GHSA-gf9r-m956-97qx
GHSA-gf9r-m956-97qx is a CWE-684 vulnerability in zebra-script. O3 Security confirms whether GHSA-gf9r-m956-97qx is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
zebrad has consensus divergence via P2SH sigop undercount in pure-Rust disabled-opcode parser
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-gf9r-m956-97qx.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
zebra-script🦀zebradReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | zebra-script | all versions | 7.0.0 |
| 🦀crates.io | zebrad | all versions | 4.5.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for zebra-script. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update zebra-script to 7.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-gf9r-m956-97qx is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-gf9r-m956-97qx is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-gf9r-m956-97qx. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-gf9r-m956-97qx in your dependencies?
O3 detects GHSA-gf9r-m956-97qx across crates.io dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.