CVE-2026-52735: zebrad has consensus divergence via P2SH sigop undercount in pure-Rust disabled-opcode parser

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.5%, percentile 42: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 42% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes zebra-script (crates.io (Rust)) <= 6.0.1, fixed in 7.0.0; zebrad (crates.io (Rust)) <= 4.4.1, fixed in 4.5.0. Check your lockfile, including transitive dependencies.

What it is

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, p2shinputsigopcount used the pure-Rust script::Code::sigopcount path, whose tryfold parser stops at disabled opcodes such as OPCODESEPARATOR 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 MAXBLOCKSIGOPS 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.

From the GitHub advisory

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

Weakness type: CWE-684.

Product: zebra-script (crates.io (Rust)) · Assigned by: GitHub_M · Published: 2026-08-18

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact partial.
• FIRST EPSS (model of 2026-10-21): 0.51% probability of exploitation activity in the next 30 days; percentile 42.

Timeline

• 2026-07-02 — GitHub-reviewed advisory GHSA-gf9r-m956-97qx published (critical)
• 2026-08-18 — CVE record last updated
• 2026-08-18 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-gf9r-m956-97qx github.com
2. CVE.org record CVE-2026-52735 cve.org
3. NVD entry CVE-2026-52735 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com
6. github.com github.com

Critical crates.io (Rust) advisories · Today’s Security Digest