Electrum has released an urgent 4.8.3 update after a critical vulnerability was discovered in previous versions; at least one user has already lost funds.

Threat: Outgoing Transaction Replacement via Malicious Server

If not updated, a malicious server can intercept a sent transaction, prevent it from reaching the network, and substitute it with a fake transaction that sends funds to the attacker's address.

Electrum: Lightweight BTC Wallet Without Altcoin Support

Electrum is one of the oldest and most well-known lightweight Bitcoin wallets; it supports only BTC and does not work with altcoins.