## Description ### Am I affected You are affected if: 1. You run any version of `zebrad` up to and including `v4.4.1`. 2. Your node accepts inbound P2P connections (`network.listen_addr` is set, which is the default). 3. Your node processes blocks past the checkpoint height (non-finalized state is active). All default configurations are affected. ### Summary Zebra records a block hash in `non_finalized_block_write_sent_hashes` when the block is sent to the write task, before contextual validation completes. If validation fails, the hash is not removed. A remote unauthenticated peer can deliver a poisoned block body that shares a header hash with a later valid canonical block. The poisoned body is rejected, but the hash remains cached. When the valid canonical block arrives, Zebra treats it as a duplicate and rejects it. The node cannot advance past that height until restart or a reorg event. ### Details ZIP-244 defines `txid_v5` without binding transparent input `scriptSig`, which lives in `auth_digest` and is committed to by `hashBlockCommitments` in the block header. Because `merkle_root` is computed over txids (not auth digests), and the block hash is computed over the header, an attacker can construct two blocks with identical header hashes but different transaction bodies by mutating the coinbase scriptSig. The attack flow over P2P: 1. Attacker observes a new block header (from any peer). 2. Attacker constructs a poisoned body by flipping a byte of the coinbase scriptSig extra-data section. The block hash is unchanged. 3. Attacker advertises the block hash via `inv` to the target node. 4. Target requests the block via `getdata`; attacker serves the poisoned body. 5. Zebra adds the hash to `non_finalized_block_write_sent_hashes` before validation. 6. The write task rejects the body at `block_commitment_is_valid_for_chain_history` (auth_data_root mismatch). 7. The hash is not removed from `non_finalized_block_write_sent_hashes`. 8. When the valid canonical block arrives (from honest peers or RPC), `queue_and_commit_to_non_finalized_state` sees the hash in the cache and returns `KnownBlock::WriteChannel` duplicate. 9. The node is stuck at height N-1. A secondary variant exists where chain pruning (via `MAX_NON_FINALIZED_CHAIN_FORKS`) removes a chain from `chain_set` but leaves its block hashes in `non_finalized_block_write_sent_hashes`, producing the same lockout for children of the pruned fork. ### Patches Patched in Zebra 4.4.2. The fix removes stale entries from `non_finalized_block_write_sent_hashes` on every failed non-finalized write path. ### Workarounds There is no complete configuration-level workaround. Reducing the node's inbound peer count (`network.peerset_initial_target_size`) narrows the attack surface but does not eliminate it. Restarting the node clears the in-memory cache and allows the valid block to be re-fetched. ### Impact A remote unauthenticated P2P peer can permanently stall a targeted Zebra node at a specific block height. The node diverges from the network tip; downstream consumers (lightwalletd, wallets, explorers, mining infrastructure) relying on the node see a stalled chain. The attack requires winning a propagation race: delivering the poisoned block body before honest peers deliver the canonical block. A well-positioned attacker (low-latency connection to the target, observation of new blocks from other peers) can reliably win this race. In sustained form, the attacker repeats for each new block, keeping the target permanently behind. Recovery requires restarting the node (which clears the in-memory sent-hash cache) or waiting for a reorg at the affected height (rare on the canonical chain). ### Credit Reported independently by `@ipwning` (primary, with ZIP-244 malleability analysis and zcashd cross-reference) and `@x15-eth` (first reporter, with E2E reproduction and control experiment).

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