.

WebIDLpartial interface Navigator {
  [SecureContext]
  Promise <MIDIAccess> requestMIDIAccess(optional MIDIOptions options = {});
};

requestMIDIAccess() method

When invoked, returns a Promise object representing a request for access to MIDI devices on the user's system.

Requesting MIDI access SHOULD prompt the user for access to MIDI devices, particularly if System Exclusive access is requested. In some scenarios, this permission may have already been implicitly or explicitly granted, in which case this prompt may not appear. If the user gives express permission or the call is otherwise approved, the vended Promise is resolved. The underlying system may choose to allow the user to select specific MIDI interfaces to expose to this API (i.e. pick and choose interfaces on an individual basis), although this is not required. The system may also choose to prompt (or not) based on whether System Exclusive support is requested, as System Exclusive access has greater privacy and security implications.

If the user declines or the call is denied for any other reason, the Promise is rejected with a DOMException parameter.

When the requestMIDIAccess() method is called, the user agent MUST run the following steps:

1. Let promise be a new Promise object and resolver be its associated resolver.

2. Return promise and run the following steps asynchronously.

3. Let document be the calling context's Document.

4. If document is not allowed to use the policy-controlled feature named midi, jump to the step labeled failure below.

5. Optionally, e.g. based on a previously-established user preference, for security reasons, or due to platform limitations, jump to the step labeled failure below.

6. Optionally, e.g. based on a previously-established user preference, jump to the step labeled success below.

7. Prompt the user in a user-agent-specific manner for permission to provide the entry script's origin with a MIDIAccess object representing control over user's MIDI devices. This prompt may be contingent upon whether System Exclusive support was requested, and may allow the user to enable or disable that access.

   If permission is denied, jump to the step labeled failure below. If the user never responds, this algorithm will never progress beyond this step. If permission is granted, continue the following steps.

8. success: Let access be a new MIDIAccess object. (It is possible to call requestMIDIAccess() multiple times; this may prompt the user multiple times, so it may not be best practice, and the same instance of MIDIAccess will not be returned each time.)

9. Call resolver's accept(value) method with access as value argument.

10. Terminate these steps.

11. failure: Let error be a new DOMException. This exception's .name should be "NotAllowedError" if the user or their security settings denied the application from creating a MIDIAccess instance with the requested options, or if the error is the result of document not being allowed to use the feature, "AbortError" if the page is going to be closed for a user navigation, "InvalidStateError" if the underlying systems raise any errors, or otherwise it should be "NotSupportedError".

12. Call resolver's reject(value) method with error as value argument.

WebIDLdictionary MIDIOptions {
  boolean sysex;
  boolean software;
};

WebIDL[SecureContext, Exposed=(Window,Worker)] interface MIDIInputMap {
  readonly maplike <DOMString, MIDIInput>;
};

The MIDIInputMap is a maplike interface whose value is a MIDIInput instance and key is its ID.

This type is used to represent all the currently available MIDI input ports.

WebIDL[SecureContext, Exposed=(Window,Worker)] interface MIDIOutputMap {
  readonly maplike <DOMString, MIDIOutput>;
};

The MIDIOutputMap is a maplike interface whose value is a MIDIOutput instance and key is its ID.

This type is used to represent all the currently available MIDI output ports.

This interface provides the methods to list MIDI input and output devices, and obtain access to an individual device.

WebIDL[SecureContext, Exposed=(Window,Worker), Transferable] interface MIDIAccess: EventTarget {
  readonly attribute MIDIInputMap inputs;
  readonly attribute MIDIOutputMap outputs;
  attribute EventHandler onstatechange;
  readonly attribute boolean sysexEnabled;
};

inputs

The MIDI input ports available to the system.

outputs

The MIDI output ports available to the system.

onstatechange

The handler called when a new port is connected or an existing port changes the state attribute.

This event handler, of type MIDIConnectionEvent, MUST be supported by all objects implementing the MIDIAccess interface.

Note

It is important to understand that leaving an EventHandler attached to this object will prevent it from being garbage-collected; when finished using the MIDIAccess, you should remove any onstatechange listeners.

Whenever a previously unavailable MIDI port becomes available for use, or an existing port changes the state attribute, the user agent SHOULD run the following steps:

1. Let port be the MIDIPort corresponding to the newly-available, or the existing port.
2. Fire an event named "statechange" at the MIDIAccess, using MIDIConnectionEvent with port set to port.

sysexEnabled

This attribute informs the user whether System Exclusive support is enabled on this MIDIAccess.

This interface represents a MIDI input or output port.

WebIDL[SecureContext, Exposed=(Window,Worker)] interface MIDIPort: EventTarget {
  readonly attribute DOMString id;
  readonly attribute DOMString? manufacturer;
  readonly attribute DOMString? name;
  readonly attribute MIDIPortType type;
  readonly attribute DOMString? version;
  readonly attribute MIDIPortDeviceState state;
  readonly attribute MIDIPortConnectionState connection;
  attribute EventHandler onstatechange;
  Promise <MIDIPort> open();
  Promise <MIDIPort> close();
};

id

A unique ID of the port. This can be used by developers to remember ports the user has chosen for their application. The User Agent MUST ensure that the id is unique to only that port. The User Agent SHOULD ensure that the id is maintained across instances of the application - e.g., when the system is rebooted - and when a device is removed from the system. Applications may want to cache these ids locally to re-create a MIDI setup. Some systems may not support completely unique persistent identifiers; in such cases, it will be more challenging to maintain identifiers when another interface is added or removed from the system. (This might throw off the index of the requested port.) It is expected that the system will do the best it can to match a port across instances of the MIDI API: for example, an implementation may opaquely use some form of hash of the port interface manufacturer, name and index as the id, so that a reference to that port id is likely to match the port when plugged in. Applications may use the comparison of id of MIDIPorts to test for equality.

manufacturer

The manufacturer of the port.

name

The system name of the port.

type

A descriptor property to distinguish whether the port is an input or an output port. For MIDIOutput, this MUST be "output". For MIDIInput, this MUST be "input".

version

The version of the port.

state

The state of the device.

connection

The state of the connection to the device.

onstatechange

The handler called when an existing port changes its state or connection attributes.

This event handler, of type "statechange", MUST be supported by all objects implementing MIDIPort interface.

Note

It is important to understand that leaving an EventHandler attached to this object will prevent it from being garbage-collected; when finished using the MIDIPort, you should remove any onstatechange listeners.

open

Makes the MIDI device corresponding to the MIDIPort explicitly available. Note that this call is NOT required in order to use the MIDIPort- calling send() on a MIDIOutput, attaching a MIDIMessageEvent EventHandler on a MIDIInput, or adding a MIDIMessageEvent EventListener on a MIDIInput will cause an implicit open(). The underlying implementation may not need to do anything in response to this call. However, some underlying implementations may not be able to support shared access to MIDI devices, so using explicit open() and close() calls will enable MIDI applications to predictably control this exclusive access to devices.

When invoked, this method returns a Promise object representing a request for access to the given MIDI port on the user's system.

If the port device has a state of "connected", when access to the port has been obtained (and the port is ready for input or output), the vended Promise is resolved.

If access to a connected port is not available (for example, the port is already in use in an exclusive-access-only platform), the Promise is rejected (if any) is invoked.

If open() is called on a port that is "disconnected", the port's .connection will transition to "pending", until the port becomes "connected" or all references to it are dropped.

When this method is called, the user agent MUST run the algorithm to open a MIDIPort:

1. Let promise be a new Promise object and resolver be its associated resolver.

2. Return promise and run the following steps asynchronously.

3. Let port be the given MIDIPort object.

4. If the device's connection is already "open" (e.g. open() has already been called on this MIDIPort, or the port has been implicitly opened), jump to the step labeled success below.

5. If the device's connection is "pending" (i.e. the connection had been opened and the device was subsequently disconnected), jump to the step labeled success below.

6. If the device's state is "disconnected", change the connection attribute of the MIDIPort to "pending", and enqueue a new MIDIConnectionEvent to the statechange handler of the MIDIAccess and to the statechange handler of the MIDIPort and jump to the step labeled success below.

7. Attempt to obtain access to the given MIDI device in the system. If the device is unavailable (e.g. is already in use by another process and cannot be opened, or is disconnected), jump to the step labeled failure below. If the device is available and access is obtained, continue the following steps.

8. Change the connection attribute of the MIDIPort to "open", and enqueue a new MIDIConnectionEvent to the statechange handler of the MIDIAccess and to the statechange handler of the MIDIPort.

9. If this port is an output port and has any pending data that is waiting to be sent, asynchronously begin sending that data.

10. success: Call resolver's accept(value) method with port as value argument.

11. Terminate these steps.

12. failure: Let error be a new DOMException. This exception's .name should be "InvalidAccessError" if the port is unavailable.

13. Call resolver's reject(value) method with error as value argument.

close

Makes the MIDI device corresponding to the MIDIPort explicitly unavailable (subsequently changing the state from "open" to "closed"). Note that successful invocation of this method will result in MIDI messages no longer being delivered to MIDIMessageEvent handlers on a MIDIInput (although setting a new handler will cause an implicit open()).

The underlying implementation may not need to do anything in response to this call. However, some underlying implementations may not be able to support shared access to MIDI devices, and the explicit close() call enables MIDI applications to ensure other applications can gain access to devices.

When invoked, this method returns a Promise object representing a request for access to the given MIDI port on the user's system. When the port has been closed (and therefore, in exclusive access systems, the port is available to other applications), the vended Promise is resolved. If the port is disconnected, the Promise is rejected.

When the close() method is called, the user agent MUST run the following steps:

1. Let promise be a new Promise object and resolver be its associated resolver.

2. Return promise and run the following steps asynchronously.

3. Let port be the given MIDIPort object.

4. If the port is already closed (its .connection is "closed" - e.g. the port has not yet been implicitly or explicitly opened, or close() has already been called on this MIDIPort), jump to the step labeled closed below.

5. If the port is an input port, skip to the next step. If the output port's .state is not "connected", clear all pending send data and skip to the next step. Clear any pending send data in the system with timestamps in the future, then finish sending any send messages with no timestamp or with a timestamp in the past or present, prior to proceeding to the next step.

6. Close access to the port in the underlying system if open, and release any blocking resources in the underlying system.

7. Change the connection attribute of the MIDIPort to "closed", and enqueue a new MIDIConnectionEvent to the statechange handler of the MIDIAccess and to the statechange handler of the MIDIPort.

8. closed: Call resolver's accept(value) method with port as value argument.

9. Terminate these steps.

Whenever the MIDI port corresponding to the MIDIPort changes the state attribute, the user agent SHOULD run the following steps:

1. Let port be the MIDIPort.

2. Fire an event named statechange at the MIDIPort, and statechange at the MIDIAccess, using MIDIConnectionEvent with the port attribute set to port.

WebIDL[SecureContext, Exposed=(Window,Worker)] interface MIDIInput: MIDIPort {
  attribute EventHandler onmidimessage;
};

WebIDL[SecureContext, Exposed=(Window,Worker)] interface MIDIOutput : MIDIPort {
  undefined send(sequence<octet> data, optional DOMHighResTimeStamp timestamp = 0);
  undefined clear();
};

WebIDLenum MIDIPortType {
  "input",
  "output",
};

WebIDLenum MIDIPortDeviceState {
  "disconnected",
  "connected",
};

WebIDLenum MIDIPortConnectionState {
  "open",
  "closed",
  "pending",
};

An event object implementing this interface is passed to a MIDIInput's onmidimessage handler when MIDI messages are received. Note that the DOM Event timeStamp attribute is defined as a DOMHighResTimeStamp, and represents the high-resolution time of when the event was received or is to be sent.

WebIDL[SecureContext, Exposed=(Window,Worker)]
interface MIDIMessageEvent : Event {
  constructor(DOMString type, optional MIDIMessageEventInit eventInitDict = {});
  readonly attribute Uint8Array? data;
};

data

A Uint8Array containing the MIDI data bytes of a single MIDI message.

WebIDLdictionary MIDIMessageEventInit: EventInit {
  Uint8Array data;
};

An event object implementing this interface is passed to a MIDIAccess' onstatechange handler when a new port becomes available (for example, when a MIDI device is first plugged in to the computer), when a previously-available port becomes unavailable, or becomes available again (for example, when a MIDI interface is disconnected, then reconnected) and (if present) is also passed to the onstatechange handlers for any MIDIPorts referencing the port.

When a MIDIPort is in the "pending" state and the device is reconnected to the host system, prior to firing a statechange event the algorithm to open a MIDIPort is run on it to attempt to reopen the port. If this transition fails (e.g. the Port is reserved by something else in the underlying system, and therefore unavailable for use), the connection state moves to "closed", else it transitions back to "open". This is done prior to the statechange event for the device state change so that the event will reflect the final connection state as well as the device state.

Some underlying systems may not provide notification events for device connection status; such systems may have long time delays as they poll for new devices infrequently. As such, it is suggested that heavy reliance on connection events not be used.

WebIDL[SecureContext, Exposed=(Window,Worker)]
interface MIDIConnectionEvent : Event {
  constructor(DOMString type, optional MIDIConnectionEventInit eventInitDict = {});
  readonly attribute MIDIPort? port;
};

port

The port that has been connected or disconnected.

WebIDLdictionary MIDIConnectionEventInit: EventInit {
  MIDIPort port;
};

One concerning theoretical attack involves malicious firmware updates for USB-MIDI devices. USB devices in general can do things based on their device descriptor, which is sent from the USB device itself. If a USB-MIDI device's firmware can modify what descriptor is sent, it could make itself act as a human interface device. This could allow a malicious website to read or inject keystrokes or other events on the host computer, which could lead to a total compromise of the system.

The attack would proceed as follows:

1. A malicious site tricks the user into granting Web MIDI permission.
2. The malicious site enumerates the MIDI devices connected to the user's machine, and identifies a vulnerable device.
3. The malicious site sends a pre-crafted set of MIDI messages to the vulnerable device, compromising the device by overwriting its firmware and adding a human interface device to its USB descriptor.
4. The compromised device injects keystrokes to download or otherwise reproduce a pre-crafted security exploit and execute it, compromising the system.

In order to enable to the above attack, a MIDI device would need all of the following to be true:

• Be vulnerable to malicious firmware updates. All of the following must be true:
  • Have user-programmable firmware. Many MIDI devices have re-programmable firmware. Those that do not are not vulnerable.
  • Allow firmware updates via sending MIDI messages. Many MIDI devices use System Exclusive messages to perform firmware updates. This is a common use of System Exclusive messages. It is technically possible to make a device that uses non-System Exclusive messages to perform firmware updates, but this is not an intended use of non-System Exclusive messages. Other devices may use out-of-band USB communication or require the user to connect a storage device directly to the MIDI device to perform firmware updates, which cannot be done using the Web MIDI API, and are not vulnerable.
  • Allow firmware updates without explicit user interaction on the device. There are known MIDI devices which do not require any user interaction to initiate a firmware update. Most MIDI devices require the user to place them in a special update mode first, such as by holding down a button or selecting a menu option, and are not vulnerable unless the attacker tricks the user into cooperating.
• Be a USB-MIDI device. Most modern MIDI devices have a USB MIDI interface. Some older MIDI devices only have a serial connection, and would not enable this attack.
• The MIDI device's firmware would have to be able to modify the USB controller's firmware. Many MIDI devices have USB controller firmware that is not addressable from the main firmware, and would not enable this attack.

MIDI devices that are vulnerable to malicious firmware updates but do not satisfy the other conditions cannot be used with this attack to compromise the host system. A malicious firmware update could still cause these MIDI devices to stop working or behave in undesired ways.

To mitigate this risk, implementers should emphasize the following in their implementations:

• Implement requestMIDIAccess() in a way that informs users of the risks of firmware updates, such as through text in permission prompts.
• Implement SecureContext as specified to prevent user-initiated legitimate firmware updates from being modified.
• Handle the sysex parameter as specified, to encourage developers to only request System Exclusive messages when necessary, since most firmware updates use System Exclusive messages.

Explicitly allowing or blocking lists of known MIDI devices may also help mitigate this specific attack, but many small companies and individuals build MIDI devices, and many MIDI devices are no longer supported, so doing this would significantly reduce the usability of the Web MIDI API.

Separate from the fingerprinting concerns of identifying the available ports are concerns around sending and receiving MIDI messages. Those issues are explored in more depth below.

MIDI messages can be divided into System Exclusive messages, and short (non-System Exclusive) messages. System Exclusive messages can be further subdivided into Universal System Exclusive messages such as the commonly recognized MIDI Time Code and MIDI Sample Dump Standard, and device-specific messages like “patch control data for a Roland Jupiter-80 synthesizer” that do not apply to other devices.

Before discussing security concerns, it's useful to examine what scenarios are enabled by MIDI using these features:

• Receiving short messages from MIDI devices - this enables getting input from keyboards, drum pads, guitars, wind controllers, DJ/controllerist controllers, and more, and using those messages as input to control instruments and features in the Web Audio API as well as other control scenarios. MIDI is the protocol of choice for the multi-billion-dollar music production industry for getting physical controllers like knobs and buttons attached to your computer, both in pro/prosumer audio and media applications as well as consumer applications like Garageband.
• Sending short messages to MIDI devices - it’s tempting to say sending is significantly less interesting, as the scenario of attached output devices like hardware synthesizers is less common in today's market. The major exception to this is that many MIDI controllers have external host control of their indicator lights, and this makes them dramatically more useful. For example, the very popular Novation Launchpad controller uses MIDI note on/off messages sent to it to turn on/off and change colors of the buttons. The same is true of nearly all DJ controllers.
• Sending and receiving System Exclusive messages - for more advanced communication with high-end hardware devices, System Exclusive messages are required. Some common MIDI commands are also sent as Universal System Exclusive messages, such as MIDI Machine Control - generic start/stop/rewind/fast-forward commands. Many devices use device-specific System Exclusive messages to program patches, send advanced controller messages, download firmware, etc., which are much-demanded scenarios for Web MIDI. Some devices use System Exclusive as a direct control protocol, as they can pack more data into a single “message”, and most devices use System Exclusive as a way to save and restore patches and configuration information on less-expensive computer storage. Several of the major music hardware producers have expressed strong interest in using Web MIDI to provide web-based configuration and programming interfaces to their hardware. In short, disabling System Exclusive altogether does not only disable high-end scenarios.

The potential security impact of each of these is as follows:

• Sending short messages to a MIDI device - sending note-on/note-off/controller messages could cause sounds to be played by attached devices, including (on Mac and Windows) any default virtual synthesizers. This by itself does not cause any concerning exposure - you can already make sounds without interaction, through <audio> or Web Audio. Some attached devices might be professional lighting control systems, so it’s possible to control stage lighting; however, this is rare, and no known system has the ability to cause lasting damage or information leakage based solely on short messages. At worst, a malicious page could flash lights, and the user could close the page and reset their lighting controller. The additional concerns about sending short messages are analogous to any audio output - you cannot overwrite user information or expose user information, but you can make sounds happen, change patches, or (in rare configurations) toggle lights - but non-destructively, and not persistently.
• Receiving short messages from a MIDI device - receiving note-on/note-off/controller messages would not cause information exposure or security issues, as there is no identifying data being received, just a stream of controller messages - all of which must be initiated by the user on that MIDI device (except clock-type messages). This is analogous to listening to keyboard, mouse, mobile/laptop accelerometer, touch input or gamepad events; there is no additional information exposed, and all messages other than clock signals must be initiated by the user.
• Sending and receiving System Exclusive messages - this is the biggest concern, because it is possible to write code that looks for system-specific responses to System Exclusive messages, which could identify the hardware available, and then use it to download data - e.g. samples stored in a sampler - or replace that data (erasing sample data or patches in the device), although both these scenarios would have to be coded for a particular device. It is also possible that some samplers might enable a System Exclusive message to start recording a sample - so if the sampler happened to have a dedicated microphone attached (uncommon in practice, but possible), it would be possible to write code specific to a particular device that could record a short sample of sound and then upload it to the network without further user intervention. You could not stream audio from the device, and most samplers have fairly limited memory, and MIDI Sample Dump sysex is a slow way to transfer data - it has to transcode into 7-bit - so it’s unlikely you could listen in for long periods. More explicit fingerprinting is a concern, as the patch information/stored samples/user configuration could uniquely identify the system. Again, this requires much device-specific code; there is not standardized “grab all patches and hash it” capability. This suggests that System Exclusive messages are in a security category of their own. Because of this less bounded potential, it seems that distinguishing requests for SysEx separately in the API is a good idea, in order to more carefully provide user security hooks. The suggested security model explicitly allows user agents to require the user's approval before giving access to MIDI devices, although it is not currently required to prompt the user for this approval - but it is also detailed that System Exclusive support must be requested as part of that request.