GHSA-hhm7-qrv5-h4r6: Zebra: Repeated Non-Finalized Shielded Transaction Aborts Zebra Before Duplicate-Nullifier Rejection

Am I affected You are affected if: 1. You run zebrad up to and including v4.4.1. 2. Your node processes blocks past the checkpoint height (non-finalized state is active). 3. The network has NU5 or later activated. All default configurations are affected. Summary Chain::push in the non-finalized state updates the transaction-location index (tx_loc_by_hash) before it runs the duplicate shielded-nullifier guard. When an invalid child block repeats a shielded transaction from its non-finalized parent, the assert_eq!(prior_pair, None, "transactions must be unique within a single chain") fires before the contextual validation that would cleanly reject the duplicate. Under Zebra's panic = "abort" release profile, this terminates the entire node process. The block should be rejected with a duplicate-nullifier contextual validation error. Instead, the ordering of index updates within Chain::push causes the process to abort. Details In zebra-state/src/service/non_finalized_state/chain.rs:1608-1628, the block push sequence is: 1. Insert transaction hash into tx_loc_by_hash with assert_eq! on uniqueness 2. Update transparent outputs and inputs 3. Update shielded data (JoinSplit, Sapling, Orchard) — including nullifier uniqueness checks The shielded nullifier uniqueness check at step 3 would correctly reject the duplicate transaction. But the assert_eq! at step 1 fires first because the transaction hash is already in tx_loc_by_hash from the parent block on the same chain. The block transaction verifier does not run the best-chain nullifier query for block transactions — that check is gated on mempool transactions only (zebra-consensus/src/transaction.rs:521-526). Initial contextual validation checks nullifiers in finalized state only (zebra-state/src/service/check.rs:407-415), but the parent transaction is still in non-finalized state. There are two attack models: Model A (two attacker blocks): The attacker mines two conse