Quoth Bitcoin release-notes-0.9.0.md:
This release contains a few fixes for transaction ID (TXID) malleability issues:
-nospendzeroconfchange command-line option, to avoid spending zero-confirmation change
IsStandard() transaction rules tightened to prevent relaying and mining of mutated transactions
Additional information in listtransactions/gettransaction output to report wallet transactions that conflict with each other because they spend the same outputs.
Bug fixes to the getbalance/listaccounts RPC commands, which would report incorrect balances for double-spent (or mutated) transactions.
New option: -zapwallettxes to rebuild the wallet's transaction information
What does "transaction rules tightened" mean and how does it prevent relaying and mining of mutated transactions? Since mutated transactions are also valid transactions, how can the client tell original transactions apart from mutated ones?
Are powerful adversaries still able to abuse this feature after the update?