Note: As of January 25, 2021, this vulnerability has been addressed in the latest version of MetaMask on all platforms. To the best of our knowledge, this attack was never exploited in the wild.

Impact

In version 8.0.x and lower of the MetaMask extension, if a user adds a custom network to MetaMask without specifying a chain ID, MetaMask requests net_version from the network's RPC endpoint at runtime, and uses the return value to sign transactions.

This can induce the user to sign transactions for unintended chains in the following ways:

1. net_version returns the network ID, which may differ the chain ID. Transactions signed with a network ID could be invalid for the intended chain, and valid for another. See EIP-155 for details.
2. A malicious or faulty endpoint could return arbitrary results for net_version at runtime.

Any user of MetaMask version 8.0.x and lower that adds or has added a custom network without specifying a chain ID is vulnerable to this attack.

Patches

Version >=8.1.0 includes 088d4c3.

Workarounds

• Update the MetaMask extension to version >=8.1.0 as soon as it's available on your platform. For most users, this will already have been done automatically.
  • Version >=8.1.0 requires chainId values to be specified by the user for all custom networks, and MetaMask will only use those values to sign transactions.

References

• EIP-155
• Corresponding vulnerability on MetaMask Mobile

For more information

If you have any questions or comments about this advisory:

• Open an issue in MetaMask/metamask-extension
• Email us at support@metamask.io