MetaMask(メタマスク) Security Guide for Beginners

MetaMask is a self-custody cryptocurrency wallet, known in Japanese as メタマスク, that keeps the cryptographic keys controlling an address on the owner’s own device instead of on a company’s servers. Self-custody means the keys sit with the person, not with a firm, and every other point in this article follows from that single fact.

What does MetaMask actually hold?

MetaMask holds cryptographic keys and the software needed to sign messages with them; MetaMask does not hold coins or tokens, which exist only as entries recorded on a public blockchain.

A wallet in this sense is closer to a keyring than to a purse. The balance associated with an address is written into a shared ledger maintained by many independent computers, and the wallet reads that ledger and displays the result. When a transfer is made, the wallet produces a digital signature demonstrating that whoever produced it knew the private key, and the network accepts or rejects the instruction on that evidence alone.

Two consequences follow from a wallet holding keys rather than money. The first consequence is reassuring: a lost or broken device destroys nothing by itself, because the ledger is unaffected and the keys can be derived again from the Secret Recovery Phrase kept elsewhere. The second consequence is less comfortable. A valid signature is the only credential the network examines. There is no name on the account, no branch office to telephone, and no clerk with discretion to reverse an entry.

Ethereum, the public network MetaMask was first built around, began operating in July 2015 and has recorded every transaction since in a form any member of the public can read. Ownership on such a network is defined by the ability to sign, not by an identity document. Self-custody software is simply the tool that turns a stored secret into a signature, and understanding that narrow role explains why the security questions worth asking are about the secret rather than about the interface.

What is a Secret Recovery Phrase?

A Secret Recovery Phrase is an ordered list of ordinary English words — commonly twelve, sometimes twenty-four — that encodes the master secret from which every private key in a wallet is mathematically derived.

The format comes from a published specification, BIP-39, introduced in 2013, which defines a fixed list of 2,048 words chosen so that no two begin with the same four letters. The words are not decoration and not a hint. The words are the secret itself, expressed in a form a human being can copy without error.

A password and a Secret Recovery Phrase differ in three ways that matter. A password protects one copy of one program on one device, can be changed at any moment, and is checked by whoever issued it. A Secret Recovery Phrase is checked by nobody, cannot be changed after it has been seen by someone else, and reconstructs the full wallet on any compatible software anywhere in the world. Once a phrase is known to another party, that party holds the wallet as completely as its owner does, and there is no revocation step available afterwards.

The practical rule that follows is narrow and absolute. A Secret Recovery Phrase belongs nowhere except inside the wallet software itself, at the moment that software legitimately needs it, and on whatever offline record its owner has made. Every other destination — a web page, a message, a helpful stranger, a page promising assistance — is either a mistake or a theft in progress.

Where must a Secret Recovery Phrase never go?

A Secret Recovery Phrase must never go into a web page, an email, a chat window, a photograph, a cloud note, a spreadsheet or a conversation with anyone claiming to represent a company, because each of those places can be read by software or by people other than the owner.

The rule is easiest to remember in its self-referential form: no person, no website, no message and no article — including this one — will ever have a legitimate reason to ask for a Secret Recovery Phrase, and anyone who asks for it is attempting theft, with no exceptions. The rule holds regardless of how plausible the request appears, how urgent it claims to be, or how closely the page resembles something familiar. Plausibility is the cheapest part of the whole operation to manufacture.

Requests arrive in recognisable shapes. Some present themselves as a routine check on an account. Some appear as assistance offered after a public complaint about a stuck transaction. Some are framed as a technical migration, a reward, or a compatibility step. The framing varies; the ask does not. Whenever a sequence of ordinary words is being solicited, the purpose is to obtain control of the wallet, and no legitimate purpose exists for which those words are required by a third party.

A photograph deserves separate mention. A phrase captured by a camera usually lands in a synchronised album, where it may be copied to a laptop, a backup service and a second telephone within minutes, and where any later compromise of that account exposes the words years afterwards. A written record kept offline, in a place its owner can describe from memory, does not travel in that way.

What is a token approval?

A token approval is a permission recorded on a blockchain that allows a named smart contract — a program stored on the blockchain itself — to move a specified token out of an address, up to a stated amount, without a fresh signature on each occasion.

The mechanism belongs to the ERC-20 token standard, first proposed in November 2015, and exists for a mundane reason: an exchange contract cannot take tokens it has not been permitted to take. Granting an approval is therefore a normal part of using many blockchain applications, and an approval is not in itself a transfer. Nothing leaves the address at the moment an approval is signed.

Two properties of approvals surprise people. The first property is duration: an approval persists until a later signed transaction reduces or cancels it, so a permission granted during a single afternoon’s activity remains in force for years unless revisited. The second property is scope: many applications request an unlimited allowance — an approval with no upper amount — and such an approval covers not only the tokens present at the time but also tokens of the same type acquired long afterwards.

Approvals matter to security because a fraudulent page rarely needs a Secret Recovery Phrase to be effective. A page that persuades someone to sign an approval obtains a lawful-looking permission that the network will honour without further question. The signature was genuine; only the understanding behind it was not. Reducing or cancelling an allowance later is possible, though doing so is itself a transaction that must be signed and paid for on the network.

What does a hardware wallet add?

A hardware wallet is a small dedicated device that keeps private keys inside its own chip and performs signing internally, so that the keys are never present in the memory of a general-purpose computer or telephone.

What a hardware wallet adds is separation. Software running on an everyday machine — including software the owner did not intend to run — cannot read a key that has never been on that machine. Most such devices also display the essential details of a transaction on their own small screen, so the description shown by the computer is not the only account of what is being signed. Both properties reduce the chance that a key is stolen by malicious software.

What a hardware wallet does not remove is the human step. The device signs what its holder confirms. A person persuaded to approve a fraudulent token allowance will approve it on a hardware wallet exactly as on any other. A Secret Recovery Phrase still exists as words on paper for such devices, and words on paper can still be photographed or disclosed. A hardware wallet also reverses nothing: once a signed instruction reaches the network and is confirmed, the device has no further influence over it.

The honest summary is that a hardware wallet changes the category of the remaining risk rather than eliminating risk. Key theft by software becomes considerably harder; deception of the person holding the device does not.

What does a phishing page look like?

A phishing page, seen from the target’s side, usually looks entirely ordinary: a familiar name, a familiar layout, an unremarkable request, and no visible sign of anything unusual until something has already been signed or disclosed.

Appearance is therefore a poor test. Copying a design costs a criminal almost nothing, and the copy is often indistinguishable from the original by eye. What tends to distinguish a fraudulent page is not how it looks but how it was reached, what it asks for, and how it treats hesitation. A page that solicits a Secret Recovery Phrase has already identified itself, whatever it looks like. A page that presents a deadline, or that treats a pause for thought as a problem to be managed, is behaving in a way legitimate software has no reason to.

The route matters more than the appearance. A bookmark saved earlier by its owner resolves to the same destination on every occasion, whereas a page reached through a message or an unfamiliar list is assembled afresh by whoever produced that list. Look-alike pages rely on that difference, because a familiar name can be reproduced beside a link far more cheaply than the destination behind it. A name shown next to a link is a claim about where the link goes, not proof of it, and the claim costs nothing to make.

The shape of the problem is consistent: the software is not defeated, the person is persuaded. Recognition therefore carries more of the weight here than any technical countermeasure.

What can MetaMask itself not do?

MetaMask cannot reverse a confirmed transaction, cannot reconstruct a Secret Recovery Phrase that no longer exists, and cannot retrieve assets sent to an address the sender did not intend to use.

The limits are structural rather than a matter of policy. A public blockchain accepts an instruction that carries a valid signature and records it permanently; the wallet publisher is not a party to the ledger and has no authority to amend it. The same design that removes the need to trust an intermediary also removes the intermediary’s ability to intervene.

A lost Secret Recovery Phrase is the clearest case. A phrase that has been destroyed, forgotten or misplaced cannot be reconstructed by the wallet publisher, by a specialist, by a court, or by anybody else, because no copy of it exists anywhere by design. Offers to retrieve lost cryptocurrency for a fee cannot do what they describe, since no copy of a Secret Recovery Phrase is held anywhere to be found, and an offer of that kind is a second fraud directed at someone who has already lost money once. Fraudulent retrieval offers typically request an advance payment, personal documents, or the very phrase whose absence is the problem.

Stating the limits plainly is not a criticism of the software. Wallet publishers have generally been open about them. Knowing in advance which mistakes are permanent is the part of the subject that repays attention most, because it determines which precautions are worth the inconvenience.

Which habits reduce risk?

Habits that reduce risk in self-custody share one characteristic: each removes a secret or a permission from a place where other parties could reach it, well before any specific threat appears.

What do the main terms mean?

The subject of this article is discussed in Japanese-language writing as メタマスク セキュリティ, and the terms below appear throughout writing on wallet security in both languages, each given here in the sense used on this page.

Self-custody wallet (セルフカストディウォレット)

A self-custody wallet is software that stores cryptographic keys under the control of the individual, so that no company can move the associated assets on the owner's instruction and no company can reissue the keys.

Secret Recovery Phrase (シークレットリカバリーフレーズ)

A Secret Recovery Phrase is an ordered sequence of common words that encodes the master secret of a wallet and can reconstruct every key in that wallet on any compatible software.

Private key (秘密鍵)

A private key is a very large secret number — 256 bits on Ethereum — whose possession allows the creation of signatures that the network treats as authorisation from the corresponding address.

Smart contract (スマートコントラクト)

A smart contract is a program stored on a blockchain that runs according to its published code when a transaction calls it, without any party being able to alter the outcome afterwards.

Token approval (トークンの承認)

A token approval is a permission recorded on the blockchain that lets a named smart contract move a stated quantity of a stated token from an address until the permission is altered.

Phishing (フィッシング)

Phishing is deception in which a page, message or conversation imitates something familiar in order to obtain a secret or a signature that the target would not otherwise have provided.

Security (セキュリティ)

Security, in the context of self-custody, describes the arrangements that keep a secret unread by anyone but its owner and a permission unsigned until its effect is understood.

Hardware wallet (ハードウェアウォレット)

A hardware wallet is a dedicated device that holds private keys internally and signs transactions on the device itself, keeping the keys off the general-purpose computer or telephone in use.

Where does the subject continue?

The subject continues in the underlying question of what a blockchain is, because the properties that make wallet security unusual — permanence, public verification and the absence of an intermediary — are properties of the ledger rather than of any particular wallet.

Metastarter publishes a companion explanation, What is blockchain and how does it work, which sets out how blocks are linked, why retroactive alteration is computationally expensive, and why a confirmed entry is treated as settled. The Bitcoin white paper, published in October 2008, describes the original design from which those properties derive. Readers who want to know who produces the material here will find the publisher’s details on the about page and the editorial contact details on the contact page.