Zebra: Finalized address balance credit-first overflow on consensus-valid blocks

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node processes blocks on any Zcash network.

Summary

The finalized transparent address balance writer processes all newly-created outputs (credits) before processing spent outputs (debits) within the same block. A consensus-valid block containing a long chain of same-address transparent self-spends can cause the intermediate per-address balance during the credit pass to exceed MAX_MONEY, triggering a panic in the finalized state writer.

Because the triggering block is consensus-valid (zcashd accepts it), the panic recurs on restart when the node re-encounters the same block. This creates a persistent chain halt that can only be resolved by a software patch.

Details

The finalized state writer at zebra-state/src/service/finalized_state/zebra_db/transparent.rs iterates all transaction outputs in a block and credits them to per-address balances before iterating inputs and debiting spent outputs. When a block contains many transparent self-spends to the same address, the intermediate credit-only balance can exceed the MAX_MONEY supply cap even though the final net balance (credits minus debits) is valid.

The code panics on the intermediate overflow via .expect() on the balance addition. Under Zebra's panic = "abort" release profile, this terminates the process. On restart, the node re-downloads and re-processes the same consensus-valid block, triggering the same panic.

An attacker with approximately 1,100–2,100 ZEC and mining capability can construct a block that permanently halts all Zebra nodes. The attacker recovers their capital (the self-spends return funds to the same address), so the net cost is the mining effort only.

Patches

Patched in Zebra 4.4.2. The fix processes credits and debits together per transaction rather than all credits then all debits, matching zcashd's approach.

Workarounds

No workaround is available. Upgrade to Zebra 4.4.2.

Impact

A single consensus-valid mined block can permanently halt all Zebra nodes on the network. The halt persists across restarts. Recovery requires deploying a patched version. Downstream consumers (light wallets, exchanges, mining infrastructure) lose service for the duration of the halt.

Credit

Reported by @sangsoo-osec.