03 — Auditing · Failure mode

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.

By MarketComp Updated 2026-09-12 v1.3 About the authors Changelog

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.

Cite this page

MarketComp (2026). Underconstrained signals. The ZK Field Manual (Version 1.3). MarketComp. https://zkpick.com/audit/failure-modes/underconstrained-signals/

@misc{zkfieldmanual-underconstrained-signals,
  title        = {Underconstrained signals — The ZK Field Manual},
  author       = {MarketComp},
  year         = {2026},
  version      = {1.3},
  howpublished = {\url{https://zkpick.com/audit/failure-modes/underconstrained-signals/}},
  note         = {Accessed: YYYY-MM-DD}
}