Litecoin Core 0.21.5.6 is an urgent maintenance release focused on MWEB security and reliability. The project recommends upgrading for all users, while miners, pools and MWEB service operators are specifically told to update before the activation height of a new consensus rule.
The release adds a node-wide limit for expensive MWEB light-client requests such as `getmwebleafset` and `getmwebutxos`, preventing reconnecting peers from repeatedly consuming excessive resources. Relay policy now also limits MWEB transaction weight and input count before expensive cryptographic verification, so oversized transactions are rejected before reaching the mempool.
Other changes reject invalid MWEB output public keys, improve handling of mutated block data and discourage peers that deliver invalid MWEB data through compact blocks. Block construction also enforces the MWEB consensus input limit so miners do not assemble oversized input sets.
At mainnet height 3,154,440, nodes running 0.21.5.6 will reject an MWEB block whose kernel signals a pegout while carrying an empty pegout list. This is a soft-forking consensus rule. Valid wallets and miners do not create that encoding, but mining operators need the update in place before activation.