CVE-2026-52735: Zebra script Zebrad Has Consensus Divergence Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-gf9r-m956-97qx",
  "modified" : "2026-07-02T19:43:36Z",
  "published" : "2026-07-02T19:43:36Z",
  "aliases" : [ "CVE-2026-52735" ],
  "summary" : "zebrad has consensus divergence via P2SH sigop undercount in pure-Rust disabled-opcode parser",
  "details" : "### Am I affected\n\nYou are affected if:\n\n1. You run any version of `zebrad` up to and including `v4.4.1`.\n2. Your node validates blocks on mainnet, testnet, or any network where both Zebra and zcashd nodes participate.\n\nAll default configurations are affected. No feature flags, non-default settings, or special build options are required.\n\n### Summary\n\nZebra'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.\n\nAn 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.\n\n### Details\n\nThe 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.\n\nThe 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.\n\nzcashd'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.\n\nA 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.\n\n### Patches\n\nPatched in Zebra 4.4.2. The fix routes the P2SH sigop counter through the same C++ FFI already used by the legacy sigop counter.\n\n### Workarounds\n\nThere 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.\n\n### Impact\n\nA 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.\n\nThe attacker does not need mining capability, RPC access, or any special privileges. The cost is the transaction fees for the funding and spending transactions.\n\n### Credit\n\nReported by `@samsulselfut` via a private GitHub Security Advisory submission.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "zebra-script"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "7.0.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.0.1"
    }
  }, {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "zebrad"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.5.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.4.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-gf9r-m956-97qx"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/AlfredoG87/zcash_script/blob/v0.4.4/src/script/iter.rs#L104-L115"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/ZcashFoundation/zebra"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ZcashFoundation/zebra/blob/d4cd662c716382f6397d2a730148025a1ca79fec/zebra-consensus/src/block.rs#L140"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/ZcashFoundation/zebra/blob/d4cd662c716382f6397d2a730148025a1ca79fec/zebra-script/src/lib.rs#L383-L400"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-684" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-02T19:43:36Z",
    "nvd_published_at" : null
  }
}