ZEBRA: Consensus divergence via P2SH sigop undercount in pure-Rust disabled-opcode parser

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

ZEBRA is a Zcash node written entirely in Rust. Prior to 4.5.0, Zebra can accept a block that zcashd rejects because the P2SH signature-operation counter undercounts redeem scripts containing a disabled opcode followed by signature opcodes. In zebra-script/src/lib.rs, p2sh_input_sigop_count used the pure-Rust script::Code::sig_op_count path, whose try_fold parser stops at disabled opcodes such as OP_CODESEPARATOR and returns only the partial count accumulated before the error. The zcashd reference implementation continues static signature-operation counting through disabled opcodes, so an attacker can broadcast P2SH spends that Zebra counts below MAX_BLOCK_SIGOPS while zcashd counts above the 20,000-operation limit. If a Zebra miner includes those transactions, Zebra validators accept the block while zcashd validators reject it, creating a consensus chain split that affects network integrity and availability without requiring the attacker to produce a block. This issue is fixed in version 4.5.0.

Source published

18 Aug 2026 19:23:10 UTC

Source modified

18 Aug 2026 19:48:45 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 4.0 · 9.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:H

ZEBRA is a Zcash node written entirely in Rust. Prior to 4.5.0, Zebra can accept a block that zcashd rejects because the P2SH signature-operation counter undercounts redeem scripts containing a disabled opcode followed by signature opcodes. In zebra-script/src/lib.rs, p2sh_input_sigop_count used the pure-Rust script::Code::sig_op_count path, whose try_fold parser stops at disabled opcodes such as OP_CODESEPARATOR and returns only the partial count accumulated before the error. The zcashd reference implementation continues static signature-operation counting through disabled opcodes, so an attacker can broadcast P2SH spends that Zebra counts below MAX_BLOCK_SIGOPS while zcashd counts above the 20,000-operation limit. If a Zebra miner includes those transactions, Zebra validators accept the block while zcashd validators reject it, creating a consensus chain split that affects network integrity and availability without requiring the attacker to produce a block. This issue is fixed in version 4.5.0.

Source published

Aug 18, 2026, 7:23:10 PM (timezone not stated)

Source modified

Aug 18, 2026, 7:48:45 PM (timezone not stated)

Observed here

7 Nov 2026 11:35:37 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 4.0 · 9.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:H

ZEBRA is a Zcash node written entirely in Rust. Prior to 4.5.0, Zebra can accept a block that zcashd rejects because the P2SH signature-operation counter undercounts redeem scripts containing a disabled opcode followed by signature opcodes. In zebra-script/src/lib.rs, p2sh_input_sigop_count used the pure-Rust script::Code::sig_op_count path, whose try_fold parser stops at disabled opcodes such as OP_CODESEPARATOR and returns only the partial count accumulated before the error. The zcashd reference implementation continues static signature-operation counting through disabled opcodes, so an attacker can broadcast P2SH spends that Zebra counts below MAX_BLOCK_SIGOPS while zcashd counts above the 20,000-operation limit. If a Zebra miner includes those transactions, Zebra validators accept the block while zcashd validators reject it, creating a consensus chain split that affects network integrity and availability without requiring the attacker to produce a block. This issue is fixed in version 4.5.0.

Source published

18 Aug 2026 20:17:17 UTC

Source modified

22 Oct 2026 22:10:00 UTC

Observed here

25 Oct 2026 13:17:10 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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CVSS 4.0 · [email protected] · Secondary · 9.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

### Am I affected You are affected if: 1. You run any version of `zebrad` up to and including `v4.4.1`. 2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate. All default configurations are affected. No feature flags, non-default settings, or special build options are required. ### Summary Zebra's P2SH sigop counter uses a pure-Rust code path that short-circuits on disabled opcodes (such as `OP_CODESEPARATOR`), returning a partial count of zero for any sigops following the disabled opcode. The reference implementation (zcashd) correctly counts through disabled opcodes in its static sigop analysis. This produces a consensus divergence: Zebra accepts blocks that zcashd rejects when the block-wide `MAX_BLOCK_SIGOPS = 20,000` threshold is crossed on one side but not the other. An attacker can exploit this without mining capability. Broadcasting transactions that spend P2SH outputs with malicious redeem scripts is sufficient; any Zebra miner who includes those transactions in a block triggers a chain split between Zebra and zcashd validators. ### Details The P2SH sigop counter at `zebra-script/src/lib.rs:399` calls `script::Code(redeemed_bytes).sig_op_count(true)`, which is a pure-Rust path through `zcash_script-0.4.4`. The legacy (non-P2SH) sigop counter at `lib.rs:282-289` correctly uses the C++ FFI via `interpreter.legacy_sigop_count_script()`. Only the P2SH path bypasses the FFI. The Rust parser in `zcash_script-0.4.4/src/opcode/mod.rs:1247-1260` treats 16 disabled opcodes (0x7e through 0xab, including `OP_CAT`, `OP_SUBSTR`, `OP_AND`, `OP_OR`, `OP_XOR`, `OP_2MUL`, `OP_2DIV`, `OP_MUL`, `OP_DIV`, `OP_MOD`, `OP_LSHIFT`, `OP_RSHIFT`, and `OP_CODESEPARATOR`) as `Err(Error::Disabled(...))`. The `sig_op_count` function at `iter.rs:104-115` uses `try_fold`, which terminates on the first `Err` and returns the partial sum accumulated so far. zcashd's `GetOp2` (`script.h:514-562`) returns `true` for all non-push opcodes including the disabled range. Its `GetSigOpCount(true)` (`script.cpp:152-174`) continues counting through disabled opcodes. zcashd rejects disabled opcodes at execution time in the interpreter, not during static sigop analysis. A redeem script of `[0xab, OP_CHECKMULTISIG x 50]` produces: Zebra = 0 sigops, zcashd = 1,000 sigops. Across 21 inputs in a block, Zebra computes 0 while zcashd computes 21,000, crossing the `MAX_BLOCK_SIGOPS = 20,000` threshold on one side only. ### Patches Patched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter. ### Workarounds There is no configuration-level workaround. All Zebra nodes validating blocks on a network shared with zcashd are affected. Upgrade as soon as the patched version is available. ### Impact A chain split between Zebra and zcashd validators. The attacker broadcasts spending transactions referencing P2SH outputs whose redeem scripts contain a disabled opcode followed by `OP_CHECKSIG` or `OP_CHECKMULTISIG` opcodes. When a Zebra miner (estimated ~30% of current network hashrate) includes these transactions in a block, Zebra validators accept the block while zcashd validators reject it with `bad-blk-sigops`. The two halves of the network diverge and every subsequent block extending the Zebra-side tip inherits the divergence. The attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions. ### Credit Reported by `@samsulselfut` via a private GitHub Security Advisory submission.

Source published

2 Jul 2026 19:43:36 UTC

Source modified

2 Jul 2026 20:11:29 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:H

Publisher rating · CRITICAL