[UTXO-BUG] MED-2: Merkle tree second-preimage vulnerability in compute_state_root()#2071
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Scottcjn merged 1 commit intoScottcjn:mainfrom Apr 5, 2026
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…e_state_root() The Merkle tree duplicates the last hash when the element count is odd (hashes.append(hashes[-1])). This is a well-known second-preimage weakness: UTXO sets [A,B,C] and [A,B,C,C] produce identical roots, meaning two nodes with different UTXO sets can agree on the state root. Fix: 1. Use domain-separated padding: SHA256(0x01 || last_hash) instead of duplicating the last element, making padding distinguishable from a real leaf. 2. Mix the UTXO count into each leaf hash so tree is bound to a specific cardinality — sets of different sizes always differ. Test added: - test_state_root_odd_count_unique All 35 tests pass. Bounty: #2819 (Medium, 50 RTC)
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Great security audit work! These fixes are critical for the protocol. |
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@Scottcjn please review |
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✅ Merged. Solid cryptographic fix — the naive Merkle tree odd-count duplication is a well-known second-preimage weakness. Domain-separated sentinel + count-binding in leaf hashes is the correct mitigation. 50 RTC paid. Wallet: |
This was referenced Apr 6, 2026
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Vulnerability Class
Medium — Merkle state root manipulation (50 RTC bounty)
The Bug
The Merkle tree duplicates the last hash when the element count is odd (
hashes.append(hashes[-1])). This is a well-known second-preimage weakness:[A, B, C]pads to[H(A), H(B), H(C), H(C)][A, B, C, C]naturally produces[H(A), H(B), H(C), H(C)]Two nodes with divergent UTXO sets can agree on state root, undermining consensus verification.
Fix
SHA256(0x01 || last_hash)instead of duplicating, making padding distinguishable from a real leafTest Added
test_state_root_odd_count_unique— verifies roots differ across 3→4→5 box transitionsAll 35 tests pass.
Files Changed
node/utxo_db.py—compute_state_root()rewritten with safe paddingnode/test_utxo_db.py— 1 test addedRef: Bounty #2819
My WALLET IS aroky-x86-miner