What to do?
Users are advised to stop using the older version of the software and upgrade to Electrum 3.0.4 before accessing the wallet. This is a security update and it fixes a vulnerability that was reported earlier today. If you don't use Electrum or a derivative, then you are not affected.
According to bitcointalk,
Update 1: If you had no wallet password set, then theft is trivial. If you had a somewhat-decent wallet password set, then it seems that an attacker could "only" get address/transaction info from your wallet and change your Electrum settings, the latter of which seems to me to have a high chance of being exploitable further. So if you had a wallet password set, you can reduce your panic by a few notches, but you should still treat this very seriously.
This is fixed now, see the release notes for details.
What is Electrum?
Electrum is one of the most popular Bitcoin wallets securing Bitcoin payments since 2011.It is fast, secure and easy to use free software released under the MIT License. It suits the needs of a wide spectrum of users. It's focus is speed, with low resource usage and simplifying Bitcoin. No single entity controls the network. Anyone can run an Electrum server. It supports hardware wallets - Ledger, Trezor, Keepkey.
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