What Is Bluetooth

Bluetooth is a communication standard for connecting devices to each other wirelessly over short distances. It is widely used to link a phone with earphones, keyboards, speakers or a smartwatch. Its biggest advantage is that no cable is needed.

The range is often quoted as about 10 m, but that figure is only a rough guide. Bluetooth SIG lists the physical layer (PHY) in use, receiver sensitivity and transmit power (-20 dBm to +20 dBm), and antenna gain together with path loss as the factors that decide range, and explains that the usable distance runs from under a meter to more than a kilometer depending on the conditions. Small devices such as earphones or a mouse transmit at low power, and the distance shrinks further when a body or a wall sits in between.

Differences Between Versions

Bluetooth comes in two families: Classic (BR/EDR), used for audio playback and file transfer, and LE (Low Energy), designed to run on very little power. When a spec sheet says a product supports Bluetooth 5, what that means depends on which of the two families is being discussed.

The higher speed and the longer range introduced with Bluetooth 5 are both extensions to the physical layer (PHY) on the LE side. As Bluetooth SIG explains, the LE 2M PHY operates at 2 Ms/s but implementing it is optional, while the LE Coded PHY can extend range to roughly four times that of Bluetooth Core 4.0 without raising transmit power. Only the original LE 1M PHY is mandatory, so a product labeled as supporting Bluetooth 5 does not necessarily offer the fast mode or the long-range mode, and neither of them applies to communication on the Classic side.

What Pairing Actually Verifies

Pairing is less an act of joining two devices than a procedure for exchanging the keys that encrypt the link. How those keys are exchanged depends on the input and output capabilities of each device, and the specification defines association models named Just Works, Numeric Comparison, Passkey Entry and OOB.

Numeric Comparison: both devices show the same six-digit number and the user confirms that the two match. If a third party has inserted itself in the middle the numbers do not agree, so impersonation is detected
Just Works: used by devices that have neither a screen nor keys, and it includes no step that checks whether the other side is genuine. The link is encrypted, but the peer stays unauthenticated while the connection completes

Devices with no screen, such as earphones and sensors, tend to end up on Just Works. In other words, the fact that pairing succeeded does not by itself mean the connection is safe: what matters is whether you knew what you were connecting at the moment you approved it, which is exactly where the advice to reject unexpected requests comes from.

Security Risks

Bluetooth carries the following security risks:

BlueBorne: a set of vulnerabilities disclosed by Armis in September 2017 that included ways to take over a device without pairing. The flaws were in the implementation in each operating system rather than in the Bluetooth specification, and fixes were distributed for the major platforms around the time of disclosure
Tracking: the risk of having your location followed through BLE advertising signals. As a mitigation, the address sent over the air can be rotated periodically so that only bonded devices holding a dedicated key (IRK) can resolve it back to the original identifier (Resolvable Private Address), though this only takes effect when the device has the feature enabled
Unauthorized pairing: the risk of approving a connection request from a device you do not know. As described above, Just Works does not confirm that what you approved is the device you meant to connect to
Misuse of item trackers: slipping a BLE tag in among someone's belongings to follow their movements. In May 2024 Apple and Google began offering alerts based on their joint Detecting Unwanted Location Trackers specification, which lets iOS 17.5 or later and Android 6.0 or later detect an unfamiliar tag that is traveling with you

Tips for Safe Usage

The first point is not to approve pairing requests you were not expecting. The device name shown in a request can be set to anything, so judge by whether you have just put that device into pairing mode rather than by the name. If the device displays digits, always check that the six digits match on both sides.

The second point is to keep the updates for your operating system and device firmware applied. Implementation flaws such as BlueBorne cannot be closed any other way. Deleting the pairing (bonding) for devices you no longer use also reduces the number of devices that still hold a key.

Turning Bluetooth off when you are not using it is reliable, since a radio that does not transmit reaches no one. Keeping it off at all times is not realistic if you use earphones or a watch every day, however, so checking the three points above is the more practical habit.

If you want a more systematic understanding of how Bluetooth works, how it differs from Wi-Fi and what to watch out for in terms of security, see also What Is Bluetooth? How It Works, Differences from Wi-Fi, and Security.

Common Misconceptions

Bluetooth is safe because of its short range: Range is decided by transmit power and by the surrounding environment, so under good conditions it reaches well beyond 10 m. Situations where strangers are nearby, such as a crowded train or a cafe, are part of everyday life, so it is safer not to treat distance itself as a barrier.
Paired devices are always safe: Pairing is a procedure for exchanging the keys that encrypt the link, and depending on the model used it does not verify that the peer is the device you intended. On top of that, a paired device is still open to flaws in its own implementation, which is why operating system and firmware updates matter.

Classic Bluetooth vs. BLE Comparison

Classic Bluetooth

Designed for music playback and file transfers. Higher power consumption but faster data speeds.

BLE

Designed for IoT devices and sensors. Low power consumption, can run on a button battery for years.

Bluetooth