---
title: "Underconstrained signals"
description: "The circuit admits witnesses that do not correspond to any correct execution: an intermediate value is computed in the witness but never constrained, or a…"
type: "failure-mode"
url: "https://zkpick.com/audit/failure-modes/underconstrained-signals/"
section: "03 Auditing a ZK protocol"
authors:
  - "MarketComp"
publisher: "MarketComp"
version: "1.3"
updated: "2026-09-12"
license: "CC-BY-4.0"
json: "https://zkpick.com/data/audit/failure-modes/underconstrained-signals.json"
---

*By MarketComp. Updated 2026-09-12. Version 1.3. CC BY 4.0.*

# Underconstrained signals

The circuit admits witnesses that do not correspond to any correct execution: an intermediate value is computed in the witness but never constrained, or a constraint is weaker than the property intended. The proof verifies; it just proves less than everyone believed. This is the single most common exploitable ZK defect class.

**Mitigation:** Run automated underconstrained-signal detection; review that every witness-assigned value is also constrained; fuzz for accepted-but-invalid witnesses rather than only checking valid ones are accepted.