PT-2026-55471 · Crates.Io · Zebra-Script+1
Published
2026-07-02
·
Updated
2026-07-02
·
CVE-2026-52735
CVSS v4.0
9.3
Critical
| Vector | AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:H/SA:H |
Am I affected
You are affected if:
- You run any version of
zebradup to and includingv4.4.1. - 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.Fix
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Weakness Enumeration
Related Identifiers
Affected Products
Zebra-Script
Zebrad