CVE-2026-54496: Zebra: Missing copy constraint in halo2_gadgets variable-base scalar multiplication allows under-constrained base, breaking Orchard Action circuit soundness

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 0.3%, percentile 23: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 23% 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 zebrad (crates.io (Rust)) <= 4.5.1, fixed in 5.0.0; halo2_gadgets (crates.io (Rust)) < 0.5.0, fixed in 0.5.0; orchard (crates.io (Rust)) < 0.14.0, fixed in 0.14.0; zcash_primitives (crates.io (Rust)) < 0.28.0, fixed in 0.28.0. Check your lockfile, including transitive dependencies.

What it is

ZEBRA is a Zcash node written entirely in Rust. Prior to zebrad 5.0.0, halo2gadgets 0.5.0, orchard 0.14.0, zcashprimitives 0.28.0, and zcashd 6.20.0, the variable-base scalar multiplication gadget in halo2gadgets/src/ecc/chip/mul/incomplete.rs used assignadvice() for the base point without a copy constraint tying it to the actual base, allowing a malicious prover to produce a valid proof for an Orchard Action with an under-constrained base point and bypass the diversified-address-integrity check that binds pkd, gd, ivk, the nullifier (nf), and the spend validating key (ak) to the note being spent. This issue is fixed in zebrad 5.0.0, halo2gadgets 0.5.0, orchard 0.14.0, zcashprimitives 0.28.0, and zcashd 6.20.0.

From the GitHub advisory

Summary

A soundness vulnerability in the variable-base scalar multiplication gadget of halo2gadgets allowed a malicious prover to produce a valid proof for an Orchard Action with an under-constrained base point. Because this gadget enforces the diversified-address-integrity condition of the Orchard Action statement, the flaw let a prover satisfy that condition for an arbitrary (pk d , g d , ivk) triple, effectively bypassing the check that binds an Action to the correct incoming viewing key — and therefore to the correct nullifier (nf) and spend validating key (ak) — of the note being spent.

The main practical consequence is that an adversary could have performed a double-spend within the Orchard pool, resulting in a balance violation: the same note could be spent multiple times, each time revealing a distinct, valid-looking nullifier. The total ZEC supply was protected by Zcash's turnstile mechanism, which bounds value flowing out of any pool, so unbounded inflation of the overall supply was not possible; the exposure was inflation of value within the Orchard pool up to the turnstile-enforced limit.

Exploiting the vulnerability via a double-spend is undetectable on-chain. Exploitation only requires setting private circuit inputs to chosen values, and nullifiers produced by a double-spend are indistinguishable from honest nullifiers, so the zero-knowledge property hides any signature of the attack.

Alternatively, an adversary could (before the fix) have stolen funds by forging a spend authorization for an existing note. To do so they would have to know the note plaintext, which in practice they might obtain by knowing the corresponding incoming viewing key. For example, this could be used to bypass the protection provided by holding spending keys on a hardware wallet, if the linked software wallet were compromised. This form of exploitation cannot be detected via the turnstile (which might be a motivation to exploit the vulnerability in this way rather than via balance violation). It can be detected by the legitimate holder of stolen funds being unable to spend them, or by seeing the note's nullifier on-chain — but this cannot necessarily be distinguished from compromise of the spending key by other means.

Weakness type: CWE-345.

Product: zebrad (crates.io (Rust)) · Assigned by: GitHub_M · Published: 2026-07-17

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: changed (can affect other components)

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): 0.32% probability of exploitation activity in the next 30 days; percentile 23.

Timeline

• 2026-07-06 — GitHub-reviewed advisory GHSA-ww9q-8r59-xv46 published (critical)
• 2026-07-17 — CVE record last updated
• 2026-07-17 — 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-ww9q-8r59-xv46 github.com
2. CVE.org record CVE-2026-54496 cve.org
3. NVD entry CVE-2026-54496 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
7. zfnd.org zfnd.org

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